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I. Anatomy of a Fish (Coloring)
Most vertebrates have the same basic body plan when it comes to internal organs. Amphibians, mammals, and birds have double-loop circulation, where blood leaves the heart, goes to the lungs, and then returns to the heart before being pumped to the body.
Color the following:
- Caudal Fin (blue)
- Kidney (green)
- Dorsal Fin (yellow)
- Swim Bladder (blue)
- Esophagus (yellow)
- Operculum (brown)
- --- Lateral Line System (black)
- << Scales (purple)
- Gills (red)
- Heart (pink)
- Pelvic Fin (green)
- Liver (brown)
- Stomach (green)
- Intestine (dark blue)
- Reproductive Organs (orange)
- Anal Fin (pink)
II. Fish Scales Tell the Age of a Fish
Look at the image of the fish scale, like a tree, scales show rings that indicate periods of growth. Rings that are farther apart occur when the fish grows well and there is lots of food - in the summer season. Rings that are close together occur when the fish does not get much food and grows slowly. On the scale you can identify the summer growth and the winter growth. (There will be several rings in each). The core represents the fish when it was first born, as a fry. The rings near the edge are the most recent periods of growth.
Color the summer growth periods green. | Color the winter growth periods blue.
How old is this fish (in years)?
III. Fish Fins Are Used for Swimming
The fins of the fish are used for swimming but each one has a specific job. The dorsal fin is sometimes split into an anterior and posterior dorsal fin. Both are used to help the fish maintain its upright position in the water. The anal fin has the same function. Pectoral and pelvic fins are used for steering and the caudal fin is used to propel the fish forward. Fish swim in a side-to-side motion. Aquatic mammals swim with an up-and-down motion, which is consistent with their evolutionary relationship with land mammals.
Label each of the fish's fins below and color.
- Pelvic Fin (green)
- Pectoral Fin (red)
- Caudal Fin (blue)
- Anal Fin (pink)
- Dorsal Fin (yellow)
Bony Fish Facts
- M.S., Resource Administration and Management, University of New Hampshire
- B.S., Natural Resources, Cornell University
Most of the world's fish species are categorized into two types: bony fish and cartilaginous fish. In simple terms, a bony fish (Osteichthyes) is one whose skeleton is made of bone, while a cartilaginous fish (Chondrichthyes) has a skeleton made of soft, flexible cartilage. A third type of fish, including eels and hagfish, is the group known as Agnatha, or jawless fish.
The cartilaginous fish include sharks, skates, and rays. Virtually all other fish fall into the class of bony fish which includes over 50,000 species.
Fast Facts: Bony Fish
- Scientific Name:Osteichthyes, Actinopterygii, Sacropterygii
- Common Names: Bony fish, ray-finned and lobe-finned fishes
- Basic Animal Group: Fish
- Size: From below a half inch to 26 feet long
- Weight: Well under an ounce to 5,000 pounds
- Lifespan: A few months to 100 years or longer
- Diet: Carnivore, Omnivore, Herbivore
- Habitat: Polar, temperate, and tropical ocean waters as well as freshwater environments
- Conservation Status: Some species are Critically Endangered and Extinct.
Fish have gills for breathing. They inhale water through their mouths, then close their mouths and force water out over the gills. Here, hemoglobin in blood circulating in the gills absorbs dissolved oxygen in the water. The gills have a gill cover, or operculum, through which the water flows out.
Many fish have a swim bladder, which is used for buoyancy. The swim bladder is a sac filled with gas that is located inside the fish. The fish can inflate or deflate the swim bladder so that it is neutrally buoyant in the water, allowing it to be at the optimal water depth.
Groups of Fish
2 Classes of Jawless fish:
- Lamprey (parasitic)
- Hagfish (scavenger)
- Both have cartilage skeleton
- Cartilage Fish
- Sharks, stingrays
- Most are predators
- Basking sharks are filter feeders
- No swim bladder, pectoral fins rigid
- Bony Fish
- Ray-finned ( Goldfish, Bass, Carp, Salmon )& Lobe Finned ( Coelacanth )
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The Anatomy of a Bony Fish - Biology
A Quick Course in Ichthyology
by Jason Buchheim
Director, Odyssey Expeditions
- FISH Definition
- FISHES- class agnatha
- FISHES- Class Chondrichthyes
- Shark Attack
- REDUCING THE RISK
- Fish are fun!
FISH : Any of a large group of cold-blooded, finned aquatic vertebrates. Fish are generally scaled and respire by passing water over gills.Modern fish are divided into three classes.
I. AGNATHA, primitive jawless fish.Lampreys and Hagfish
II. CHONDRICHTHYES, the jawed fish with cartilaginous skeletons. Sharks, Rays, Rat-Fishes
III. OSTEICHTHYES, fish with bony skeletons.Lungfish, Trout, Bass, Salmon, Perch, Parrot Fish
Fish come in all shapes and sizes, some are free swimming, while others rest on the bottom of the sea, some are herbivores and others are carnivores, and some lay eggs while others give live birth and parental care to their young.
FISH: the members of a single species FISHES: more than one species of fish FISHES- class Agnatha
- No jaws
- Cartilaginous skeleton
- Scaleless skin
- Oral sucker in place of jaws
- Predators and filter feeders
- anticoagulating saliva
- fresh and salt water
- some anadromous
- Cartilaginous skeleton
- Skin covered with denticles, not scales
- Five to seven gill slits per side
- No swim bladder
- Internal fertilization
- Spiral valve intestines
- Five to seven gill arches
- Cartilaginous jaws, loosely attached lower jaws
In fact, most sharks are entirely incapable of this feat. The largest fish of all, the Whale Shark, which can reach sizes of up to 59 feet and weigh 88,000 lb., is a very calm and approachable plankton feeder. There are many species of sharks which can inflict severe bodily injury and require the utmost of respect. The most feared of all, the Great White Shark, has been responsible for most of the fatal shark attacks off the California and Australian coastlines. While the Great White gets all the notoriety, pound for pound, the Bull Shark is probably the most ferocious. The Great White generally attacks a person because it has confused it with its favorite food, the seals and sea lions, but the Bull Shark will attack a person just because they are there. Even with these dangerous animals roaming the ocean, your chances of getting attacked by a shark are very remote.
Worldwide, there are only about three hundred documented shark attacks a year. The chances are much higher that you will be hit by a drunk driver while driving to the beach then they are that you will even encounter a dangerous shark when you get there. There are some activities that will greatly increase your chance of a shark attack, such as carrying speared fish with you while diving or collecting abalone in turbid waters. Statistics of 1,652 shark attacks show that males are much more likely to be attacked than females (10 to 1 ratio), this is probably because males are much more active in the water, surfing and going to deeper depths where sharks are more common.
The presence of large numbers of fish, or fish behaving in an unusual manner, has been reported preceding many attacks. In 40 percent of the reported shark attacks, people were pole-fishing or spear-fishing in the area of an attack. A comparison of the number of people swimming to those fishing and spear-fishing seems to show that these two pastimes have by far the highest risk of inducing an attack. While swimming, the chance of drowning is more than 1,000 times greater than that of dying from a shark attack.
Most shark attacks occur in shallow water, where most bathers are, and in 94 percent of the cases the attack was by an individual shark acting alone. About 10 percent of reported shark attacks are on divers since the number of divers in the water at one time must be much smaller than 10 percent of beach bathers, the odds of being attacked must be significantly greater for divers.
Close passes were seldom made before the attack, and in the majority of the cases there was only one strike. Few attacks involved more than one bite. This indicates that in many cases the attacking shark mistook the victim for a more usual kind of food and did not attack any further when the error was discovered. It is fortunate that sharks, in most cases, do not consider humans to be suitable food. This information also refutes the long-standing notion that fresh human blood is a powerful attractant that excites sharks into a feeding frenzy. If this were so, the presence of blood would certainly have induced that attacking shark to strike the victim repeatedly. Most wounds occur on the appendages- the hands, arms, legs, and feet. Lacerations of varying severity are the most common types of injury. About 25 percent of attacks kill the victim. The most usual cause of death is shock, combined with a severe loss of blood.
REDUCING THE RISK
Swimmers and divers can reduce the chance of being attacked by following a few simple rules: Never swim in areas where sharks are known to be common. Never enter the water where people are fishing, either from the beach or from inshore boats. If there are a number of people in the water, do not separate yourself from them. There is safety in numbers. Avoid swimming near deep channels, or where shallow water suddenly becomes deeper. Do not swim alone, or at dusk or after dark, when sharks are feeding actively and are likely to be closer to the shore. Do not enter the water, or if in the water leave immediately, if large numbers of fish are seen, or if fish seem to be acting strangely. Be alert for unusual movements in the water. Do not wear a watch or other jewelry that shines and reflects light. Do not enter the water with an open wound, and women should not swim during their menstrual periods.
Sharks are animals that are superbly adapted to their environment. Almost all are carnivores or scavengers, although the species that live close to the sea floor feed mostly on invertebrates. Most possess a keen sense of smell, a large brain, good eyesight, and highly specialized mouth and teeth. Their bodies are usually heavier than water, and they do not have an air filled swim bladder for buoyancy like most bony fishes. All sharks have an asymmetric tail fin, with the upper lobe being larger than the lower one. This feature, together with flattened pectoral fins, and an oil-filled liver compensates for the lack of a swim bladder. There are 344 known species of sharks living in all parts of the oceans, from shallow to deep water and from the tropics to the polar regions. A few even venture into fresh water and have been found in rivers and lakes. Contrary to popular belief, most sharks are harmless to humans. Sharks are classified into eight orders:
1. Sawsharks (Pristophoriformes), one family, five sp.Live on the bottom in warm temperate or tropical seas. Easily recognized because of tube, blade like snouts. Bear live young.
2. Dogfish Sharks (Squaliformes), three families, 73 sp. Bottom dwelling deep water sharks, distributed worldwide. Bear live young and eat bony fishes, crustaceans, squid and other sharks. Harmless to humans.
3. Angel Sharks (Squatiniformes), one family, 13 sp. Flattened, bottom dwelling sharks. Found on continental shelves and upper slopes of cold temperate and tropical seas. Have very sharp, awl-like teeth that are used to impale small fish and crustaceans.
4. Bullhead Sharks (Heterodontiformes), one family, 8 sp. Live on rocky reefs where there are plenty of cracks and crevices. Found in Pacific and Indian Ocean. Eat invertebrates.
6. Mackerel Sharks (Lamniformes), seven families, 16 sp. Small, highly diverse order. Found in tropical to cold temperate or even Arctic waters. Oceanic and coastal. Most very large, eat bony fish, other sharks, squid, and marine mammals. Includes the Mako and Great White and the plankton eating Megamouth and Basking Sharks.
7. Carpet Sharks (Otectolobiformes) seven families, 31 sp. Warm tropical to temperate waters. All members except whale shark live on bottom. Flattened. Most eat small fishes and invertebrates. Whale shark is plankton feeder. Some bear live young and others lay eggs.
8. Ground Sharks (Carcharhiniformes) 8 families, 193 sp. Largest order of sharks. Worldwide distribution, temperate and tropical waters. Most live near coast, although some found in deeper waters. Eat bony fishes, other sharks, squid, and small invertebrates. Includes the dangerous Tiger shark.
Sharks have numerous structural and physiological features that make them unique among the fishes. They have a simple cartilaginous skeleton with no ribs, and a cartilaginous jaw, backbone, and cranium.
Thick skin supports the flimsy skeleton. The skin is elastic and aids in movement when the tail is arched, it pulls on the skin, which pulls back like a rubber band. The jaws are not connected to the skull and become unhinged, protruding forward from the skull allowing for a wider gape when feeding. The teeth are ossified with minerals known as 'apatite'. They form a conveyer belt with as many as eight teeth in a row. When a shark looses a tooth, another one just pops up. Sharks go through up to 2,400 teeth a year.
Sharks have placoid scales which are fixed, slightly ossified and layered. They are smooth to the touch in one direction and extremely course in another. Just rubbing a shark the wrong way can inflict serious wounds.
All sharks, rays, and skates are carnivores. They have normal sensory modalities, a small brain (most of which is dedicated to the olfactory lobes giving them an acute sense of smell) and well developed eyes with color vision and adaptation to low light levels.
Some sharks lay eggs (all skates and ratfish do), but most are ovoviviparous (all rays are). The young develop with their yolk sacks within the mother, but without a placenta or umbilical cord. Some sharks (the Great White) are oviphagous the young eat the other developing young and embryos inside their mother and only the fiercest is born! A few sharks (hammerheads and reef sharks) are viviparous like mammals, the young are nourished with a placenta within the mother. The gestation period is around 22 months and 2-80 pups are born per litter. Because most sharks are ovoviviparous or viviparous, they do not produce mass numbers of young like other fish do. They are slow to develop and for this reason shark population numbers have been decreasing rapidly due to the recent popularity of shark fin soup. Fishermen are taking many more sharks than the maximum sustainable yield will allow. Some sharks will soon be endangered species. Rays
Rays in general are physiologically exactly like sharks except the rays pectoral fins are fussed to their heads. Their gills are ventrally located. They swim with their ventral fins, like wings. Their eyes are dorsally [top] located and have spericules behind them. The spericules are used to breathe in with.
Rays are modified as bottom feeders, feeding on invertebrates found in the sand. Sometimes you can watch a ray making quite a ruckus on the sand bottom in search of the invertebrates.
Manta rays are planktivores and cruise the open water filter feeding out small animals. Mantas are the largest of the rays.
Electric rays swim with their caudal fin and use their modified pectoral fins to electrically shock and stun their prey.
Sawfish look like sharks but have true fused pectoral fins and gills on the ventral surface.
Stingrays have a toxin filled spine at the base of their tail. Stingrays are not the mean creatures roaming the waters to hurt swimmers, as many people believe them to be. Stingrays are actually very approachable and can be hand fed and petted, just don't step on them!
The bony fish comprise the largest section of the vertebrates, with over 20,000 species worldwide. They are called bony fish because their skeletons are calcified, making them much harder than the cartilage bones of the chondrichthyes. The bony fishes have great maneuverability and speed, highly specialized mouths equipped with protrusible jaws, and a swim bladder to control buoyancy.
The bony fish have evolved to be of almost every imaginable shape and size, and exploit most marine and freshwater habitats on earth. Many of them have complex, recently evolved physiologies, organs, and behaviors for dealing with their environment in a sophisticated manner.
Eels -Anguilliformes 597 spp
Salmon -salmoniformes 350 spp
Deep Sea Fish -Stomiiformes 250 spp Gobies -Gobiesociformes 114 spp Trumpetfish -Syngnathiformes 257 spp
Flyingfishes -Cyprinodontiformes 845 spp
Silversides -Atheriniformes 235 spp
Squirrelfishes -Beryciformes 164 spp
Scorpionfishes -Scopaeniformes 1160 spp
Flatfish -Pleuronectiformes 538 spp
Triggerfish -Tetraodontiformes 329 spp
Perch Like -Perciformes 7791 spp, largest order
FISH SEX- how fish reproduce
Fish have come up with three modes of reproduction depending on the method they care for their eggs.
- Ovopartity -- Lay undeveloped eggs, External fertilization (90% of bony fish), Internal fertilization (some sharks and rays)
- Ovoviviparity - Internal development- without direct maternal nourishment-Advanced at birth (most sharks + rays)-Larval birth (some scorpeaniforms-rockfish)
- Viviparity - Internal development- direct nourishment from mother-Fully advanced at birth (some sharks, surf perches)
Parental care: In fishes, parental care is very rare as most fish are broadcast spawners, but there are a few instances of parental care. Male gobies guard the eggs in a nest until they are born. The male yellowhead jawfish actually guards the eggs by holding them in his mouth! Weird Fish Sex!
Some fish are very kinky creatures by human standards, displaying behavior that would probably get a human incarcerated for a long time.
- Hermaphroditism : Some fish individuals are both males and females, either simultaneously or sequentially. There is no genetic or physical reason why hermaphroditism should not be present. About 21 families of fish are hermaphrodites.
- Simultaneous hermaphrodite : There are some instances where being a member of both sexes could have its advantages. Imagine all the dates that you could have! In the deep sea, the low light levels and limited food supply make for a very low population density meaning that potential mates are few and far between. Members of the fish family Salmoniformes (eg salmon) and Serranidae (hamlets) are simultaneous hermaphrodites they can spawn with any individual encountered.
- Sequential hermaphrodite: Very strange life histories develop in species whose individuals may change sex at some time in their life. They may change from being males to females (protandry) or females to males (protogyny).
A classic example of protogyny is found in the wrasses and parrotfishes. The males in these species form harems, with one large male sequestering and defending a group of smaller females. The male enjoys spectacular reproductive success, as it has many females to mate with. The females also enjoy a limited reproductive success, producing as many eggs as they can, all fertilized by the one male. The male has the advantage over the females it has many females producing eggs for him to fertilize, whereas the females only have themselves. It is great to be the king!
The weird sex stuff comes in when we analyze what the reproductive success of a smaller male may be. As only the largest male, the 'SuperMale' gets to mate with the females, a smaller male would enjoy zero reproductive success. There is no advantage to being a small male, and this is where the hermaphrodism comes in. If all the smaller fish were females, they could all enjoy a limited reproductive success while they are growing. If the male dies, the one that has grown to be the largest female will change sexes and become the male, in turn enjoying a much greater reproductive success than if she did not switch. So there are no small males and everything is all said and done, but wait! Evolution has a keen ability in finding weaknesses in any system, and it has done so with the parrotfish. In nature, we do find smaller male parrotfish, why should this be so? It has to do with the kind of thing that if a parrotfish was a human, could get the parrotfish into a great deal of trouble. The 'supermale' has to run around all of the time keeping track of and protecting all of his females as well capturing and eating food himself, so he does not necessarily have time to pay close attention to the details. When parrotfish mate, they form a spawning aggregation where the supermale will release his sperm into the water and the many females release their eggs. The sperm and egg find each other in the water column and fertilization takes place, and this is where the weakness of the system lays. Along comes the smaller male, who has evolved to look just like a female. Most of the time the smaller male will make itself completely inconspicuous by behaving just like the females, but during the spawning aggregations, he will be releasing sperm instead of eggs. The supermale will probably not even know that he has been conned. Everything gets really mixed up as males are changing into females changing into males. FISH- Schooling Behavior
Everyone has heard of a school of fish, an aggregation of fish hanging out together but why, they are obviously not learning reading, writing, and arithmetic. Schools of fish may be either polarized (with all the fish facing the same direction) or non polarized (all going every which way)
There are some factors that can make it advantageous to hang out with other fish.
- A. Confusion effect. A large school of fish may be able to confuse a potential predator into thinking that the school is actually a much larger organism.
B. Dilution affect. If a fish hangs out with a lot of other fish and a predator does come around, the predator must usually select one prey item. With so many choices, the chances are that it will not be you. This is known as the 'selfish herd'.
Enhanced Foraging: A school of fish may have better abilities to acquire food. With many more eyes to detect food, many more meals may be found but there would also be many more mouths to feed. By working as a team, the school may be able to take larger food items than any one individual could manage to capture.
Migration: The migration abilities of fish in schools may possibly be enhanced due to better navigation, etc. Hydrodynamic efficiency: Due to the complex hydrodynamic properties of water (properties the fish probably discovered only by accident), a fish may gain a swimming advantage by being in a school. The slipstream from the fish ahead of it may make it easier to pass through the water. Good for all the fish except for the ones in front.
The density of water makes it very difficult to move in, but fish can move very smoothly and quickly.
A swimming fish is relying on its skeleton for framework, its muscles for power, and its fins for thrust and direction.
The skeleton of a fish is the most complex in all vertebrates. The skull acts as a fulcrum, the relatively stable part of the fish. The vertebral column acts as levers that operate for the movement of the fish.
The muscles provide the power for swimming and constitute up to 80% of the fish itself. The muscles are arranged in multiple directions (myomeres) that allow the fish to move in any direction. A sinusoidal wave passes down from the head to the tail. The fins provide a platform to exert the thrust from the muscles onto the water.
Diagram of forces when a fish swims.
Thrust- force in animal's direction
Lift- force opposite in right angles to the thrust
Drag- force opposite the direction of movement
- Cruisers: These are the fish that swim almost continuously in search for food, such as the tuna. Red Muscle- richly vascularized (blood-carrying capacity), rich in myoglobin (oxygen holder and transferor into the muscles active sites) * able to sustain continuous aerobic movement.
- Burst Swimmers: These fish usually stay relatively in the same place such as most reef fish.
- Caudal fin-- provides thrust, and control the fishes direction
- Pectorals-- act mostly as rudders and hydroplanes to control yaw and pitch. Also act as very important brakes by causing drag.
- Pelvic fins-- mostly controls pitch
- Dorsal/anal-- control roll
- A tuna fish which has a fusiform similar to a torpedo can cruise through the water at very high speeds.
- The attenuated shape of the eel allows it to wiggle into small crevices where it hunts prey.
- The depressed shape of the angler fish is advantageous for its "sit and wait" strategy of hunting.
- The compressed shape found on many reef fishes such as the butter fish gives the fish great agility for movement around the reef and can support sudden bursts of acceleration.
- Ectothermic: fish derive their heat from the environment
- Poikilothermic : fish conform to the heat in the environment
They maintain a higher body temperature through the use of a specialized counter-current heat exchanger called a reta mirabile. These are dense capillary beds within the swimming muscle that run next to the veins leaving the muscles. Blood passes through the veins and arteries in a counter current (opposite) direction. The heat produced from the muscle contraction flows from the exiting veins into the incoming arteries and is recycled.
Why should they bother having an elevated body temperature? To increase the speed of the fish. The higher the body temperature, the greater the muscular power. Thirty degrees Celsius is the optimum temperature for muscular speed. With increased speed, the tuna can capture the slower, cold blooded fish it prey upon. Tuna have been clocked at record speed of 50-70 mph!
Bony fish have swim bladders to help them maintain buoyancy in the water. The swim bladder is a sac inside the abdomen that contains gas. This sac may be open or closed to the gut. If you have ever caught a fish and wondered why its eyes are bulging out of its head, it is because the air in the swim bladder has expanded and is pushing against the back of the eye. Oxygen is the largest percentage of gas in the bladder nitrogen and carbon dioxide also fill in passively.
Physoclistous- swim bladder is closed to the gut. The gas gets in through a special gas gland in the front of the swim bladder. Gas leaves the bladder through an oval body in the back of the swim bladder. The system works in a pretty miraculous way. Oval body, filled by venous blood -gasses leave here
Gas gland, fed by arterial blood -gasses enter here
inside the spots= giant secretory cells- secrete lactate -in capillary clusters rete mirabile
Increased lactate levels from the giant secretory cells lower the surrounding pH, causing the blood hemoglobin to dump off its oxygen. The oxygen diffuses back into the incoming capillary, increasing the partial pressure of oxygen in the incoming capillary. This continues until the partial pressure of the oxygen in the capillary is higher than that of the swim bladder (which has a high concentration of oxygen). This complex system is necessary because the concentration of oxygen is higher in the swim bladder than it is in the blood, so simple diffusion would tend to pull the oxygen out of the bladder instead of pushing it in. If the fish wants more buoyancy, it must tell its secretory cells to release more lactate. Since oxygen diffuses easily with oxygen-poor venous blood, the gas can be forced out.
*Fish that migrate vertically tend to have high oxygen levels in their bladders because it fills in faster and leaves faster.
*Fish that maintain a stable depth tend to have more nitrogen because it is inert, enters slowly, and exits slowly.
How in the heck can a fish, which is underwater, breath if there is no air? When we go under water, we have to bring air with us to survive. Whales and dolphins have lungs that store air from the surface. Fish don't have lungs, and they rarely ever venture into the air, so how do they survive. We all know it has something to do with gills, but what exactly.
The water surrounding a fish contains a small percentage of dissolved oxygen. In the surface waters there can be about 5 ml. of oxygen per liter of water. This is much less than the 210 ml. of oxygen per liter of air that we breath, so the fish must use a special system for concentrating the oxygen in the water to meet their physiological needs. Here it comes again, a counter current exchange system, similar to the one we found in the fish's swim bladder and in the tuna's muscles.
The circulation of blood in fish is simple. The heart only has two chambers, in contrast to our heart which has four. This is because the fish heart only pumps blood in one direction. The blood enters the heart through a vein and exits through a vein on its way to the gills. In the gills, the blood picks up oxygen from the surrounding water and leaves the gills in arteries, which go to the body. The oxygen is used in the body and goes back to the heart. A very simple closed-circle circulatory system.
- The blood flows thorough the gill filaments and secondary lamellae in the opposite direction from the water passing the gills. This is very important for getting all of the available oxygen out of the water and into the blood.
- If the blood flowed in the same direction as the water passing it, then the blood would only be able to get half of the available oxygen from the water. The blood and water would reach an equilibrium in oxygen content and diffusion would no longer take place.
- By having the blood flow in the opposite direction, the gradient is always such that the water has more available oxygen than the blood, and oxygen diffusion continues to take place after the blood has acquired more than 50% of the water's oxygen content. The countercurrent exchange system gives fish an 80-90% efficiency in acquiring oxygen.
- When fish are taken out of the water, they suffocate. This is not because they cannot breathe the oxygen available in the air, but because their gill arches collapse and there is not enough surface area for diffusion to take place. There are actually some fish that can survive out of the water, such as the walking catfish (which have modified lamellae allowing them to breathe air.
- It is possible for a fish to suffocate in the water. This could happen when the oxygen in the water has been used up by another biotic source such as bacteria decomposing a red tide.
--Ram Ventilation: Swim through the water and open your mouth. Very simple, but the fish must swim continuously in order to breathe, not so simple.
Successful survival in any environment depends upon an organism's ability to acquire information from its environment through its senses. Fish have many of the same senses that we have, they can see, smell, touch, feel, and taste, and they have developed some senses that we don't have, such as electroreception. Fish can sense light, chemicals, vibrations and electricity.
Light: photoreception [Vision]. Fish have a very keen sense of vision, which helps them to find food, shelter, mates, and avoid predators. Fish vision is on par with our own vision many can see in color, and some can see in extremely dim light.
Fish eyes are different from our own. Their lenses are perfectly spherical, which enables them to see underwater because it has a higher refractive index to help them focus. They focus by moving the lens in and out instead of stretching it like we do. They cannot dilate or contract their pupils because the lens bulges through the iris. As the depth at which fish are found increases, the resident fish's eye sizes increase in order to gather the dimmer light. This process continues until the end of the photic zone, where eye size drops off as their is no light to see with. Nocturnal fish tend to have larger eyes then diurnal fish. Just look at a squirrelfish, and you will see this to be so. Some fish have a special eye structure known as the Tapetum lucidum, which amplifies the incoming light. It is a layer of guanine crystals which glow at night. Photons which pass the retina get bounced back to be detected again. If the photons are still not absorbed, they are reflected back out of the eye. On a night dive, you may see these reflections as you shine your light around!
Chemicals: chemoreception [Smell and Taste]. Chemoreception is very well developed in the fishes, especially the sharks and eels which rely upon this to detect their prey. Fish have two nostrils on each side of their head, and there is no connection between the nostrils and the throat. The olfactory rosette is the organ that detects the chemicals. The size of the rosette is proportional to the fish's ability to smell. Some fish (such as sharks, rays, eels, and salmon) can detect chemical levels as low as 1 part per billion.
Fish also have the ability to taste. They have taste buds on their lips, tongue, and all over their mouths. Some fish, such as the goatfish or catfish, have barbels, which are whiskers that have taste structures. Goatfish can be seen digging through the sand with their barbels looking for invertebrate worms to eat and can taste them before they even reach their mouths.
Vibrations: mechanoreception [Hearing and touch]. Have you ever seen a fish's ear. Probably not, but they do have them, located within their bodies as well as a lateral line system that actually lets them feel their surroundings.
Fish do not have external ears, but sound vibrations readily transmit from the water through the fish's body to its internal ears. The ears are divided into two sections, an upper section (pars superior) and a lower section (utriculus) The pars superior is divided into three semicircular canals and give the fish its sense of balance. It is fluid-filled with sensory hairs. The sensory hairs detect the rotational acceleration of the fluid. The canals are arranged so that one gives yaw, another pitch, and the last- roll. The utriculus gives the fish its ability to hear. It has two large otoliths which vibrate with the sound and stimulate surrounding hair cells.
Fish posses another sense of mechanoreception that is kind of like a cross between hearing and touch. The organ responsible for this is the neuromast, a cluster of hair cells which have their hairs linked in a glob of jelly known as 'cupala'. All fish posses free neuromasts, which come in contact directly with the water. Most fish have a series of neuromasts not in direct contact with the water. These are arranged linearly and form the fishes lateral lines. A free neuromast gives the fish directional input.
A lateral line receives signals stimulated in a sequence, and gives the fish much more information (feeling the other fish around it for polarized schooling, and short-range prey detection 'the sense of distant touch').
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Derived from the Greek ἀνατομή anatomē "dissection" (from ἀνατέμνω anatémnō "I cut up, cut open" from ἀνά aná "up", and τέμνω témnō "I cut"),  anatomy is the scientific study of the structure of organisms including their systems, organs and tissues. It includes the appearance and position of the various parts, the materials from which they are composed, their locations and their relationships with other parts. Anatomy is quite distinct from physiology and biochemistry, which deal respectively with the functions of those parts and the chemical processes involved. For example, an anatomist is concerned with the shape, size, position, structure, blood supply and innervation of an organ such as the liver while a physiologist is interested in the production of bile, the role of the liver in nutrition and the regulation of bodily functions. 
The discipline of anatomy can be subdivided into a number of branches including gross or macroscopic anatomy and microscopic anatomy.  Gross anatomy is the study of structures large enough to be seen with the naked eye, and also includes superficial anatomy or surface anatomy, the study by sight of the external body features. Microscopic anatomy is the study of structures on a microscopic scale, along with histology (the study of tissues), and embryology (the study of an organism in its immature condition). 
Anatomy can be studied using both invasive and non-invasive methods with the goal of obtaining information about the structure and organization of organs and systems.  Methods used include dissection, in which a body is opened and its organs studied, and endoscopy, in which a video camera-equipped instrument is inserted through a small incision in the body wall and used to explore the internal organs and other structures. Angiography using X-rays or magnetic resonance angiography are methods to visualize blood vessels.    
The term "anatomy" is commonly taken to refer to human anatomy. However, substantially the same structures and tissues are found throughout the rest of the animal kingdom and the term also includes the anatomy of other animals. The term zootomy is also sometimes used to specifically refer to non-human animals. The structure and tissues of plants are of a dissimilar nature and they are studied in plant anatomy. 
The kingdom Animalia contains multicellular organisms that are heterotrophic and motile (although some have secondarily adopted a sessile lifestyle). Most animals have bodies differentiated into separate tissues and these animals are also known as eumetazoans. They have an internal digestive chamber, with one or two openings the gametes are produced in multicellular sex organs, and the zygotes include a blastula stage in their embryonic development. Metazoans do not include the sponges, which have undifferentiated cells. 
Unlike plant cells, animal cells have neither a cell wall nor chloroplasts. Vacuoles, when present, are more in number and much smaller than those in the plant cell. The body tissues are composed of numerous types of cell, including those found in muscles, nerves and skin. Each typically has a cell membrane formed of phospholipids, cytoplasm and a nucleus. All of the different cells of an animal are derived from the embryonic germ layers. Those simpler invertebrates which are formed from two germ layers of ectoderm and endoderm are called diploblastic and the more developed animals whose structures and organs are formed from three germ layers are called triploblastic.  All of a triploblastic animal's tissues and organs are derived from the three germ layers of the embryo, the ectoderm, mesoderm and endoderm.
Animal tissues can be grouped into four basic types: connective, epithelial, muscle and nervous tissue.
Connective tissue Edit
Connective tissues are fibrous and made up of cells scattered among inorganic material called the extracellular matrix. Connective tissue gives shape to organs and holds them in place. The main types are loose connective tissue, adipose tissue, fibrous connective tissue, cartilage and bone. The extracellular matrix contains proteins, the chief and most abundant of which is collagen. Collagen plays a major part in organizing and maintaining tissues. The matrix can be modified to form a skeleton to support or protect the body. An exoskeleton is a thickened, rigid cuticle which is stiffened by mineralization, as in crustaceans or by the cross-linking of its proteins as in insects. An endoskeleton is internal and present in all developed animals, as well as in many of those less developed. 
Epithelial tissue is composed of closely packed cells, bound to each other by cell adhesion molecules, with little intercellular space. Epithelial cells can be squamous (flat), cuboidal or columnar and rest on a basal lamina, the upper layer of the basement membrane,  the lower layer is the reticular lamina lying next to the connective tissue in the extracellular matrix secreted by the epithelial cells.  There are many different types of epithelium, modified to suit a particular function. In the respiratory tract there is a type of ciliated epithelial lining in the small intestine there are microvilli on the epithelial lining and in the large intestine there are intestinal villi. Skin consists of an outer layer of keratinized stratified squamous epithelium that covers the exterior of the vertebrate body. Keratinocytes make up to 95% of the cells in the skin.  The epithelial cells on the external surface of the body typically secrete an extracellular matrix in the form of a cuticle. In simple animals this may just be a coat of glycoproteins.  In more advanced animals, many glands are formed of epithelial cells. 
Muscle tissue Edit
Muscle cells (myocytes) form the active contractile tissue of the body. Muscle tissue functions to produce force and cause motion, either locomotion or movement within internal organs. Muscle is formed of contractile filaments and is separated into three main types smooth muscle, skeletal muscle and cardiac muscle. Smooth muscle has no striations when examined microscopically. It contracts slowly but maintains contractibility over a wide range of stretch lengths. It is found in such organs as sea anemone tentacles and the body wall of sea cucumbers. Skeletal muscle contracts rapidly but has a limited range of extension. It is found in the movement of appendages and jaws. Obliquely striated muscle is intermediate between the other two. The filaments are staggered and this is the type of muscle found in earthworms that can extend slowly or make rapid contractions.  In higher animals striated muscles occur in bundles attached to bone to provide movement and are often arranged in antagonistic sets. Smooth muscle is found in the walls of the uterus, bladder, intestines, stomach, oesophagus, respiratory airways, and blood vessels. Cardiac muscle is found only in the heart, allowing it to contract and pump blood round the body.
Nervous tissue Edit
Nervous tissue is composed of many nerve cells known as neurons which transmit information. In some slow-moving radially symmetrical marine animals such as ctenophores and cnidarians (including sea anemones and jellyfish), the nerves form a nerve net, but in most animals they are organized longitudinally into bundles. In simple animals, receptor neurons in the body wall cause a local reaction to a stimulus. In more complex animals, specialized receptor cells such as chemoreceptors and photoreceptors are found in groups and send messages along neural networks to other parts of the organism. Neurons can be connected together in ganglia.  In higher animals, specialized receptors are the basis of sense organs and there is a central nervous system (brain and spinal cord) and a peripheral nervous system. The latter consists of sensory nerves that transmit information from sense organs and motor nerves that influence target organs.   The peripheral nervous system is divided into the somatic nervous system which conveys sensation and controls voluntary muscle, and the autonomic nervous system which involuntarily controls smooth muscle, certain glands and internal organs, including the stomach. 
All vertebrates have a similar basic body plan and at some point in their lives, mostly in the embryonic stage, share the major chordate characteristics a stiffening rod, the notochord a dorsal hollow tube of nervous material, the neural tube pharyngeal arches and a tail posterior to the anus. The spinal cord is protected by the vertebral column and is above the notochord and the gastrointestinal tract is below it.  Nervous tissue is derived from the ectoderm, connective tissues are derived from mesoderm, and gut is derived from the endoderm. At the posterior end is a tail which continues the spinal cord and vertebrae but not the gut. The mouth is found at the anterior end of the animal, and the anus at the base of the tail.  The defining characteristic of a vertebrate is the vertebral column, formed in the development of the segmented series of vertebrae. In most vertebrates the notochord becomes the nucleus pulposus of the intervertebral discs. However, a few vertebrates, such as the sturgeon and the coelacanth retain the notochord into adulthood.  Jawed vertebrates are typified by paired appendages, fins or legs, which may be secondarily lost. The limbs of vertebrates are considered to be homologous because the same underlying skeletal structure was inherited from their last common ancestor. This is one of the arguments put forward by Charles Darwin to support his theory of evolution. 
Fish anatomy Edit
The body of a fish is divided into a head, trunk and tail, although the divisions between the three are not always externally visible. The skeleton, which forms the support structure inside the fish, is either made of cartilage, in cartilaginous fish, or bone in bony fish. The main skeletal element is the vertebral column, composed of articulating vertebrae which are lightweight yet strong. The ribs attach to the spine and there are no limbs or limb girdles. The main external features of the fish, the fins, are composed of either bony or soft spines called rays, which with the exception of the caudal fins, have no direct connection with the spine. They are supported by the muscles which compose the main part of the trunk.  The heart has two chambers and pumps the blood through the respiratory surfaces of the gills and on round the body in a single circulatory loop.  The eyes are adapted for seeing underwater and have only local vision. There is an inner ear but no external or middle ear. Low frequency vibrations are detected by the lateral line system of sense organs that run along the length of the sides of fish, and these respond to nearby movements and to changes in water pressure. 
Sharks and rays are basal fish with numerous primitive anatomical features similar to those of ancient fish, including skeletons composed of cartilage. Their bodies tend to be dorso-ventrally flattened, they usually have five pairs of gill slits and a large mouth set on the underside of the head. The dermis is covered with separate dermal placoid scales. They have a cloaca into which the urinary and genital passages open, but not a swim bladder. Cartilaginous fish produce a small number of large, yolky eggs. Some species are ovoviviparous and the young develop internally but others are oviparous and the larvae develop externally in egg cases. 
The bony fish lineage shows more derived anatomical traits, often with major evolutionary changes from the features of ancient fish. They have a bony skeleton, are generally laterally flattened, have five pairs of gills protected by an operculum, and a mouth at or near the tip of the snout. The dermis is covered with overlapping scales. Bony fish have a swim bladder which helps them maintain a constant depth in the water column, but not a cloaca. They mostly spawn a large number of small eggs with little yolk which they broadcast into the water column. 
Amphibian anatomy Edit
Amphibians are a class of animals comprising frogs, salamanders and caecilians. They are tetrapods, but the caecilians and a few species of salamander have either no limbs or their limbs are much reduced in size. Their main bones are hollow and lightweight and are fully ossified and the vertebrae interlock with each other and have articular processes. Their ribs are usually short and may be fused to the vertebrae. Their skulls are mostly broad and short, and are often incompletely ossified. Their skin contains little keratin and lacks scales, but contains many mucous glands and in some species, poison glands. The hearts of amphibians have three chambers, two atria and one ventricle. They have a urinary bladder and nitrogenous waste products are excreted primarily as urea. Amphibians breathe by means of buccal pumping, a pump action in which air is first drawn into the buccopharyngeal region through the nostrils. These are then closed and the air is forced into the lungs by contraction of the throat.  They supplement this with gas exchange through the skin which needs to be kept moist. 
In frogs the pelvic girdle is robust and the hind legs are much longer and stronger than the forelimbs. The feet have four or five digits and the toes are often webbed for swimming or have suction pads for climbing. Frogs have large eyes and no tail. Salamanders resemble lizards in appearance their short legs project sideways, the belly is close to or in contact with the ground and they have a long tail. Caecilians superficially resemble earthworms and are limbless. They burrow by means of zones of muscle contractions which move along the body and they swim by undulating their body from side to side. 
Reptile anatomy Edit
Reptiles are a class of animals comprising turtles, tuataras, lizards, snakes and crocodiles. They are tetrapods, but the snakes and a few species of lizard either have no limbs or their limbs are much reduced in size. Their bones are better ossified and their skeletons stronger than those of amphibians. The teeth are conical and mostly uniform in size. The surface cells of the epidermis are modified into horny scales which create a waterproof layer. Reptiles are unable to use their skin for respiration as do amphibians and have a more efficient respiratory system drawing air into their lungs by expanding their chest walls. The heart resembles that of the amphibian but there is a septum which more completely separates the oxygenated and deoxygenated bloodstreams. The reproductive system has evolved for internal fertilization, with a copulatory organ present in most species. The eggs are surrounded by amniotic membranes which prevents them from drying out and are laid on land, or develop internally in some species. The bladder is small as nitrogenous waste is excreted as uric acid. 
Turtles are notable for their protective shells. They have an inflexible trunk encased in a horny carapace above and a plastron below. These are formed from bony plates embedded in the dermis which are overlain by horny ones and are partially fused with the ribs and spine. The neck is long and flexible and the head and the legs can be drawn back inside the shell. Turtles are vegetarians and the typical reptile teeth have been replaced by sharp, horny plates. In aquatic species, the front legs are modified into flippers. 
Tuataras superficially resemble lizards but the lineages diverged in the Triassic period. There is one living species, Sphenodon punctatus. The skull has two openings (fenestrae) on either side and the jaw is rigidly attached to the skull. There is one row of teeth in the lower jaw and this fits between the two rows in the upper jaw when the animal chews. The teeth are merely projections of bony material from the jaw and eventually wear down. The brain and heart are more primitive than those of other reptiles, and the lungs have a single chamber and lack bronchi. The tuatara has a well-developed parietal eye on its forehead. 
Lizards have skulls with only one fenestra on each side, the lower bar of bone below the second fenestra having been lost. This results in the jaws being less rigidly attached which allows the mouth to open wider. Lizards are mostly quadrupeds, with the trunk held off the ground by short, sideways-facing legs, but a few species have no limbs and resemble snakes. Lizards have moveable eyelids, eardrums are present and some species have a central parietal eye. 
Snakes are closely related to lizards, having branched off from a common ancestral lineage during the Cretaceous period, and they share many of the same features. The skeleton consists of a skull, a hyoid bone, spine and ribs though a few species retain a vestige of the pelvis and rear limbs in the form of pelvic spurs. The bar under the second fenestra has also been lost and the jaws have extreme flexibility allowing the snake to swallow its prey whole. Snakes lack moveable eyelids, the eyes being covered by transparent "spectacle" scales. They do not have eardrums but can detect ground vibrations through the bones of their skull. Their forked tongues are used as organs of taste and smell and some species have sensory pits on their heads enabling them to locate warm-blooded prey. 
Crocodilians are large, low-slung aquatic reptiles with long snouts and large numbers of teeth. The head and trunk are dorso-ventrally flattened and the tail is laterally compressed. It undulates from side to side to force the animal through the water when swimming. The tough keratinized scales provide body armour and some are fused to the skull. The nostrils, eyes and ears are elevated above the top of the flat head enabling them to remain above the surface of the water when the animal is floating. Valves seal the nostrils and ears when it is submerged. Unlike other reptiles, crocodilians have hearts with four chambers allowing complete separation of oxygenated and deoxygenated blood. 
Bird anatomy Edit
Birds are tetrapods but though their hind limbs are used for walking or hopping, their front limbs are wings covered with feathers and adapted for flight. Birds are endothermic, have a high metabolic rate, a light skeletal system and powerful muscles. The long bones are thin, hollow and very light. Air sac extensions from the lungs occupy the centre of some bones. The sternum is wide and usually has a keel and the caudal vertebrae are fused. There are no teeth and the narrow jaws are adapted into a horn-covered beak. The eyes are relatively large, particularly in nocturnal species such as owls. They face forwards in predators and sideways in ducks. 
The feathers are outgrowths of the epidermis and are found in localized bands from where they fan out over the skin. Large flight feathers are found on the wings and tail, contour feathers cover the bird's surface and fine down occurs on young birds and under the contour feathers of water birds. The only cutaneous gland is the single uropygial gland near the base of the tail. This produces an oily secretion that waterproofs the feathers when the bird preens. There are scales on the legs, feet and claws on the tips of the toes. 
Mammal anatomy Edit
Mammals are a diverse class of animals, mostly terrestrial but some are aquatic and others have evolved flapping or gliding flight. They mostly have four limbs but some aquatic mammals have no limbs or limbs modified into fins and the forelimbs of bats are modified into wings. The legs of most mammals are situated below the trunk, which is held well clear of the ground. The bones of mammals are well ossified and their teeth, which are usually differentiated, are coated in a layer of prismatic enamel. The teeth are shed once (milk teeth) during the animal's lifetime or not at all, as is the case in cetaceans. Mammals have three bones in the middle ear and a cochlea in the inner ear. They are clothed in hair and their skin contains glands which secrete sweat. Some of these glands are specialized as mammary glands, producing milk to feed the young. Mammals breathe with lungs and have a muscular diaphragm separating the thorax from the abdomen which helps them draw air into the lungs. The mammalian heart has four chambers and oxygenated and deoxygenated blood are kept entirely separate. Nitrogenous waste is excreted primarily as urea. 
Mammals are amniotes, and most are viviparous, giving birth to live young. The exception to this are the egg-laying monotremes, the platypus and the echidnas of Australia. Most other mammals have a placenta through which the developing foetus obtains nourishment, but in marsupials, the foetal stage is very short and the immature young is born and finds its way to its mother's pouch where it latches on to a nipple and completes its development. 
Most bony fishes (adult flounder and some bonito are an exception) have a swim bladder for buoyancy control. The amount of gas contained within the bladder is adjusted to allow the fish to move up and down in the water column while conserving energy. In some species the swim bladder is also used in hearing and sound production.
Sharks don’t have a swim bladder. Instead sharks have oily livers that are lighter than water which assist the shark in achieving neutral buoyancy.
The otolith (sometimes referred to as the ear bone) is the fish’s inner ear, assisting with balance and enabling them to listen to sound waves that travel through the water. Fishery scientists are able to determine the age of a fish by counting the number of growth rings that are deposited in the otolith.
How do fish hear? (Video courtesy of Woodside)
The heart is a muscular organ that pumps blood throughout the body. In fishes, blood is circulated by a 2-chambered heart – deoxygenated blood enters the first chamber of the heart from the body. It is then pumped to the second chamber before passing through the gills where it loses carbon dioxide and receives a fresh supply of oxygen. The oxygenated blood is carried back to the body by blood vessels called arteries. The arteries branch out into capillaries, which then collect into larger vessels, veins, carrying deoxygenated blood and dissolved carbon dioxide back to the heart to complete the cycle.
Nearly all fish species are cold blooded poikilothermic organisms, which means that their blood temperature varies with the water temperature around them. Some fish species, such as the white shark and yellowfin tuna, are partially warm blooded and are able to raise their body temperature a few degrees above the temperature of the surrounding water. In some fish species, such as swordfish, they have adapted a specialised blood vessel structure that allows them to increase the temperature of their brains and eyes, presumably to assist with hunting in deep, cold water.
The liver produces enzymes to aid in digestion. It is also a storage area for fats, blood sugars and vitamins and breaks down toxins and old blood cells.
In sharks, the liver is particularly large and rich in oil.
The stomach is where digestion commences. All fish have a one-way digestive system. Food enters the mouth and travels via the oesophagus to the stomach. From the stomach, it is passed into the intestine for further digestion. Digested wastes are eliminated from the intestine via the anus.
The intestine is the main site of digestion and absorption of nutrients. Undigested material exits the body through the anus.
As a general rule, bony fish with short intestines are carnivorous and those with long, coiled intestines are herbivorous, as fibrous plant materials are harder to break down.
The intestine of cartilaginous fish contain an area called the spiral valve that acts to increase the internal surface area of the intestine.
The reproductive organs of bony fish are referred to as gonads. These organs are usually paired. Female gonads are called ovaries and produce eggs. They are usually pink, red or orange in colour and covered in numerous blood vessels. Male gonads are called testes and produce sperm. They are usually white/cream or grey in colour.
Figure 15. Left – ripe male gonads. Right – ripe female gonads.
The kidneys are one of the organs that assist with excretion and regulation of water balance. In bony fish, salts that are absorbed are excreted by the kidneys. They also conserve water by only producing small amount of urine.
In cartilaginous fish, the kidneys also control the amount of urea retained in the blood that aids in making the blood concentration closer to the of seawater.
Castro, P. & Huber, M.E. 2008 Marine Biology, 7 th Ed. McGraw-Hill.
Chen et al.1997 and Chen et al.2002. Cited in Rowat & Brooks (2012) A review of the biology, fisheries and conservation of the whale shark Rhincodon typus. Journal of Fish Biology, Vol. 80 (5), pp 1019-1056.
Helfman, G.S., Collette, B.B., Douglas, E.F. & Bowen, B.W. 2009, The diversity of fishes: Biology, Evolution and Ecology, 2 nd Ed. Wiley-Blackwell.
Moffatt, B., Ryan, T. & Zann, L. 2003, Marine Science for Australian students. Wet Paper Publications, Queensland.
Parish, S. 1997, Amazing facts about Australian Marine Life. Steve Parish Publishing, Fortitude Valley, Queensland.
Pair of fins situated just behind the head in fishes that help control the direction of movement
Associated with the surface or middle depths of a body of water.
The first pair of ventral fins of fishes.
The class Osteichthyes (literally ȫony fish") gets its name from the bony skeleton and scales of its members. The group comprises nearly all living fish, with notable exceptions being sharks and other cartilaginous fish, and the primitive lampreys and their kin. Bony skeletons and scales are the primary features that differentiate these fish from other cartilaginous fish, whose skeleton is composed of cartilage and whose skin is leathery. Other important differences include the swim bladder, a lunglike, gas-filled organ that helps bony fish to regulate their buoyancy.
Osteichthyes is the most numerous and diverse group of vertebrates, occupying virtually all large bodies of water, from polar seas to hot undersea vents to land-locked lakes. Because of their numbers (more than 20,000 species), diversity, and range, the bony fish play a major role in virtually all marine and freshwater ecosystems . They range from the tiny seahorses to giant sunfish (weighing thousands of pounds) to the salmon on one's dinner plate.
Most of the bony fish—thirty-nine of the forty-two orders𠅊re rayfinned fish subclass Actinopterygii. The other three orders are fleshy-finned fish, members of the subclass Sacropterygii. Although much less numerous and diverse than the ray-fins, the fleshy finned fish are still interesting and important. In two orders, the fleshy finned fish have lungs instead of swim bladders, and can survive their ponds drying up by burrowing into the mud. The final order contains only one species: the coelacanth (pronounced SEE-low-kanth), an ancient species of fish once thought to be long extinct. Living coelacanths, virtually identical to its fossil relatives that lived 20 million years ago, were first found in 1938. This discovery was doubly important because the coelacanth is a close relative of the fish from which amphibians evolved, making it closely related to the ancestors of all terrestrial vertebrates.
In many respects, fish anatomy is different from mammalian anatomy. However, it still shares the same basic body plan from which all vertebrates have evolved: a notochord, rudimentary vertebrae, and a well-defined head and tail.  
Fish have a variety of different body plans. At the broadest level, their body is divided into head, trunk, and tail, although the divisions are not always externally visible. The body is often fusiform, a streamlined body plan often found in fast-moving fish. They may also be filiform (eel-shaped) or vermiform (worm-shaped). Fish are often either compressed (laterally thin) or depressed (dorso-ventrally flat).
There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body. The skeleton of the fish is made of either cartilage (cartilaginous fishes) or bone (bony fishes). The fins are made up of bony fin rays and, except for the caudal fin, have no direct connection with the spine. They are supported only by the muscles. The ribs attach to the spine.
Bones are rigid organs that form part of the endoskeleton of vertebrates. They function to move, support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue. Bones come in a variety of shapes and have a complex internal and external structure. They are lightweight, yet strong and hard, in addition to fulfilling their many other biological functions.
Fish are vertebrates. All vertebrates are built along the basic chordate body plan: a stiff rod running through the length of the animal (vertebral column or notochord),  with a hollow tube of nervous tissue (the spinal cord) above it and the gastrointestinal tract below. In all vertebrates, the mouth is found at, or right below, the anterior end of the animal, while the anus opens to the exterior before the end of the body. The remaining part of the body beyond the anus forms a tail with vertebrae and the spinal cord, but no gut. 
The defining characteristic of a vertebrate is the vertebral column, in which the notochord (a stiff rod of uniform composition) found in all chordates has been replaced by a segmented series of stiffer elements (vertebrae) separated by mobile joints (intervertebral discs, derived embryonically and evolutionarily from the notochord). However, a few fish have secondarily [ clarification needed ] lost this anatomy, retaining the notochord into adulthood, such as the sturgeon. 
The vertebral column consists of a centrum (the central body or spine of the vertebra), vertebral arches which protrude from the top and bottom of the centrum, and various processes which project from the centrum or arches. An arch extending from the top of the centrum is called a neural arch, while the haemal arch or chevron is found underneath the centrum in the caudal vertebrae of fish. The centrum of a fish is usually concave at each end (amphicoelous), which limits the motion of the fish. In contrast, the centrum of a mammal is flat at each end (acoelous), a shape that can support and distribute compressive forces.
The vertebrae of lobe-finned fishes consist of three discrete bony elements. The vertebral arch surrounds the spinal cord, and is broadly similar in form to that found in most other vertebrates. Just beneath the arch lies the small plate-like pleurocentrum, which protects the upper surface of the notochord. Below that, a larger arch-shaped intercentrum protects the lower border. Both of these structures are embedded within a single cylindrical mass of cartilage. A similar arrangement was found in primitive tetrapods, but in the evolutionary line that led to reptiles, mammals and birds, the intercentrum became partially or wholly replaced by an enlarged pleurocentrum, which in turn became the bony vertebral body. 
In most ray-finned fishes, including all teleosts, these two structures are fused with and embedded within a solid piece of bone superficially resembling the vertebral body of mammals. In living amphibians, there is simply a cylindrical piece of bone below the vertebral arch, with no trace of the separate elements present in the early tetrapods. 
In cartilaginous fish such as sharks, the vertebrae consist of two cartilaginous tubes. The upper tube is formed from the vertebral arches, but also includes additional cartilaginous structures filling in the gaps between the vertebrae, enclosing the spinal cord in an essentially continuous sheath. The lower tube surrounds the notochord and has a complex structure, often including multiple layers of calcification. 
Lampreys have vertebral arches, but nothing resembling the vertebral bodies found in all higher vertebrates. Even the arches are discontinuous, consisting of separate pieces of arch-shaped cartilage around the spinal cord in most parts of the body, changing to long strips of cartilage above and below in the tail region. Hagfishes lack a true vertebral column, and are therefore not properly considered vertebrates, but a few tiny neural arches are present in the tail.   Hagfishes do, however, possess a cranium. For this reason, the vertebrate subphylum is sometimes referred to as "Craniata" when discussing morphology. Molecular analysis [ specify ] since 1992 has suggested that the hagfishes are most closely related to lampreys,  and so also are vertebrates in a monophyletic sense. Others consider them a sister group of vertebrates in the common taxon of Craniata. 
The head or skull includes the skull roof (a set of bones covering the brain, eyes and nostrils), the snout (from the eye to the forward-most point of the upper jaw), the operculum or gill cover (absent in sharks and jawless fish), and the cheek, which extends from the eye to the preopercle. The operculum and preopercle may or may not have spines. In sharks and some primitive bony fish the spiracle, a small extra gill opening, is found behind each eye.
The skull in fishes is formed from a series of only loosely connected bones. Jawless fish and sharks only possess a cartilaginous endocranium, with the upper and lower jaws of cartilaginous fish being separate elements not attached to the skull. Bony fishes have additional dermal bone, forming a more or less coherent skull roof in lungfish and holost fish. The lower jaw defines a chin.
In lampreys, the mouth is formed into an oral disk. In most jawed fish, however, there are three general configurations. The mouth may be on the forward end of the head (terminal), may be upturned (superior), or may be turned downwards or on the bottom of the fish (subterminal or inferior). The mouth may be modified into a suckermouth adapted for clinging onto objects in fast-moving water.
The simpler structure is found in jawless fish, in which the cranium is represented by a trough-like basket of cartilaginous elements only partially enclosing the brain and associated with the capsules for the inner ears and the single nostril. Distinctively, these fish have no jaws. 
Cartilaginous fish such as sharks also have simple, and presumably primitive, skull structures. The cranium is a single structure forming a case around the brain, enclosing the lower surface and the sides, but always at least partially open at the top as a large fontanelle. The most anterior part of the cranium includes a forward plate of cartilage, the rostrum, and capsules to enclose the olfactory organs. Behind these are the orbits, and then an additional pair of capsules enclosing the structure of the inner ear. Finally, the skull tapers towards the rear, where the foramen magnum lies immediately above a single condyle, articulating with the first vertebra. Smaller foramina for the cranial nerves can be found at various points throughout the cranium. The jaws consist of separate hoops of cartilage, almost always distinct from the cranium proper. 
In the ray-finned fishes, there has also been considerable modification from the primitive pattern. The roof of the skull is generally well formed, and although the exact relationship of its bones to those of tetrapods is unclear, they are usually given similar names for convenience. Other elements of the skull, however, may be reduced there is little cheek region behind the enlarged orbits, and little if any bone in between them. The upper jaw is often formed largely from the premaxilla, with the maxilla itself located further back, and an additional bone, the sympletic, linking the jaw to the rest of the cranium. 
Although the skulls of fossil lobe-finned fish resemble those of the early tetrapods, the same cannot be said of those of the living lungfishes. The skull roof is not fully formed, and consists of multiple, somewhat irregularly shaped bones with no direct relationship to those of tetrapods. The upper jaw is formed from the pterygoid bones and vomers alone, all of which bear teeth. Much of the skull is formed from cartilage, and its overall structure is reduced. 
The head may have several fleshy structures known as barbels, which may be very long and resemble whiskers. Many fish species also have a variety of protrusions or spines on the head. The nostrils or nares of almost all fishes do not connect to the oral cavity, but are pits of varying shape and depth.
Skull of Tiktaalik, a genus of extinct sarcopterygian (lobe-finned "fish") from the late Devonian period
The vertebrate jaw probably originally evolved in the Silurian period and appeared in the Placoderm fish which further diversified in the Devonian. Jaws are thought to derive from the pharyngeal arches that support the gills in fish. The two most anterior of these arches are thought to have become the jaw itself (see hyomandibula) and the hyoid arch, which braces the jaw against the braincase and increases mechanical efficiency. While there is no fossil evidence directly to support this theory, it makes sense in light of the numbers of pharyngeal arches that are visible in extant jawed animals (the gnathostomes), which have seven arches, and primitive jawless vertebrates (the Agnatha), which have nine. [ citation needed ]
It is thought that the original selective advantage garnered by the jaw was not related to feeding, but to increase respiration efficiency. The jaws were used in the buccal pump (observable in modern fish and amphibians) that pumps water across the gills of fish or air into the lungs of amphibians. Over evolutionary time, the more familiar use of jaws in feeding was selected for and became a very important function in vertebrates.
Linkage systems are widely distributed in animals. The most thorough overview of the different types of linkages in animals has been provided by M. Muller,  who also designed a new classification system which is especially well suited for biological systems. Linkage mechanisms are especially frequent and various in the head of bony fishes, such as wrasses, which have evolved many specialized aquatic feeding mechanisms. Especially advanced are the linkage mechanisms of jaw protrusion. For suction feeding a system of connected four-bar linkages is responsible for the coordinated opening of the mouth and 3-D expansion of the buccal cavity. Other linkages are responsible for protrusion of the premaxilla.
Fish eyes are similar to terrestrial vertebrates like birds and mammals, but have a more spherical lens. Their retinas generally have both rod cells and cone cells (for scotopic and photopic vision), and most species have colour vision. Some fish can see ultraviolet and some can see polarized light. Amongst jawless fish, the lamprey has well-developed eyes, while the hagfish has only primitive eyespots.  The ancestors of modern hagfish, thought to be protovertebrate,  were evidently pushed to very deep, dark waters, where they were less vulnerable to sighted predators and where it is advantageous to have a convex eyespot, which gathers more light than a flat or concave one. Unlike humans, fish normally adjust focus by moving the lens closer to or further from the retina. 
The gills, located under the operculum, are a respiratory organ for the extraction of oxygen from water and for the excretion of carbon dioxide. They are not usually visible, but can be seen in some species, such as the frilled shark. The labyrinth organ of Anabantoidei and Clariidae is used to allow the fish to extract oxygen from the air. Gill rakers are finger-like projections off the gill arch which function in filter feeders to retain filtered prey. They may be bony or cartilaginous.
The epidermis of fish consists entirely of live cells, with only minimal quantities of keratin in the cells of the superficial layer. It is generally permeable. The dermis of bony fish typically contains relatively little of the connective tissue found in tetrapods. Instead, in most species, it is largely replaced by solid, protective bony scales. Apart from some particularly large dermal bones that form parts of the skull, these scales are lost in tetrapods, although many reptiles do have scales of a different kind, as do pangolins. Cartilaginous fish have numerous tooth-like denticles embedded in their skin in place of true scales.
Sweat glands and sebaceous glands are both unique to mammals, but other types of skin glands are found in fish. Fish typically have numerous individual mucus-secreting skin cells that aid in insulation and protection, but may also have venom glands, photophores, or cells that produce a more watery serous fluid.  Melanin colours the skin of many species, but in fish the epidermis is often relatively colourless. Instead, the colour of the skin is largely due to chromatophores in the dermis, which, in addition to melanin, may contain guanine or carotenoid pigments. Many species, such as flounders, change the colour of their skin by adjusting the relative size of their chromatophores. 
The outer body of many fish is covered with scales, which are part of the fish's integumentary system. The scales originate from the mesoderm (skin), and may be similar in structure to teeth. Some species are covered instead by scutes. Others have no outer covering on the skin. Most fish are covered in a protective layer of slime (mucus).
There are four principal types of fish scales.
- , also called dermal denticles, are similar to teeth in that they are made of dentin covered by enamel. They are typical of sharks and rays. are flat, basal-looking scales that cover a fish's body with little overlapping. They are typical of gar and bichirs. are small, oval-shaped scales with growth rings like the rings of a tree. Bowfin and remora have cycloid scales. are similar to cycloid scales, also having growth rings. They are distinguished by spines that cover one edge. Halibut have this type of scale.
Another less common type of scale is the scute, which may be an external, shield-like bony plate a modified, thickened scale that is often keeled or spiny or a projecting, modified (rough and strongly ridged) scale. Scutes are usually associated with the lateral line, but may be found on the caudal peduncle (where they form caudal keels) or along the ventral profile. Some fish, such as pineconefish, are completely or partially covered in scutes.
Lateral line Edit
The lateral line is a sense organ used to detect movement and vibration in the surrounding water. For example, fish can use their lateral line system to follow the vortices produced by fleeing prey. In most species, it consists of a line of receptors running along each side of the fish.
Photophores are light-emitting organs which appear as luminous spots on some fishes. The light can be produced from compounds during the digestion of prey, from specialized mitochondrial cells in the organism called photocytes, or from symbiotic bacteria. Photophores are used for attracting food or confusing predators.
Fins are the most distinctive features of fish. They are either composed of bony spines or rays protruding from the body with skin covering them and joining them together, either in a webbed fashion as seen in most bony fish, or similar to a flipper as seen in sharks. Apart from the tail or caudal fin, fins have no direct connection with the spine and are supported by muscles only. Their principal function is to help the fish swim. Fins can also be used for gliding or crawling, as seen in the flying fish and frogfish. Fins located in different places on the fish serve different purposes, such as moving forward, turning, and keeping an upright position. For every fin, there are a number of fish species in which this particular fin has been lost during evolution. [ citation needed ]
Spines and rays Edit
In bony fish, most fins may have spines or rays. A fin may contain only spiny rays, only soft rays, or a combination of both. If both are present, the spiny rays are always anterior. Spines are generally stiff, sharp and unsegmented. Rays are generally soft, flexible, segmented, and may be branched. This segmentation of rays is the main difference that distinguishes them from spines spines may be flexible in certain species, but never segmented.
Spines have a variety of uses. In catfish, they are used as a form of defense many catfish have the ability to lock their spines outwards. Triggerfish also use spines to lock themselves in crevices to prevent them being pulled out.
Lepidotrichia are bony, bilaterally-paired, segmented fin rays found in bony fishes. They develop around actinotrichia as part of the dermal exoskeleton. Lepidotrichia may have some cartilage or bone in them as well. They are actually segmented and appear as a series of disks stacked one on top of another. The genetic basis for the formation of the fin rays is thought to be genes coding for the proteins actinodin 1 and actinodin 2. 
Types of fin Edit
- : Located on the back of the fish, dorsal fins serve to prevent the fish from rolling and assist in sudden turns and stops. Most fishes have one dorsal fin, but some fishes have two or three. In anglerfish, the anterior of the dorsal fin is modified into an illicium and esca, a biological equivalent to a fishing rod and lure. The two to three bones that support the dorsal fin are called the proximal, middle, and distalpterygiophores. In spinous fins, the distal pterygiophore is often fused to the middle or not present at all.
- Caudal/Tail fins: Also called the tail fins, caudal fins are attached to the end of the caudal peduncle and used for propulsion. The caudal peduncle is the narrow part of the fish's body. The hypural joint is the joint between the caudal fin and the last of the vertebrae. The hypural is often fan-shaped. The tail may be heterocercal, reversed heterocercal, protocercal, diphycercal, or homocercal.
- Heterocercal: vertebrae extend into the upper lobe of the tail, making it longer (as in sharks)
- Reversed heterocercal: vertebrae extend into the lower lobe of the tail, making it longer (as in the Anaspida)
- Protocercal: vertebrae extend to the tip of the tail the tail is symmetrical but not expanded (as in lancelets)
- Diphycercal: vertebrae extend to the tip of the tail the tail is symmetrical and expanded (as in the bichir, lungfish, lamprey and coelacanth). Most Palaeozoic fishes had a diphycercal heterocercal tail. 
- Homocercal: vertebrae extend a very short distance into the upper lobe of the tail tail still appears superficially symmetric. Most fish have a homocercal tail, but it can be expressed in a variety of shapes. The tail fin can be rounded at the end, truncated (almost vertical edge, as in salmon), forked (ending in two prongs), emarginate (with a slight inward curve), or continuous (dorsal, caudal, and anal fins attached, as in eels).
- "Cephalic fins": The "horns" of manta rays and their relatives, sometimes called cephalic fins, are actually a modification of the anterior portion of the pectoral fin.
As with other vertebrates, the intestines of fish consist of two segments, the small intestine and the large intestine. In most higher vertebrates, the small intestine is further divided into the duodenum and other parts. In fish, the divisions of the small intestine are not as clear, and the terms anterior intestine or proximal intestine may be used instead of duodenum.  In bony fish, the intestine is relatively short, typically around one and a half times the length of the fish's body. It commonly has a number of pyloric caeca, small pouch-like structures along its length that help to increase the overall surface area of the organ for digesting food. There is no ileocaecal valve in teleosts, with the boundary between the small intestine and the rectum being marked only by the end of the digestive epithelium.  There is no small intestine as such in non-teleost fish, such as sharks, sturgeons, and lungfish. Instead, the digestive part of the gut forms a spiral intestine, connecting the stomach to the rectum. In this type of gut, the intestine itself is relatively straight, but has a long fold running along the inner surface in a spiral fashion, sometimes for dozens of turns. This fold creates a valve-like structure that greatly increases both the surface area and the effective length of the intestine. The lining of the spiral intestine is similar to that of the small intestine in teleosts and non-mammalian tetrapods.  In lampreys, the spiral valve is extremely small, possibly because their diet requires little digestion. Hagfish have no spiral valve at all, with digestion occurring for almost the entire length of the intestine, which is not subdivided into different regions. 
Pyloric caeca Edit
The pyloric caecum is a pouch, usually peritoneal, at the beginning of the large intestine. It receives faecal material from the ileum, and connects to the ascending colon of the large intestine. It is present in most amniotes, and also in lungfish.  Many fish in addition have a number of small outpocketings, also called pyloric caeca, along their intestine despite the name they are not homologous to the caecum of amniotes. Their purpose is to increase the overall surface area of the digestive epithelium, therefore optimizing the absorption of sugars, amino acids, and dipeptides, among other nutrients.  
As with other vertebrates, the relative positions of the esophageal and duodenal openings to the stomach remain relatively constant. As a result, the stomach always curves somewhat to the left before curving back to meet the pyloric sphincter. However, lampreys, hagfishes, chimaeras, lungfishes, and some teleost fish have no stomach at all, with the esophagus opening directly into the intestine. These fish consume diets that either require little storage of food, no pre-digestion with gastric juices, or both. 
The kidneys of fish are typically narrow, elongated organs, occupying a significant portion of the trunk. They are similar to the mesonephros of higher vertebrates (reptiles, birds, and mammals). The kidneys contain clusters of nephrons, serviced by collecting ducts which usually drain into a mesonephric duct. However, the situation is not always so simple. In cartilaginous fish, there is also a shorter duct which drains the posterior (metanephric) parts of the kidney, and joins with the mesonephric duct at the bladder or cloaca. Indeed, in many cartilaginous fish, the anterior portion of the kidney may degenerate or cease to function altogether in the adult.  Hagfish and lamprey kidneys are unusually simple. They consist of a row of nephrons, each emptying directly into the mesonephric duct.  Like the Nile tilapia, the kidney of some fish shows its three parts head, trunk, and tail kidneys.  Fish do not have a discrete adrenal gland with distinct cortex and medulla, similar to those found in mammals. The interrenal and chromaffin cells are located within the head kidney 
The spleen is found in nearly all vertebrates. It is a non-vital organ, similar in structure to a large lymph node. It acts primarily as a blood filter, and plays important roles in regards to red blood cells and the immune system.  In cartilaginous and bony fish it consists primarily of red pulp and is normally a somewhat elongated organ as it actually lies inside the serosal lining of the intestine.  The only vertebrates lacking a spleen are the lampreys and hagfishes. Even in these animals, there is a diffuse layer of haematopoietic tissue within the gut wall, which has a similar structure to red pulp, and is presumed to be homologous to the spleen of higher vertebrates. 
The liver is a large vital organ present in all fish. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion. It is very susceptible to contamination by organic and inorganic compounds because they can accumulate over time and cause potentially life-threatening conditions. Because of the liver's capacity for detoxification and storage of harmful components, it is often used as an environmental biomarker. 
Fish have what is often described as a two-chambered heart,  consisting of one atrium to receive blood and one ventricle to pump it,  in contrast to three chambers (two atria, one ventricle) of amphibian and most reptile hearts and four chambers (two atria, two ventricles) of mammal and bird hearts.  However, the fish heart has entry and exit compartments that may be called chambers, so it is also sometimes described as three-chambered,  or four-chambered,  depending on what is counted as a chamber. The atrium and ventricle are sometimes considered "true chambers", while the others are considered "accessory chambers". 
The four compartments are arranged sequentially:
- : A thin-walled sac or reservoir with some cardiac muscle that collects deoxygenated blood through the incoming hepatic and cardinal veins. [verification needed] 
- Atrium: A thicker-walled, muscular chamber that sends blood to the ventricle. 
- Ventricle: A thick-walled, muscular chamber that pumps the blood to the fourth part, the outflow tract.  The shape of the ventricle varies considerably, usually tubular in fish with elongated bodies, pyramidal with a triangular base in others, or sometimes sac-like in some marine fish. 
- Outflow tract (OFT): Goes to the ventral aorta and consists of the tubular conus arteriosus, bulbus arteriosus, or both.  The conus arteriosus, typically found in more primitive species of fish, contracts to assist blood flow to the aorta, while the bulbus anteriosus does not. 
Ostial valves, consisting of flap-like connective tissues, prevent blood from flowing backward through the compartments.  The ostial valve between the sinus venosus and atrium is called the sino-atrial valve, which closes during ventricular contraction.  Between the atrium and ventricle is an ostial valve called the atrioventricular valve, and between the bulbus arteriosus and ventricle is an ostial valve called the bulbo-ventricular valve.  The conus arteriosus has a variable number of semilunar valves. 
The ventral aorta delivers blood to the gills where it is oxygenated and flows, through the dorsal aorta, into the rest of the body. (In tetrapods, the ventral aorta is divided in two one half forms the ascending aorta, while the other forms the pulmonary artery). 
The circulatory systems of all vertebrates are closed. Fish have the simplest circulatory system, consisting of only one circuit, with the blood being pumped through the capillaries of the gills and on to the capillaries of the body tissues. This is known as single cycle circulation. 
In the adult fish, the four compartments are not arranged in a straight row, instead forming an S-shape with the latter two compartments lying above the former two. This relatively simpler pattern is found in cartilaginous fish and in the ray-finned fish. In teleosts, the conus arteriosus is very small and can more accurately be described as part of the aorta rather than of the heart proper. The conus arteriosus is not present in any amniotes, presumably having been absorbed into the ventricles over the course of evolution. Similarly, while the sinus venosus is present as a vestigial structure in some reptiles and birds, it is otherwise absorbed into the right atrium and is no longer distinguishable. 
Swim bladder Edit
The swim bladder or gas bladder is an internal organ that contributes to the ability of a fish to control its buoyancy, and thus to stay at the current water depth, ascend, or descend without having to waste energy in swimming. The bladder is found only in the bony fishes. In the more primitive groups like some Leuciscinae, bichirs and lungfish, the bladder is open to the esophagus and doubles as a lung. It is often absent in fast swimming fishes such as the tuna and mackerel families. Fish with bladders open to the esophagus are called physostomes, while fish with the bladder closed are called physoclists. In the latter, the gas content of the bladder is controlled through a rete mirabilis, a network of blood vessels affecting gas exchange between the bladder and the blood. 
Weberian apparatus Edit
Fishes of the superorder Ostariophysi possess a structure called the Weberian apparatus, a modification which allows them to hear better. This ability may explain the marked success of ostariophysian fishes.  The apparatus is made up of a set of bones known as Weberian ossicles, a chain of small bones that connect the auditory system to the swim bladder of fishes.  The ossicles connect the gas bladder wall with Y-shaped lymph sinus that is next to the lymph-filled transverse canal joining the saccules of the right and left ears. This allows the transmission of vibrations to the inner ear. A fully functioning Weberian apparatus consists of the swim bladder, the Weberian ossicles, a portion of the anterior vertebral column, and some muscles and ligaments. 
Fish reproductive organs include testes and ovaries. In most species, gonads are paired organs of similar size, which can be partially or totally fused.  There may also be a range of secondary organs that increase reproductive fitness. The genital papilla is a small, fleshy tube behind the anus in some fishes from which the sperm or eggs are released the sex of a fish often can be determined by the shape of its papilla. [ citation needed ]
Most male fish have two testes of similar size. In the case of sharks, the testis on the right side is usually larger. The primitive jawless fish have only a single testis located in the midline of the body, although even this forms from the fusion of paired structures in the embryo. 
Under a tough membranous shell, the tunica albuginea, the testis of some teleost fish, contains very fine coiled tubes called seminiferous tubules. The tubules are lined with a layer of cells (germ cells) that from puberty into old age, develop into sperm cells (also known as spermatozoa or male gametes). The developing sperm travel through the seminiferous tubules to the rete testis located in the mediastinum testis, to the efferent ducts, and then to the epididymis where newly created sperm cells mature (see spermatogenesis). The sperm move into the vas deferens, and are eventually expelled through the urethra and out of the urethral orifice through muscular contractions.
However, most fish do not possess seminiferous tubules. Instead, the sperm are produced in spherical structures called sperm ampullae. These are seasonal structures, releasing their contents during the breeding season and then being reabsorbed by the body. Before the next breeding season, new sperm ampullae begin to form and ripen. The ampullae are otherwise essentially identical to the seminiferous tubules in higher vertebrates, including the same range of cell types. 
In terms of spermatogonia distribution, the structure of teleost testes have two types: in the most common, spermatogonia occur all along the seminiferous tubules, while in Atherinomorpha, they are confined to the distal portion of these structures. Fish can present cystic or semi-cystic spermatogenesis [ definition needed ] in relation to the release phase of germ cells in cysts to the lumen of the seminiferous tubules. 
Many of the features found in ovaries are common to all vertebrates, including the presence of follicular cells and tunica albuginea There may be hundreds or even millions of fertile eggs present in the ovary of a fish at any given time. Fresh eggs may be developing from the germinal epithelium throughout life. Corpora lutea are found only in mammals, and in some elasmobranch fish in other species, the remnants of the follicle are quickly resorbed by the ovary.  The ovary of teleosts is often contains a hollow, lymph-filled space which opens into the oviduct, and into which the eggs are shed.  Most normal female fish have two ovaries. In some elasmobranchs, only the right ovary develops fully. In the primitive jawless fish and some teleosts, there is only one ovary, formed by the fusion of the paired organs in the embryo. 
Fish ovaries may be of three types: gymnovarian, secondary gymnovarian or cystovarian. In the first type, the oocytes are released directly into the coelomic cavity and then enter the ostium, then through the oviduct and are eliminated. Secondary gymnovarian ovaries shed ova into the coelom from which they go directly into the oviduct. In the third type, the oocytes are conveyed to the exterior through the oviduct.  Gymnovaries are the primitive condition found in lungfish, sturgeon, and bowfin. Cystovaries characterize most teleosts, where the ovary lumen has continuity with the oviduct.  Secondary gymnovaries are found in salmonids and a few other teleosts.
Central nervous system Edit
Fish typically have quite small brains relative to body size compared with other vertebrates, typically one-fifteenth the brain mass of a similarly sized bird or mammal.  However, some fish have relatively large brains, most notably mormyrids and sharks, which have brains about as massive relative to body weight as birds and marsupials. 
Fish brains are divided into several regions. At the front are the olfactory lobes, a pair of structures that receive and process signals from the nostrils via the two olfactory nerves.  Similar to the way humans smell chemicals in the air, fish smell chemicals in the water by tasting them. The olfactory lobes are very large in fish that hunt primarily by smell, such as hagfish, sharks, and catfish. Behind the olfactory lobes is the two-lobed telencephalon, the structural equivalent to the cerebrum in higher vertebrates. In fish the telencephalon is concerned mostly with olfaction.  Together these structures form the forebrain.
The forebrain is connected to the midbrain via the diencephalon (in the diagram, this structure is below the optic lobes and consequently not visible). The diencephalon performs functions associated with hormones and homeostasis.  The pineal body lies just above the diencephalon. This structure detects light, maintains circadian rhythms, and controls color changes.  The midbrain or mesencephalon contains the two optic lobes. These are very large in species that hunt by sight, such as rainbow trout and cichlids. 
The hindbrain or metencephalon is particularly involved in swimming and balance.  The cerebellum is a single-lobed structure that is typically the biggest part of the brain.  Hagfish and lampreys have relatively small cerebella, while the mormyrid cerebellum is massive and apparently involved in their electrical sense. 
The brain stem or myelencephalon is the brain's posterior.  As well as controlling some muscles and body organs, in bony fish at least, the brain stem governs respiration and osmoregulation. 
Vertebrates are the only chordate group to exhibit a proper brain. A slight swelling of the anterior end of the dorsal nerve cord is found in the lancelet, though it lacks the eyes and other complex sense organs comparable to those of vertebrates. Other chordates do not show any trends towards cephalisation.  The central nervous system is based on a hollow nerve tube running along the length of the animal, from which the peripheral nervous system branches out to innervate the various systems. The front end of the nerve tube is expanded by a thickening of the walls and expansion of the central canal of spinal cord into three primary brain vesicles the prosencephalon (forebrain), mesencephalon (midbrain) and rhombencephalon (hindbrain) then further differentiated in the various vertebrate groups.  Two laterally placed eyes form around outgrows from the midbrain, except in hagfish, though this may be a secondary loss.   The forebrain is well developed and subdivided in most tetrapods, while the midbrain dominates in many fish and some salamanders. Vesicles of the forebrain are usually paired, giving rise to hemispheres like the cerebral hemispheres in mammals.  The resulting anatomy of the central nervous system, with a single, hollow ventral nerve cord topped by a series of (often paired) vesicles is unique to vertebrates. 
The circuits in the cerebellum are similar across all classes of vertebrates, including fish, reptiles, birds, and mammals.  There is also an analogous brain structure in cephalopods with well-developed brains, such as octopuses.  This has been taken as evidence that the cerebellum performs functions important to all animal species with a brain.
There is considerable variation in the size and shape of the cerebellum in different vertebrate species. In amphibians, lampreys, and hagfish, the cerebellum is little developed in the latter two groups, it is barely distinguishable from the brain-stem. Although the spinocerebellum is present in these groups, the primary structures are small paired nuclei corresponding to the vestibulocerebellum. 
The cerebellum of cartilaginous and bony fishes is extraordinarily large and complex. In at least one important respect, it differs in internal structure from the mammalian cerebellum: The fish cerebellum does not contain discrete deep cerebellar nuclei. Instead, the primary targets of Purkinje cells are a distinct type of cell distributed across the cerebellar cortex, a type not seen in mammals. In mormyrids (a family of weakly electrosensitive freshwater fish), the cerebellum is considerably larger than the rest of the brain put together. The largest part of it is a special structure called the valvula, which has an unusually regular architecture and receives much of its input from the electrosensory system. 
Most species of fish and amphibians possess a lateral line system that senses pressure waves in water. One of the brain areas that receives primary input from the lateral line organ, the medial octavolateral nucleus, has a cerebellum-like structure, with granule cells and parallel fibers. In electrosensitive fish, the input from the electrosensory system goes to the dorsal octavolateral nucleus, which also has a cerebellum-like structure. In ray-finned fishes (by far the largest group), the optic tectum has a layer—the marginal layer—that is cerebellum-like. 
Identified neurons Edit
A neuron is "identified" if it has properties that distinguish it from every other neuron in the same animal—properties such as location, neurotransmitter, gene expression pattern, and connectivity—and if every individual organism belonging to the same species has one and only one neuron with the same set of properties.  In vertebrate nervous systems, very few neurons are "identified" in this sense (in humans, there are believed to be none). In simpler nervous systems, some or all neurons may be thus unique. 
In vertebrates, the best known identified neurons are the gigantic Mauthner cells of fish.  Every fish has two Mauthner cells, located in the bottom part of the brainstem, one on the left side and one on the right. Each Mauthner cell has an axon that crosses over, innervating neurons at the same brain level and then travelling down through the spinal cord, making numerous connections as it goes. The synapses generated by a Mauthner cell are so powerful that a single action potential gives rise to a major behavioral response: within milliseconds the fish curves its body into a C-shape, then straightens, thereby propelling itself rapidly forward. Functionally, this is a fast escape response, triggered most easily by a strong sound wave or pressure wave impinging on the lateral line organ of the fish. Mauthner cells are not the only identified neurons in fish—there are about 20 more types, including pairs of "Mauthner cell analogs" in each spinal segmental nucleus. Although a Mauthner cell is capable of bringing about an escape response all by itself, in the context of ordinary behavior, other types of cells usually contribute to shaping the amplitude and direction of the response.
Mauthner cells have been described as command neurons. A command neuron is a special type of identified neuron, defined as a neuron that is capable of driving a specific behavior all by itself.  Such neurons appear most commonly in the fast escape systems of various species—the squid giant axon and squid giant synapse, used for pioneering experiments in neurophysiology because of their enormous size, both participate in the fast escape circuit of the squid. The concept of a command neuron has, however, become controversial, because of studies showing that some neurons that initially appeared to fit the description were really only capable of evoking a response in a limited set of circumstances. 
Immune organs vary by type of fish.  In the jawless fish (lampreys and hagfish), true lymphoid organs are absent. These fish rely on regions of lymphoid tissue within other organs to produce immune cells. For example, erythrocytes, macrophages and plasma cells are produced in the anterior kidney (or pronephros) and some areas of the gut (where granulocytes mature). They resemble primitive bone marrow in hagfish.
Cartilaginous fish (sharks and rays) have a more advanced immune system. They have three specialized organs that are unique to chondrichthyes the epigonal organs (lymphoid tissues similar to mammalian bone) that surround the gonads, the Leydig's organ within the walls of their esophagus, and a spiral valve in their intestine. These organs house typical immune cells (granulocytes, lymphocytes and plasma cells). They also possess an identifiable thymus and a well-developed spleen (their most important immune organ) where various lymphocytes, plasma cells and macrophages develop and are stored.
Chondrostean fish (sturgeons, paddlefish and bichirs) possess a major site for the production of granulocytes within a mass that is associated with the meninges, the membranes surrounding the central nervous system. Their heart is frequently covered with tissue that contains lymphocytes, reticular cells and a small number of macrophages. The chondrostean kidney is an important hemopoietic organ it is where erythrocytes, granulocytes, lymphocytes and macrophages develop.
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