How do single-celled predators chase other cells?

How do single-celled predators chase other cells?

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From my understanding, single celled organisms have been seen avoiding, and chasing, potential food or other organisms.

How do they accomplish this? They do not have eyes or ears or a nervous system. My understanding is that chemical reactions are involved somehow.

How do single-celled predators chase other cells? What happens on a chemical level?

Edit:To help keep a more constrained question, the single-celled organism I am considering is Paramecium

Detection of and movement according to a gradient of a chemical species is a strategy that single cells use to track a target across space. There are very many strategies of movement depending on the cell and its environment, but a common problem that while cells can sense the concentration of chemicals, they cannot sense its gradient (the direction in which it increases) because they are of too small a size to rely on the difference of concentration on one side and the other. Howard Berg and his co-workers have discovered a mechanism that allows single cells to move toward increasing concentration zones by a combination of concentration sampling and movement. See his webpage,


Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill the host) and parasitoidism (which always does, eventually). It is distinct from scavenging on dead prey, though many predators also scavenge it overlaps with herbivory, as seed predators and destructive frugivores are predators.

Predators may actively search for or pursue prey or wait for it, often concealed. When prey is detected, the predator assesses whether to attack it. This may involve ambush or pursuit predation, sometimes after stalking the prey. If the attack is successful, the predator kills the prey, removes any inedible parts like the shell or spines, and eats it.

Predators are adapted and often highly specialized for hunting, with acute senses such as vision, hearing, or smell. Many predatory animals, both vertebrate and invertebrate, have sharp claws or jaws to grip, kill, and cut up their prey. Other adaptations include stealth and aggressive mimicry that improve hunting efficiency.

Predation has a powerful selective effect on prey, and the prey develop antipredator adaptations such as warning coloration, alarm calls and other signals, camouflage, mimicry of well-defended species, and defensive spines and chemicals. Sometimes predator and prey find themselves in an evolutionary arms race, a cycle of adaptations and counter-adaptations. Predation has been a major driver of evolution since at least the Cambrian period.

How Cells Build Organisms

Live-cell imaging shows the dynamic environment and extent of cell movement that occurs as the nascent spinal cord forms in a developing zebrafish embryo.Video: Tony Tsai/Sean Megason.

Under a microscope, the first few hours of every multicellular organism’s life seem incongruously chaotic. After fertilization, a once tranquil single-celled egg divides again and again, quickly becoming a visually tumultuous mosh pit of cells jockeying for position inside the rapidly growing embryo.

Yet, amid this apparent pandemonium, cells begin to self-organize. Soon, spatial patterns emerge, serving as the foundation for the construction of tissues, organs and elaborate anatomical structures from brains to toes and everything in between. For decades, scientists have intensively studied this process, called morphogenesis, but it remains in many ways enigmatic.

Now, researchers at Harvard Medical School and the Institute of Science and Technology (IST) Austria have discovered a key control mechanism that cells use to self-organize in early embryonic development. The findings, published in Science on Oct. 2, shed light on a process fundamental to multicellular life and open new avenues for improved tissue and organ engineering strategies.

Studying spinal cord formation in zebrafish embryos, a team co-led by Sean Megason, professor of systems biology in the Blavatnik Institute at HMS, revealed that different cell types express unique combinations of adhesion molecules in order to self-sort during morphogenesis. These “adhesion codes” determine which cells prefer to stay connected, and how strongly they do so, even as widespread cellular rearrangements occur in the developing embryo.

The researchers found that adhesion codes are regulated by morphogens, master signaling molecules long known to govern cell fate and pattern formation in development. The results suggest that the interplay of morphogens and adhesion properties allows cells to organize with the precision and consistency required to construct an organism.

From One, Many

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Engineering Reality

“My lab’s goal is to understand the basic design principles of biological form,” said Megason, co-corresponding author on the study. “Our findings represent a new way of approaching the question of morphogenesis, which is one of the oldest and most important in embryology. We see this as the tip of the iceberg for such efforts.”

Insights into how cells self-organize in early development could also aid efforts to engineer tissues and organs for clinical uses such as transplantation, the authors said.

“Constructing artificial tissues for research or medical applications is a critically important goal, but currently one of the biggest problems is inconsistency,” said lead study author Tony Tsai, research fellow in systems biology in the Blavatnik Institute. “There is a clear lesson to learn from understanding and reverse engineering how cells in a developing embryo are able to build the components of an organism in such a robust and reproducible way.”

Fuzzy flag

Spearheaded by Tsai and in collaboration with Carl-Philipp Heisenberg and colleagues at IST Austria, the research team first looked at one of the most well-established frameworks for morphogenesis, the French flag model.

In this model, morphogens are released from localized sources in the embryo, exposing nearby cells to higher levels of the signaling molecule than cells farther away. The amount of morphogen a cell is exposed to activates different cellular programs, particularly those that determine cell fate. Concentration gradients of morphogens therefore “paint” patterns onto groups of cells, evocative of the distinct color bands of the French flag.

This model has limitations, however. Previous studies from the Megason lab used live-cell imaging and single-cell tracking in whole zebrafish embryos to show that morphogen signals can be noisy and imprecise, particularly at the boundaries of the “flag.” In addition, cells in a developing embryo are constantly dividing and in motion, which can scramble the morphogen signal. This results in an initial mixed patterning of cell types.

Nevertheless, cells self-sort into precise patterns, even with a noisy start, and in the current study, the team set out to understand how. They focused on a hypothesis proposed over 50 years ago, known as differential adhesion. This model suggests that cells adhere to certain other cell types, self-sorting in a way similar to how oil and vinegar separates over time. But there was little evidence that this plays a role in patterning.

A cellular tug-of-war assay. Video: Tony Tsai/Sean Megason

To investigate, Megason, Tsai and colleagues developed a method to measure the force by which cells adhere to one another. They placed two individual cells together and then pulled on each cell with precisely controlled suction pressure from two micropipettes. This allowed the researchers to measure the precise amount of force needed to pull the cells apart. By analyzing three cells at once, they could also establish adhesion preferences.

The team used this technique to study the patterning of three different types of neural progenitor cells involved in building the nascent spinal cord in zebrafish embryos.

The experiments revealed that cells of a similar type strongly and preferentially adhered to one another. To identify the relevant adhesion molecule-encoding genes, the researchers analyzed the gene expression profile of each cell type using RNA sequencing. They then used CRISPR-Cas9 to block the expression of candidate genes, one at a time. If pattern formation became disrupted, they applied the pulling assay to see how much the molecule contributed to adhesion.

Adhesion code

Three genes—N-cadherin, cadherin 11 and protocadherin 19—emerged as essential for normal patterning. The expression of different combinations and different levels of these genes was responsible for differences in adhesion preference, representing what the team dubbed an adhesion code. This code was unique to each of the cell types and determined which other cells each cell type stays connected to during morphogenesis.

“All three adhesion molecules we looked at are expressed in different amounts in each cell type,” Tsai said. “Cells use this code to preferentially adhere to cells of their own type, which is what allows different cell types to separate during pattern formation. But cells also maintain some level of adhesion with other cell types since they have to collaborate to form tissues. By piecing together these local interaction rules, we can illuminate the global picture.”

Because the adhesion code is cell-type specific, the researchers hypothesized that it is likely controlled by the same processes that determine cell fate—namely, morphogen signaling. They looked at how perturbations to one the most well-known morphogens, Sonic hedgehog (Shh), affected cell type and corresponding adhesion-molecule gene expression.

The analyses revealed that both cell type and adhesion-molecule gene expression were highly correlated, both in level and spatial position. This held true across the entire nascent spinal cord, where patterns of gene expression for cell type and adhesion molecule changed together in response to differences in Shh activity.

“What we found is that this morphogen not only controls cell fate, it controls cell adhesion,” Megason said. “The French flag model gives a rough sketch, and differential adhesion then forms the precise pattern. Combining these different strategies appears to be how cells build patterns in 3D space and time as the embryo is forming.”


Cells are of two types: eukaryotic, which contain a nucleus, and prokaryotic, which do not. Prokaryotes are single-celled organisms, while eukaryotes can be either single-celled or multicellular.

Prokaryotic cells

Prokaryotes include bacteria and archaea, two of the three domains of life. Prokaryotic cells were the first form of life on Earth, characterized by having vital biological processes including cell signaling. They are simpler and smaller than eukaryotic cells, and lack a nucleus, and other membrane-bound organelles. The DNA of a prokaryotic cell consists of a single circular chromosome that is in direct contact with the cytoplasm. The nuclear region in the cytoplasm is called the nucleoid. Most prokaryotes are the smallest of all organisms ranging from 0.5 to 2.0 μm in diameter. [13]

A prokaryotic cell has three regions:

  • Enclosing the cell is the cell envelope – generally consisting of a plasma membrane covered by a cell wall which, for some bacteria, may be further covered by a third layer called a capsule. Though most prokaryotes have both a cell membrane and a cell wall, there are exceptions such as Mycoplasma (bacteria) and Thermoplasma (archaea) which only possess the cell membrane layer. The envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective filter. The cell wall consists of peptidoglycan in bacteria, and acts as an additional barrier against exterior forces. It also prevents the cell from expanding and bursting (cytolysis) from osmotic pressure due to a hypotonic environment. Some eukaryotic cells (plant cells and fungal cells) also have a cell wall.
  • Inside the cell is the cytoplasmic region that contains the genome (DNA), ribosomes and various sorts of inclusions. [4] The genetic material is freely found in the cytoplasm. Prokaryotes can carry extrachromosomal DNA elements called plasmids, which are usually circular. Linear bacterial plasmids have been identified in several species of spirochete bacteria, including members of the genus Borrelia notably Borrelia burgdorferi, which causes Lyme disease. [14] Though not forming a nucleus, the DNA is condensed in a nucleoid. Plasmids encode additional genes, such as antibiotic resistance genes.
  • On the outside, flagella and pili project from the cell's surface. These are structures (not present in all prokaryotes) made of proteins that facilitate movement and communication between cells.

Eukaryotic cells

Plants, animals, fungi, slime moulds, protozoa, and algae are all eukaryotic. These cells are about fifteen times wider than a typical prokaryote and can be as much as a thousand times greater in volume. The main distinguishing feature of eukaryotes as compared to prokaryotes is compartmentalization: the presence of membrane-bound organelles (compartments) in which specific activities take place. Most important among these is a cell nucleus, [4] an organelle that houses the cell's DNA. This nucleus gives the eukaryote its name, which means "true kernel (nucleus)". Other differences include:

  • The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls may or may not be present.
  • The eukaryotic DNA is organized in one or more linear molecules, called chromosomes, which are associated with histone proteins. All chromosomal DNA is stored in the cell nucleus, separated from the cytoplasm by a membrane. [4] Some eukaryotic organelles such as mitochondria also contain some DNA.
  • Many eukaryotic cells are ciliated with primary cilia. Primary cilia play important roles in chemosensation, mechanosensation, and thermosensation. Each cilium may thus be "viewed as a sensory cellular antennae that coordinates a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation." [15]
  • Motile eukaryotes can move using motile cilia or flagella. Motile cells are absent in conifers and flowering plants. [16] Eukaryotic flagella are more complex than those of prokaryotes. [17]

All cells, whether prokaryotic or eukaryotic, have a membrane that envelops the cell, regulates what moves in and out (selectively permeable), and maintains the electric potential of the cell. Inside the membrane, the cytoplasm takes up most of the cell's volume. All cells (except red blood cells which lack a cell nucleus and most organelles to accommodate maximum space for hemoglobin) possess DNA, the hereditary material of genes, and RNA, containing the information necessary to build various proteins such as enzymes, the cell's primary machinery. There are also other kinds of biomolecules in cells. This article lists these primary cellular components, then briefly describes their function.


The cell membrane, or plasma membrane, is a biological membrane that surrounds the cytoplasm of a cell. In animals, the plasma membrane is the outer boundary of the cell, while in plants and prokaryotes it is usually covered by a cell wall. This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a double layer of phospholipids, which are amphiphilic (partly hydrophobic and partly hydrophilic). Hence, the layer is called a phospholipid bilayer, or sometimes a fluid mosaic membrane. Embedded within this membrane is a macromolecular structure called the porosome the universal secretory portal in cells and a variety of protein molecules that act as channels and pumps that move different molecules into and out of the cell. [4] The membrane is semi-permeable, and selectively permeable, in that it can either let a substance (molecule or ion) pass through freely, pass through to a limited extent or not pass through at all. Cell surface membranes also contain receptor proteins that allow cells to detect external signaling molecules such as hormones.


The cytoskeleton acts to organize and maintain the cell's shape anchors organelles in place helps during endocytosis, the uptake of external materials by a cell, and cytokinesis, the separation of daughter cells after cell division and moves parts of the cell in processes of growth and mobility. The eukaryotic cytoskeleton is composed of microtubules, intermediate filaments and microfilaments. In the cytoskeleton of a neuron the intermediate filaments are known as neurofilaments. There are a great number of proteins associated with them, each controlling a cell's structure by directing, bundling, and aligning filaments. [4] The prokaryotic cytoskeleton is less well-studied but is involved in the maintenance of cell shape, polarity and cytokinesis. [19] The subunit protein of microfilaments is a small, monomeric protein called actin. The subunit of microtubules is a dimeric molecule called tubulin. Intermediate filaments are heteropolymers whose subunits vary among the cell types in different tissues. But some of the subunit protein of intermediate filaments include vimentin, desmin, lamin (lamins A, B and C), keratin (multiple acidic and basic keratins), neurofilament proteins (NF–L, NF–M).

Genetic material

Two different kinds of genetic material exist: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Cells use DNA for their long-term information storage. The biological information contained in an organism is encoded in its DNA sequence. [4] RNA is used for information transport (e.g., mRNA) and enzymatic functions (e.g., ribosomal RNA). Transfer RNA (tRNA) molecules are used to add amino acids during protein translation.

Prokaryotic genetic material is organized in a simple circular bacterial chromosome in the nucleoid region of the cytoplasm. Eukaryotic genetic material is divided into different, [4] linear molecules called chromosomes inside a discrete nucleus, usually with additional genetic material in some organelles like mitochondria and chloroplasts (see endosymbiotic theory).

A human cell has genetic material contained in the cell nucleus (the nuclear genome) and in the mitochondria (the mitochondrial genome). In humans the nuclear genome is divided into 46 linear DNA molecules called chromosomes, including 22 homologous chromosome pairs and a pair of sex chromosomes. The mitochondrial genome is a circular DNA molecule distinct from the nuclear DNA. Although the mitochondrial DNA is very small compared to nuclear chromosomes, [4] it codes for 13 proteins involved in mitochondrial energy production and specific tRNAs.

Foreign genetic material (most commonly DNA) can also be artificially introduced into the cell by a process called transfection. This can be transient, if the DNA is not inserted into the cell's genome, or stable, if it is. Certain viruses also insert their genetic material into the genome.


Organelles are parts of the cell which are adapted and/or specialized for carrying out one or more vital functions, analogous to the organs of the human body (such as the heart, lung, and kidney, with each organ performing a different function). [4] Both eukaryotic and prokaryotic cells have organelles, but prokaryotic organelles are generally simpler and are not membrane-bound.

There are several types of organelles in a cell. Some (such as the nucleus and golgi apparatus) are typically solitary, while others (such as mitochondria, chloroplasts, peroxisomes and lysosomes) can be numerous (hundreds to thousands). The cytosol is the gelatinous fluid that fills the cell and surrounds the organelles.


  • Cell nucleus: A cell's information center, the cell nucleus is the most conspicuous organelle found in a eukaryotic cell. It houses the cell's chromosomes, and is the place where almost all DNA replication and RNA synthesis (transcription) occur. The nucleus is spherical and separated from the cytoplasm by a double membrane called the nuclear envelope. The nuclear envelope isolates and protects a cell's DNA from various molecules that could accidentally damage its structure or interfere with its processing. During processing, DNA is transcribed, or copied into a special RNA, called messenger RNA (mRNA). This mRNA is then transported out of the nucleus, where it is translated into a specific protein molecule. The nucleolus is a specialized region within the nucleus where ribosome subunits are assembled. In prokaryotes, DNA processing takes place in the cytoplasm. [4]
  • Mitochondria and chloroplasts: generate energy for the cell. Mitochondria are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. [4]Respiration occurs in the cell mitochondria, which generate the cell's energy by oxidative phosphorylation, using oxygen to release energy stored in cellular nutrients (typically pertaining to glucose) to generate ATP. Mitochondria multiply by binary fission, like prokaryotes. Chloroplasts can only be found in plants and algae, and they capture the sun's energy to make carbohydrates through photosynthesis.
  • Endoplasmic reticulum: The endoplasmic reticulum (ER) is a transport network for molecules targeted for certain modifications and specific destinations, as compared to molecules that float freely in the cytoplasm. The ER has two forms: the rough ER, which has ribosomes on its surface that secrete proteins into the ER, and the smooth ER, which lacks ribosomes. [4] The smooth ER plays a role in calcium sequestration and release.
  • Golgi apparatus: The primary function of the Golgi apparatus is to process and package the macromolecules such as proteins and lipids that are synthesized by the cell.
  • Lysosomes and peroxisomes: Lysosomes contain digestive enzymes (acid hydrolases). They digest excess or worn-out organelles, food particles, and engulfed viruses or bacteria. Peroxisomes have enzymes that rid the cell of toxic peroxides. The cell could not house these destructive enzymes if they were not contained in a membrane-bound system. [4]
  • Centrosome: the cytoskeleton organiser: The centrosome produces the microtubules of a cell – a key component of the cytoskeleton. It directs the transport through the ER and the Golgi apparatus. Centrosomes are composed of two centrioles, which separate during cell division and help in the formation of the mitotic spindle. A single centrosome is present in the animal cells. They are also found in some fungi and algae cells.
  • Vacuoles: Vacuoles sequester waste products and in plant cells store water. They are often described as liquid filled space and are surrounded by a membrane. Some cells, most notably Amoeba, have contractile vacuoles, which can pump water out of the cell if there is too much water. The vacuoles of plant cells and fungal cells are usually larger than those of animal cells.

Eukaryotic and prokaryotic

  • Ribosomes: The ribosome is a large complex of RNA and protein molecules. [4] They each consist of two subunits, and act as an assembly line where RNA from the nucleus is used to synthesise proteins from amino acids. Ribosomes can be found either floating freely or bound to a membrane (the rough endoplasmatic reticulum in eukaryotes, or the cell membrane in prokaryotes). [20]

Many cells also have structures which exist wholly or partially outside the cell membrane. These structures are notable because they are not protected from the external environment by the semipermeable cell membrane. In order to assemble these structures, their components must be carried across the cell membrane by export processes.

Cell wall

Many types of prokaryotic and eukaryotic cells have a cell wall. The cell wall acts to protect the cell mechanically and chemically from its environment, and is an additional layer of protection to the cell membrane. Different types of cell have cell walls made up of different materials plant cell walls are primarily made up of cellulose, fungi cell walls are made up of chitin and bacteria cell walls are made up of peptidoglycan.



A gelatinous capsule is present in some bacteria outside the cell membrane and cell wall. The capsule may be polysaccharide as in pneumococci, meningococci or polypeptide as Bacillus anthracis or hyaluronic acid as in streptococci. Capsules are not marked by normal staining protocols and can be detected by India ink or methyl blue which allows for higher contrast between the cells for observation. [21] : 87


Flagella are organelles for cellular mobility. The bacterial flagellum stretches from cytoplasm through the cell membrane(s) and extrudes through the cell wall. They are long and thick thread-like appendages, protein in nature. A different type of flagellum is found in archaea and a different type is found in eukaryotes.


A fimbria (plural fimbriae also known as a pilus, plural pili) is a short, thin, hair-like filament found on the surface of bacteria. Fimbriae are formed of a protein called pilin (antigenic) and are responsible for the attachment of bacteria to specific receptors on human cells (cell adhesion). There are special types of pili involved in bacterial conjugation.


Cell division involves a single cell (called a mother cell) dividing into two daughter cells. This leads to growth in multicellular organisms (the growth of tissue) and to procreation (vegetative reproduction) in unicellular organisms. Prokaryotic cells divide by binary fission, while eukaryotic cells usually undergo a process of nuclear division, called mitosis, followed by division of the cell, called cytokinesis. A diploid cell may also undergo meiosis to produce haploid cells, usually four. Haploid cells serve as gametes in multicellular organisms, fusing to form new diploid cells.

DNA replication, or the process of duplicating a cell's genome, [4] always happens when a cell divides through mitosis or binary fission. This occurs during the S phase of the cell cycle.

In meiosis, the DNA is replicated only once, while the cell divides twice. DNA replication only occurs before meiosis I. DNA replication does not occur when the cells divide the second time, in meiosis II. [22] Replication, like all cellular activities, requires specialized proteins for carrying out the job. [4]

DNA repair

In general, cells of all organisms contain enzyme systems that scan their DNA for damages and carry out repair processes when damages are detected. [23] Diverse repair processes have evolved in organisms ranging from bacteria to humans. The widespread prevalence of these repair processes indicates the importance of maintaining cellular DNA in an undamaged state in order to avoid cell death or errors of replication due to damages that could lead to mutation. E. coli bacteria are a well-studied example of a cellular organism with diverse well-defined DNA repair processes. These include: (1) nucleotide excision repair, (2) DNA mismatch repair, (3) non-homologous end joining of double-strand breaks, (4) recombinational repair and (5) light-dependent repair (photoreactivation).

Growth and metabolism

Between successive cell divisions, cells grow through the functioning of cellular metabolism. Cell metabolism is the process by which individual cells process nutrient molecules. Metabolism has two distinct divisions: catabolism, in which the cell breaks down complex molecules to produce energy and reducing power, and anabolism, in which the cell uses energy and reducing power to construct complex molecules and perform other biological functions. Complex sugars consumed by the organism can be broken down into simpler sugar molecules called monosaccharides such as glucose. Once inside the cell, glucose is broken down to make adenosine triphosphate (ATP), [4] a molecule that possesses readily available energy, through two different pathways.

Protein synthesis

Cells are capable of synthesizing new proteins, which are essential for the modulation and maintenance of cellular activities. This process involves the formation of new protein molecules from amino acid building blocks based on information encoded in DNA/RNA. Protein synthesis generally consists of two major steps: transcription and translation.

Transcription is the process where genetic information in DNA is used to produce a complementary RNA strand. This RNA strand is then processed to give messenger RNA (mRNA), which is free to migrate through the cell. mRNA molecules bind to protein-RNA complexes called ribosomes located in the cytosol, where they are translated into polypeptide sequences. The ribosome mediates the formation of a polypeptide sequence based on the mRNA sequence. The mRNA sequence directly relates to the polypeptide sequence by binding to transfer RNA (tRNA) adapter molecules in binding pockets within the ribosome. The new polypeptide then folds into a functional three-dimensional protein molecule.


Unicellular organisms can move in order to find food or escape predators. Common mechanisms of motion include flagella and cilia.

In multicellular organisms, cells can move during processes such as wound healing, the immune response and cancer metastasis. For example, in wound healing in animals, white blood cells move to the wound site to kill the microorganisms that cause infection. Cell motility involves many receptors, crosslinking, bundling, binding, adhesion, motor and other proteins. [24] The process is divided into three steps – protrusion of the leading edge of the cell, adhesion of the leading edge and de-adhesion at the cell body and rear, and cytoskeletal contraction to pull the cell forward. Each step is driven by physical forces generated by unique segments of the cytoskeleton. [25] [26]

Navigation, control and communication

In August 2020, scientists described one way cells – in particular cells of a slime mold and mouse pancreatic cancer–derived cells – are able to navigate efficiently through a body and identify the best routes through complex mazes: generating gradients after breaking down diffused chemoattractants which enable them to sense upcoming maze junctions before reaching them, including around corners. [27] [28] [29]

Fun Facts about the Sea Anemone!

Sea anemones are fascinating organisms and have long drawn the interest of researchers and ocean lovers alike. They have many unique properties including their symbiotic relationships with various organisms as well as some fascinating regeneration abilities.

Related to Jellyfish and Coral

As a member of the phylum Cnidaria, sea anemones are related to corals, jellyfish, tube-dwelling anemones, and Hydra spp. When one considers the polyp-shape of corals or the notorious presence of cnidocytes in jellyfish, this is not overly surprising.

All sea anemones belong to the class Anthozoa, of which there are three subclasses: Octocorallia, Ceriantharia, and Hexacorallia. To the latter belong the order Actiniaria and all 45 of its sea anemone families. The phylogenetic relationships within the class Anthozoa are not well understood, particularly within these three subclasses. Indeed, recent DNA analysis suggests that some species of anemone may indeed exist outside of the order Actiniaria. This only further highlights the complexity of studying phylogenetic relationships, particularly in species with virtually no hard parts that can be preserved in the fossil record.

Cnidocytes and Nemadocysts

Much like jellyfish, which are famous for their ability to ‘sting’ their prey and potential predators, sea anemones possess cnidocytes – specialized ‘stinging cells’. Within each cnidocyte are organelles (small organ-like structures) known as nematocysts. These are the ‘stingers’ and in anemones are present mostly in the area of the oral disc as well as the tentacles.

Within each cnidocyte, the nematocyst contains many coiled, barbed threads. On the outside of the cnidocyte, a hairlike structure known as a scnidocils sits. These structures are sensitive to touch and, once touched, trigger the nematocysts to uncoil and penetrate their prey (or predator). In many cases, these nematocysts also deliver toxins known as actinotoxins, that can immobilize the victim.

Actinotoxins are highly toxic to prey species of fish and crustaceans. However, Amphiprioninae (clownfish), small banded fish in various colors, are not affected by their host anemone’s sting and shelter themselves from predators among its tentacles. Several other species have similar adaptions and are also unaffected.

At the base of the tentacles in some species are elongated, inflatable, tentacle-like organs known as acrorhagi. These are armed with cnidocytes and are used to fend off other encroaching anemones.

Most sea anemones are harmless to humans, but some highly toxic species do exist. Actinodendron arboreum, Phyllodiscus semoni, and Stichodactyla spp., for example, are known to cause severe injuries and are even potentially lethal to humans.


Beyond their symbiosis with clownfish and other species immune to the anemone’s stinging-cells, they are also well known for another symbiotic relationship that they form with plants. Indeed, single-celled algae known as zooxanthellae and zoochlorellae are often observed living within the dermal cells of anemones. These are normally concentrated in the tips of their tentacles and other upward-facing parts of the anemone’s bodies.

This relationship is known to be mutually beneficial to both species, as the algae avoid being consumed by herbivorous species as they float through the sea while the anemone is able to utilize some of the oxygen produced by the algal cells as well as its energy in the form of glycerol, glucose, and alanine. This forms an important supplement to the anemone’s diet which is otherwise carnivorous, using its stinging cells to paralyze and consume various prey species.

Properties of Life

Figure 1. This female monarch butterfly represents a highly organized structure consisting of cells, tissues, organs, and organ systems

All living organisms share several key characteristics or functions: order, sensitivity or response to the environment, reproduction, growth and development, regulation, homeostasis, and energy processing. When viewed together, these characteristics serve to define life.


Organisms are highly organized, coordinated structures that consist of one or more cells. Even very simple, single-celled organisms are remarkably complex: inside each cell, atoms make up molecules these in turn make up cell organelles and other cellular inclusions.

In multicellular organisms (Figure 1), similar cells form tissues. Tissues, in turn, collaborate to create organs (body structures with a distinct function). Organs work together to form organ systems.

Sensitivity or Response to Stimuli

Figure 2.The leaves of this sensitive plant (Mimosa pudica) will instantly droop and fold when touched. After a few minutes, the plant returns to normal.

Organisms respond to diverse stimuli. For example, plants can bend toward a source of light, climb on fences and walls, or respond to touch (Figure 2). Even tiny bacteria can move toward or away from chemicals (a process called chemotaxis) or light (phototaxis). Movement toward a stimulus is considered a positive response, while movement away from a stimulus is considered a negative response.

Watch this video to see how plants respond to a stimulus—from opening to light, to wrapping a tendril around a branch, to capturing prey.


Single-celled organisms reproduce by first duplicating their DNA, and then dividing it equally as the cell prepares to divide to form two new cells. Multicellular organisms often produce specialized reproductive germline cells that will form new individuals. When reproduction occurs, genes containing DNA are passed along to an organism’s offspring. These genes ensure that the offspring will belong to the same species and will have similar characteristics, such as size and shape.

Growth and Development

Figure 3. Although no two look alike, these puppies have inherited genes from both parents and share many of the same characteristics.

Organisms grow and develop following specific instructions coded for by their genes. These genes provide instructions that will direct cellular growth and development, ensuring that a species’ young (Figure 3) will grow up to exhibit many of the same characteristics as its parents.


Even the smallest organisms are complex and require multiple regulatory mechanisms to coordinate internal functions, respond to stimuli, and cope with environmental stresses. Two examples of internal functions regulated in an organism are nutrient transport and blood flow. Organs (groups of tissues working together) perform specific functions, such as carrying oxygen throughout the body, removing wastes, delivering nutrients to every cell, and cooling the body.


Figure 4. Polar bears (Ursus maritimus) and other mammals living in ice-covered regions maintain their body temperature by generating heat and reducing heat loss through thick fur and a dense layer of fat under their skin.

In order to function properly, cells need to have appropriate conditions such as proper temperature, pH, and appropriate concentration of diverse chemicals. These conditions may, however, change from one moment to the next. Organisms are able to maintain internal conditions within a narrow range almost constantly, despite environmental changes, through homeostasis (literally, “steady state”)—the ability of an organism to maintain constant internal conditions. For example, an organism needs to regulate body temperature through a process known as thermoregulation. Organisms that live in cold climates, such as the polar bear (Figure 4), have body structures that help them withstand low temperatures and conserve body heat. Structures that aid in this type of insulation include fur, feathers, blubber, and fat. In hot climates, organisms have methods (such as perspiration in humans or panting in dogs) that help them to shed excess body heat.

Energy Processing

All organisms use a source of energy for their metabolic activities. Some organisms capture energy from the sun and convert it into chemical energy in food (photosynthesis) others use chemical energy in molecules they take in as food (cellular respiration).

Figure 5. The California condor (Gymnogyps californianus) uses chemical energy derived from food to power flight. California condors are an endangered species this bird has a wing tag that helps biologists identify the individual.

Flashes bright when squeezed tight: How single-celled organisms light up the oceans

Research explains how a unicellular marine organism generates light as a response to mechanical stimulation, lighting up breaking waves at night.

Every few years, a bloom of microscopic organisms called dinoflagellates transforms the coasts around the world by endowing breaking waves with an eerie blue glow. This year's spectacular bloom in southern California was a particularly striking example. In a new study published in the journal Physical Review Letters, researchers have identified the underlying physics that results in light production in one species of these organisms.

The international team, led by the University of Cambridge, developed unique experimental tools based on micromanipulation and high-speed imaging to visualize light production on the single-cell level. They showed how a single-celled organism of the species Pyrocystis lunula produces a flash of light when its cell wall is deformed by mechanical forces. Through systematic experimentation, they found that the brightness of the flash depends both on the depth of the deformation and the rate at which it is imposed.

Known as a 'viscoelastic' response, this behavior is found in many complex materials such as fluids with suspended polymers. In the case of organisms like Pyrocystis lunula, known as dinoflagellates, this mechanism is most likely related to ion channels, which are specialized proteins distributed on the cell membrane. When the membrane is stressed, these channels open up, allowing calcium to move between compartments in the cell, triggering a biochemical cascade that produces light.

"Despite decades of scientific research, primarily within the field of biochemistry, the physical mechanism by which fluid flow triggers light production has remained unclear," said Professor Raymond E. Goldstein, the Schlumberger Professor of Complex Physical Systems in the Department of Applied Mathematics and Theoretical Physics, who led the research.

"Our findings reveal the physical mechanism by which the fluid flow triggers light production and show how elegant decision-making can be on a single-cell level," said Dr Maziyar Jalaal, the paper's first author.

Bioluminescence has been of interest to humankind for thousands of years, as it is visible as the glow of night-time breaking waves in the ocean or the spark of fireflies in the forest. Many authors and philosophers have written about bioluminescence, from Aristotle to Shakespeare, who in Hamlet wrote about the 'uneffectual fire' of the glow-worm a reference to the production of light without heat:

To prick and sting her. Fare thee well at once.

The glowworm shows the matin to be near,

And 'gins to pale his uneffectual fire.

Adieu, adieu, adieu. Remember me."

The bioluminescence in the ocean is, however, not 'uneffectual.' In contrast, it is used for defense, offense, and mating. In the case of dinoflagellates, they use light production to scare off predators.

The results of the current study show that when the deformation of the cell wall is small, the light intensity is small no matter how rapidly the indentation is made, and it is also small when the indentation is large but applied slowly. Only when both the amplitude and rate are large is the light intensity maximized. The group developed a mathematical model that was able to explain these observations quantitatively, and they suggest that this behavior can act as a filter to avoid spurious light flashes from being triggered.

In the meantime, the researchers plan to analyze more quantitatively the distribution of forces over the entire cells in the fluid flow, a step towards understanding the light prediction in a marine context.

Other members of the research team were postdoctoral researcher Hélène de Maleprade, visiting students Nico Schramma from the Max-Planck Institute for Dynamics and Self-Organization in Göttingen, Germany and Antoine Dode from the Ècole Polytechnique in France, and visiting professor Christophe Raufaste from the Institut de Physique de Nice, France.

The work was supported by the Marine Microbiology Initiative of the Gordon and Betty Moore Foundation, the Schlumberger Chair Fund, the French National Research Agency, and the Wellcome Trust.

Interesting Insights from the Water Buffalo!

The water buffalo is not just a peaceful bovid from Asia – it is also the perfect example of several important biological concepts.

The Even-Toed Ungulates: Water buffaloes and Whales

First, the mud serves as a physical barrier to protect the water buffalo from biting insects. Not only do biting insects carry many diseases, but they also cause uncomfortable itching and inflammation – even when your skin is as thick as a water buffalo’s!

Second, wet mud provides effective relief against the prevalent heat of the tropics. Water buffalo will roll in the mud, coating themselves effectively. As the water evaporates from the mud, it cools the remaining mud. Plus, it keeps the buffalo’s skin out of direct sunlight. This protects the buffalo from quite a bit of heat and UV damage.

The Evolution of Domestication

While wild water buffalo are in danger of extinction, domestic water buffalo are not. Like other domestic animals, water buffalos have been changed through thousands of years of coevolution with humans. Much like dogs still resemble wolves, water buffalo still resemble their wild counterparts. However, due to their more docile nature, domestic water buffaloes are the preferred working animal for many farmers across parts of Asia and the Middle East.

The fact that the wild water buffalo species are all Endangered is not surprising. The wild counterpart of the cow – known as the auroch – is now completely extinct globally. Cows, however, outnumber almost all wild animal species besides some insects. Many domesticated species have outlived their wild counterparts by being essential to certain cultures, which continue to reproduce them and protect them from harm.

A domesticated Asian Water BuffaloThis isn’t true of all domestic animals, though. Domestic pigs are still highly related to their wild relatives – the feral pigs. Feral pigs are actually so prevalent in some parts of the United States and South America that they are treated as a pest species. The smaller size of species like the pig may have contributed to their survival into the human era – since humans wiped out the last auroch ages ago.

Animal-Like Protists: Protozoa

Animal-like protists are commonly called protozoa (singular, protozoan). Most protozoa consist of a single cell. They are animal-like because they are heterotrophs, and are capable of moving. Although protozoa are not animals, they are thought to be the ancestors of animals.

Ecology of Protozoa

Protozoa generally feed by engulfing and digesting other organisms. As consumers, they have various roles in food chains and webs. Some are predators. They prey upon other single-celled organisms, such as bacteria. In fact, protozoa predators keep many bacterial populations under control. Other protozoa are herbivores. They graze on algae. Still others are decomposers. They consume dead organic matter. There are also parasitic protozoa that live in or on living hosts. For example, the protozoan that causes malaria lives inside a human host. Protozoa are also important food sources for many larger organisms, including insectsand worms.

Classification of Protozoa

Protozoa can be classified on the basis of how they move. As shown in Table below, protozoa move in three different ways. Only sporozoa cannot move. Note that this classification is based only on differences in movement. It does not represent phylogenetic relationships.


Among animal phyla, the Ctenophores are more complex than sponges, about as complex as cnidarians (jellyfish, sea anemones, etc.), and less complex than bilaterians (which include almost all other animals). Unlike sponges, both ctenophores and cnidarians have: cells bound by inter-cell connections and carpet-like basement membranes muscles nervous systems and some have sensory organs. Ctenophores are distinguished from all other animals by having colloblasts, which are sticky and adhere to prey, although a few ctenophore species lack them. [16] [17]

Like sponges and cnidarians, ctenophores have two main layers of cells that sandwich a middle layer of jelly-like material, which is called the mesoglea in cnidarians and ctenophores more complex animals have three main cell layers and no intermediate jelly-like layer. Hence ctenophores and cnidarians have traditionally been labelled diploblastic, along with sponges. [16] [18] Both ctenophores and cnidarians have a type of muscle that, in more complex animals, arises from the middle cell layer, [19] and as a result some recent text books classify ctenophores as triploblastic, [20] while others still regard them as diploblastic. [16] The comb jellies have more than 80 different cell types, exceeding the numbers from other groups like placozoans, sponges, cnidarians, and some deep-branching bilaterians. [21]

Ranging from about 1 millimeter (0.04 in) to 1.5 meters (5 ft) in size, [20] [22] ctenophores are the largest non-colonial animals that use cilia ("hairs") as their main method of locomotion. [20] Most species have eight strips, called comb rows, that run the length of their bodies and bear comb-like bands of cilia, called "ctenes", stacked along the comb rows so that when the cilia beat, those of each comb touch the comb below. [20] The name "ctenophora" means "comb-bearing", from the Greek κτείς (stem-form κτεν-) meaning "comb" and the Greek suffix -φορος meaning "carrying". [23]

Comparison with other major animal groups
Sponges [24] [25] Cnidarians [16] [18] [26] Ctenophores [16] [20] Bilateria [16]
Cnidocytes No Yes Only in some species (obtained by ingesting cnidarians) Only in some species (obtained by ingesting cnidarians)
microRNA Yes Yes No Yes
Hox genes No Yes No Yes
Colloblasts No In most species [17] No
Digestive and circulatory organs No Yes
Anal pores No Yes Only in some flatworms
Number of main cell layers Two, with jelly-like layer between them Debate about whether two [16] or three [19] [20] Three
Cells in each layer bound together No, except that Homoscleromorpha have basement membranes. [27] Yes: Inter-cell connections basement membranes
Sensory organs No Yes
Eyes (e.g. ocelli) No Yes No Yes
Apical organ No Yes No In species with primary ciliated larvae
Number of cells in middle "jelly" layer Many Few (Not applicable)
Cells in outer layers can move inwards and change functions Yes No (Not applicable)
Nervous system No Yes, simple Simple to complex
Muscles None Mostly epitheliomuscular Mostly myoepithelial Mostly myocytes

For a phylum with relatively few species, ctenophores have a wide range of body plans. [20] Coastal species need to be tough enough to withstand waves and swirling sediment particles, while some oceanic species are so fragile that it is very difficult to capture them intact for study. [17] In addition, oceanic species do not preserve well, [17] and are known mainly from photographs and from observers' notes. [28] Hence most attention has until recently concentrated on three coastal genera – Pleurobrachia, Beroe and Mnemiopsis. [17] [29] At least two textbooks base their descriptions of ctenophores on the cydippid Pleurobrachia. [16] [20]

Since the body of many species is almost radially symmetrical, the main axis is oral to aboral (from the mouth to the opposite end.) However, since only two of the canals near the statocyst terminate in anal pores, ctenophores have no mirror-symmetry, although many have rotational symmetry. In other words, if the animal rotates in a half-circle it looks the same as when it started. [30]

Common features Edit

The Ctenophore phylum has a wide range of body forms, including the flattened, deep-sea platyctenids, in which the adults of most species lack combs, and the coastal beroids, which lack tentacles and prey on other ctenophores by using huge mouths armed with groups of large, stiffened cilia that act as teeth.

Body layers Edit

Like those of cnidarians, (jellyfish, sea anemones, etc.), ctenophores' bodies consist of a relatively thick, jelly-like mesoglea sandwiched between two epithelia, layers of cells bound by inter-cell connections and by a fibrous basement membrane that they secrete. [16] [20] The epithelia of ctenophores have two layers of cells rather than one, and some of the cells in the upper layer have several cilia per cell. [20]

The outer layer of the epidermis (outer skin) consists of: sensory cells cells that secrete mucus, which protects the body and interstitial cells, which can transform into other types of cell. In specialized parts of the body, the outer layer also contains colloblasts, found along the surface of tentacles and used in capturing prey, or cells bearing multiple large cilia, for locomotion. The inner layer of the epidermis contains a nerve net, and myoepithelial cells that act as muscles. [20]

The internal cavity forms: a mouth that can usually be closed by muscles a pharynx ("throat") a wider area in the center that acts as a stomach and a system of internal canals. These branch through the mesoglea to the most active parts of the animal: the mouth and pharynx the roots of the tentacles, if present all along the underside of each comb row and four branches around the sensory complex at the far end from the mouth – two of these four branches terminate in anal pores. The inner surface of the cavity is lined with an epithelium, the gastrodermis. The mouth and pharynx have both cilia and well-developed muscles. In other parts of the canal system, the gastrodermis is different on the sides nearest to and furthest from the organ that it supplies. The nearer side is composed of tall nutritive cells that store nutrients in vacuoles (internal compartments), germ cells that produce eggs or sperm, and photocytes that produce bioluminescence. The side furthest from the organ is covered with ciliated cells that circulate water through the canals, punctuated by ciliary rosettes, pores that are surrounded by double whorls of cilia and connect to the mesoglea. [20]

Feeding, excretion and respiration Edit

When prey is swallowed, it is liquefied in the pharynx by enzymes and by muscular contractions of the pharynx. The resulting slurry is wafted through the canal system by the beating of the cilia, and digested by the nutritive cells. The ciliary rosettes in the canals may help to transport nutrients to muscles in the mesoglea. The anal pores may eject unwanted small particles, but most unwanted matter is regurgitated via the mouth. [20]

Little is known about how ctenophores get rid of waste products produced by the cells. The ciliary rosettes in the gastrodermis may help to remove wastes from the mesoglea, and may also help to adjust the animal's buoyancy by pumping water into or out of the mesoglea. [20]

Locomotion Edit

The outer surface bears usually eight comb rows, called swimming-plates, which are used for swimming. The rows are oriented to run from near the mouth (the "oral pole") to the opposite end (the "aboral pole"), and are spaced more or less evenly around the body, [16] although spacing patterns vary by species and in most species the comb rows extend only part of the distance from the aboral pole towards the mouth. The "combs" (also called "ctenes" or "comb plates") run across each row, and each consists of thousands of unusually long cilia, up to 2 millimeters (0.079 in). Unlike conventional cilia and flagella, which has a filament structure arranged in a 9 + 2 pattern, these cilia are arranged in a 9 + 3 pattern, where the extra compact filament is suspected to have a supporting function. [31] These normally beat so that the propulsion stroke is away from the mouth, although they can also reverse direction. Hence ctenophores usually swim in the direction in which the mouth is eating, unlike jellyfish. [20] When trying to escape predators, one species can accelerate to six times its normal speed [32] some other species reverse direction as part of their escape behavior, by reversing the power stroke of the comb plate cilia.

It is uncertain how ctenophores control their buoyancy, but experiments have shown that some species rely on osmotic pressure to adapt to the water of different densities. [33] Their body fluids are normally as concentrated as seawater. If they enter less dense brackish water, the ciliary rosettes in the body cavity may pump this into the mesoglea to increase its bulk and decrease its density, to avoid sinking. Conversely, if they move from brackish to full-strength seawater, the rosettes may pump water out of the mesoglea to reduce its volume and increase its density. [20]

Nervous system and senses Edit

Ctenophores have no brain or central nervous system, but instead have a nerve net (rather like a cobweb) that forms a ring round the mouth and is densest near structures such as the comb rows, pharynx, tentacles (if present) and the sensory complex furthest from the mouth. [20] Their nerve cells arise from the same progenitor cells as the colloblasts. [34]

The largest single sensory feature is the aboral organ (at the opposite end from the mouth). Its main component is a statocyst, a balance sensor consisting of a statolith, a tiny grain of calcium carbonate, supported on four bundles of cilia, called "balancers", that sense its orientation. The statocyst is protected by a transparent dome made of long, immobile cilia. A ctenophore does not automatically try to keep the statolith resting equally on all the balancers. Instead, its response is determined by the animal's "mood", in other words, the overall state of the nervous system. For example, if a ctenophore with trailing tentacles captures prey, it will often put some comb rows into reverse, spinning the mouth towards the prey. [20]

Research supports the hypothesis that the ciliated larvae in cnidarians and bilaterians share an ancient and common origin. [35] The larvae's apical organ is involved in the formation of the nervous system. [36] The aboral organ of comb jellies is not homologous with the apical organ in other animals, and the formation of their nervous system has therefore a different embryonic origin. [37]

Ctenophore nerve cells and nervous system have different biochemistry as compared to other animals. For instance, they lack the genes and enzymes required to manufacture neurotransmitters like serotonin, dopamine, nitric oxide, octopamine, noradrenaline, and others, otherwise seen in all other animals with a nervous system, with the genes coding for the receptors for each of these neurotransmitters missing. [38] They have been found to use L-glutamate as a neurotransmitter, and have an unusually high variety of ionotropic glutamate receptors and genes for glutamate synthesis and transport compared to other metazoans. [39] The genomic content of the nervous system genes is the smallest known of any animal, and could represent the minimum genetic requirements for a functional nervous system. [40] Therefore, if ctenophores are the sister group to all other metazoans, nervous systems may have either been lost in sponges and placozoans, or arisen more than once among metazoans. [41]

Cydippids Edit

Cydippid ctenophores have bodies that are more or less rounded, sometimes nearly spherical and other times more cylindrical or egg-shaped the common coastal "sea gooseberry", Pleurobrachia, sometimes has an egg-shaped body with the mouth at the narrow end, [20] although some individuals are more uniformly round. From opposite sides of the body extends a pair of long, slender tentacles, each housed in a sheath into which it can be withdrawn. [16] Some species of cydippids have bodies that are flattened to various extents so that they are wider in the plane of the tentacles. [20]

The tentacles of cydippid ctenophores are typically fringed with tentilla ("little tentacles"), although a few genera have simple tentacles without these sidebranches. The tentacles and tentilla are densely covered with microscopic colloblasts that capture prey by sticking to it. Colloblasts are specialized mushroom-shaped cells in the outer layer of the epidermis, and have three main components: a domed head with vesicles (chambers) that contain adhesive a stalk that anchors the cell in the lower layer of the epidermis or in the mesoglea and a spiral thread that coils round the stalk and is attached to the head and to the root of the stalk. The function of the spiral thread is uncertain, but it may absorb stress when prey tries to escape, and thus prevent the collobast from being torn apart. [20]

In addition to colloblasts, members of the genus Haeckelia, which feed mainly on jellyfish, incorporate their victims' stinging nematocytes into their own tentacles – some cnidaria-eating nudibranchs similarly incorporate nematocytes into their bodies for defense. [42] The tentilla of Euplokamis differ significantly from those of other cydippids: they contain striated muscle, a cell type otherwise unknown in the phylum Ctenophora and they are coiled when relaxed, while the tentilla of all other known ctenophores elongate when relaxed. Euplokamis' tentilla have three types of movement that are used in capturing prey: they may flick out very quickly (in 40 to 60 milliseconds) they can wriggle, which may lure prey by behaving like small planktonic worms and they coil round prey. The unique flicking is an uncoiling movement powered by contraction of the striated muscle. The wriggling motion is produced by smooth muscles, but of a highly specialized type. Coiling around prey is accomplished largely by the return of the tentilla to their inactive state, but the coils may be tightened by smooth muscle. [43]

There are eight rows of combs that run from near the mouth to the opposite end, and are spaced evenly round the body. [16] The "combs" beat in a metachronal rhythm rather like that of a Mexican wave. [44] From each balancer in the statocyst a ciliary groove runs out under the dome and then splits to connect with two adjacent comb rows, and in some species runs along the comb rows. This forms a mechanical system for transmitting the beat rhythm from the combs to the balancers, via water disturbances created by the cilia. [45]

Lobates Edit

The Lobata has a pair of lobes, which are muscular, cuplike extensions of the body that project beyond the mouth. Their inconspicuous tentacles originate from the corners of the mouth, running in convoluted grooves and spreading out over the inner surface of the lobes (rather than trailing far behind, as in the Cydippida). Between the lobes on either side of the mouth, many species of lobates have four auricles, gelatinous projections edged with cilia that produce water currents that help direct microscopic prey toward the mouth. This combination of structures enables lobates to feed continuously on suspended planktonic prey. [20]

Lobates have eight comb-rows, originating at the aboral pole and usually not extending beyond the body to the lobes in species with (four) auricles, the cilia edging the auricles are extensions of cilia in four of the comb rows. Most lobates are quite passive when moving through the water, using the cilia on their comb rows for propulsion, [20] although Leucothea has long and active auricles whose movements also contribute to propulsion. Members of the lobate genera Bathocyroe and Ocyropsis can escape from danger by clapping their lobes, so that the jet of expelled water drives them back very quickly. [46] Unlike cydippids, the movements of lobates' combs are coordinated by nerves rather than by water disturbances created by the cilia, yet combs on the same row beat in the same Mexican wave style as the mechanically coordinated comb rows of cydippids and beroids. [45] This may have enabled lobates to grow larger than cydippids and to have less egg-like shapes. [44]

An unusual species first described in 2000, Lobatolampea tetragona, has been classified as a lobate, although the lobes are "primitive" and the body is medusa-like when floating and disk-like when resting on the sea-bed. [28]

Beroids Edit

The Beroida, also known as Nuda, have no feeding appendages, but their large pharynx, just inside the large mouth and filling most of the saclike body, bears "macrocilia" at the oral end. These fused bundles of several thousand large cilia are able to "bite" off pieces of prey that are too large to swallow whole – almost always other ctenophores. [47] In front of the field of macrocilia, on the mouth "lips" in some species of Beroe, is a pair of narrow strips of adhesive epithelial cells on the stomach wall that "zip" the mouth shut when the animal is not feeding, by forming intercellular connections with the opposite adhesive strip. This tight closure streamlines the front of the animal when it is pursuing prey. [48]

Other body forms Edit

The Ganeshida has a pair of small oral lobes and a pair of tentacles. The body is circular rather than oval in cross-section, and the pharynx extends over the inner surfaces of the lobes. [20]

The Thalassocalycida, only discovered in 1978 and known from only one species, [49] are medusa-like, with bodies that are shortened in the oral-aboral direction, and short comb-rows on the surface furthest from the mouth, originating from near the aboral pole. They capture prey by movements of the bell and possibly by using two short tentacles. [20]

The Cestida ("belt animals") are ribbon-shaped planktonic animals, with the mouth and aboral organ aligned in the middle of opposite edges of the ribbon. There is a pair of comb-rows along each aboral edge, and tentilla emerging from a groove all along the oral edge, which stream back across most of the wing-like body surface. Cestids can swim by undulating their bodies as well as by the beating of their comb-rows. There are two known species, with worldwide distribution in warm, and warm-temperate waters: Cestum veneris ("Venus' girdle") is among the largest ctenophores – up to 1.5 meters (4.9 ft) long, and can undulate slowly or quite rapidly. Velamen parallelum, which is typically less than 20 centimeters (0.66 ft) long, can move much faster in what has been described as a "darting motion". [20] [50]

Most Platyctenida have oval bodies that are flattened in the oral-aboral direction, with a pair of tentilla-bearing tentacles on the aboral surface. They cling to and creep on surfaces by everting the pharynx and using it as a muscular "foot". All but one of the known platyctenid species lack comb-rows. [20] Platyctenids are usually cryptically colored, live on rocks, algae, or the body surfaces of other invertebrates, and are often revealed by their long tentacles with many side branches, seen streaming off the back of the ctenophore into the current.

Reproduction and development Edit

Adults of most species can regenerate tissues that are damaged or removed, [51] although only platyctenids reproduce by cloning, splitting off from the edges of their flat bodies fragments that develop into new individuals. [20]

The last common ancestor (LCA) of the ctenophores was hermaphroditic. [52] Some are simultaneous hermaphrodites, which can produce both eggs and sperm at the same time, while others are sequential hermaphrodites, in which the eggs and sperm mature at different times. At least three species are known to have evolved separate sexes (dioecy) Ocyropsis crystallina and Ocyropsis maculata in the genus Ocyropsis and Bathocyroe fosteri in the genus Bathocyroe. [53] The gonads are located in the parts of the internal canal network under the comb rows, and eggs and sperm are released via pores in the epidermis. Fertilization is generally external, but platyctenids use internal fertilization and keep the eggs in brood chambers until they hatch. Self-fertilization has occasionally been seen in species of the genus Mnemiopsis, [20] and it is thought that most of the hermaphroditic species are self-fertile. [17]

Development of the fertilized eggs is direct there is no distinctive larval form. Juveniles of all groups are generally planktonic, and most species resemble miniature adult cydippids, gradually developing their adult body forms as they grow. In the genus Beroe, however, the juveniles have large mouths and, like the adults, lack both tentacles and tentacle sheaths. In some groups, such as the flat, bottom-dwelling platyctenids, the juveniles behave more like true larvae. They live among the plankton and thus occupy a different ecological niche from their parents, only attaining the adult form by a more radical metamorphosis [20] after dropping to the sea-floor. [17]

At least in some species, juvenile ctenophores appear capable of producing small quantities of eggs and sperm while they are well below adult size, and adults produce eggs and sperm for as long as they have sufficient food. If they run short of food, they first stop producing eggs and sperm, and then shrink in size. When the food supply improves, they grow back to normal size and then resume reproduction. These features make ctenophores capable of increasing their populations very quickly. [17] Members of the Lobata and Cydippida also have a reproduction form called dissogeny two sexually mature stages, first as larva and later as juveniles and adults. During their time as larva they are capable of releasing gametes periodically. After their reproductive larval period is over they will not produce more gametes again until after metamorphosis. A population of Mertensia ovum in the central Baltic Sea have become paedogenetic, and consist solely of sexually mature larvae less than 1.6 mm. [54] [55]

Colors and bioluminescence Edit

Most ctenophores that live near the surface are mostly colorless and almost transparent. However some deeper-living species are strongly pigmented, for example the species known as "Tortugas red" [56] (see illustration here), which has not yet been formally described. [17] Platyctenids generally live attached to other sea-bottom organisms, and often have similar colors to these host organisms. [17] The gut of the deep-sea genus Bathocyroe is red, which hides the bioluminescence of copepods it has swallowed. [46]

The comb rows of most planktonic ctenophores produce a rainbow effect, which is not caused by bioluminescence but by the scattering of light as the combs move. [17] [57] Most species are also bioluminescent, but the light is usually blue or green and can only be seen in darkness. [17] However some significant groups, including all known platyctenids and the cydippid genus Pleurobrachia, are incapable of bioluminescence. [58]

When some species, including Bathyctena chuni, Euplokamis stationis and Eurhamphaea vexilligera, are disturbed, they produce secretions (ink) that luminesce at much the same wavelengths as their bodies. Juveniles will luminesce more brightly in relation to their body size than adults, whose luminescence is diffused over their bodies. Detailed statistical investigation has not suggested the function of ctenophores' bioluminescence nor produced any correlation between its exact color and any aspect of the animals' environments, such as depth or whether they live in coastal or mid-ocean waters. [59]

In ctenophores, bioluminescence is caused by the activation of calcium-activated proteins named photoproteins in cells called photocytes, which are often confined to the meridional canals that underlie the eight comb rows. In the genome of Mnemiopsis leidyi ten genes encode photoproteins. These genes are co-expressed with opsin genes in the developing photocytes of Mnemiopsis leidyi, raising the possibility that light production and light detection may be working together in these animals. [60]

Distribution Edit

Ctenophores are found in most marine environments: from polar waters to the tropics near coasts and in mid-ocean from the surface waters to the ocean depths. [17] The best-understood are the genera Pleurobrachia, Beroe and Mnemiopsis, as these planktonic coastal forms are among the most likely to be collected near shore. [29] [46] No ctenophores have been found in fresh water.

In 2013, the marine ctenophore Mnemiopsis leidyi was recorded in a lake in Egypt, accidentally introduced by the transport of fish (mullet) fry this was the first record from a true lake, though other species are found in the brackish water of coastal lagoons and estuaries. [61]

Ctenophores may be abundant during the summer months in some coastal locations, but in other places, they are uncommon and difficult to find.

In bays where they occur in very high numbers, predation by ctenophores may control the populations of small zooplanktonic organisms such as copepods, which might otherwise wipe out the phytoplankton (planktonic plants), which are a vital part of marine food chains.

Prey and predators Edit

Almost all ctenophores are predators – there are no vegetarians and only one genus that is partly parasitic. [46] If food is plentiful, they can eat 10 times their own weight per day. [62] While Beroe preys mainly on other ctenophores, other surface-water species prey on zooplankton (planktonic animals) ranging in size from the microscopic, including mollusc and fish larvae, to small adult crustaceans such as copepods, amphipods, and even krill. Members of the genus Haeckelia prey on jellyfish and incorporate their prey's nematocysts (stinging cells) into their own tentacles instead of colloblasts. [17] Ctenophores have been compared to spiders in their wide range of techniques for capturing prey – some hang motionless in the water using their tentacles as "webs", some are ambush predators like Salticid jumping spiders, and some dangle a sticky droplet at the end of a fine thread, as bolas spiders do. This variety explains the wide range of body forms in a phylum with rather few species. [46] The two-tentacled "cydippid" Lampea feeds exclusively on salps, close relatives of sea-squirts that form large chain-like floating colonies, and juveniles of Lampea attach themselves like parasites to salps that are too large for them to swallow. [46] Members of the cydippid genus Pleurobrachia and the lobate Bolinopsis often reach high population densities at the same place and time because they specialize in different types of prey: Pleurobrachia's long tentacles mainly capture relatively strong swimmers such as adult copepods, while Bolinopsis generally feeds on smaller, weaker swimmers such as rotifers and mollusc and crustacean larvae. [63]

Ctenophores used to be regarded as "dead ends" in marine food chains because it was thought their low ratio of organic matter to salt and water made them a poor diet for other animals. It is also often difficult to identify the remains of ctenophores in the guts of possible predators, although the combs sometimes remain intact long enough to provide a clue. Detailed investigation of chum salmon, Oncorhynchus keta, showed that these fish digest ctenophores 20 times as fast as an equal weight of shrimps, and that ctenophores can provide a good diet if there are enough of them around. Beroids prey mainly on other ctenophores. Some jellyfish and turtles eat large quantities of ctenophores, and jellyfish may temporarily wipe out ctenophore populations. Since ctenophores and jellyfish often have large seasonal variations in population, most fish that prey on them are generalists and may have a greater effect on populations than the specialist jelly-eaters. This is underlined by an observation of herbivorous fishes deliberately feeding on gelatinous zooplankton during blooms in the Red Sea. [64] The larvae of some sea anemones are parasites on ctenophores, as are the larvae of some flatworms that parasitize fish when they reach adulthood. [65]

Ecological impacts Edit

Most species are hermaphrodites, and juveniles of at least some species are capable of reproduction before reaching the adult size and shape. This combination of hermaphroditism and early reproduction enables small populations to grow at an explosive rate.

Ctenophores may balance marine ecosystems by preventing an over-abundance of copepods from eating all the phytoplankton (planktonic plants), [66] which are the dominant marine producers of organic matter from non-organic ingredients. [67]

On the other hand, in the late 1980s the Western Atlantic ctenophore Mnemiopsis leidyi was accidentally introduced into the Black Sea and Sea of Azov via the ballast tanks of ships, and has been blamed for causing sharp drops in fish catches by eating both fish larvae and small crustaceans that would otherwise feed the adult fish. [66] Mnemiopsis is well equipped to invade new territories (although this was not predicted until after it so successfully colonized the Black Sea), as it can breed very rapidly and tolerate a wide range of water temperatures and salinities. [68] The impact was increased by chronic overfishing, and by eutrophication that gave the entire ecosystem a short-term boost, causing the Mnemiopsis population to increase even faster than normal [69] – and above all by the absence of efficient predators on these introduced ctenophores. [68] Mnemiopsis populations in those areas were eventually brought under control by the accidental introduction of the Mnemiopsis-eating North American ctenophore Beroe ovata, [70] and by a cooling of the local climate from 1991 to 1993, [69] which significantly slowed the animal's metabolism. [68] However the abundance of plankton in the area seems unlikely to be restored to pre-Mnemiopsis levels. [71]

In the late 1990s Mnemiopsis appeared in the Caspian Sea. Beroe ovata arrived shortly after, and is expected to reduce but not eliminate the impact of Mnemiopsis there. Mnemiopsis also reached the eastern Mediterranean in the late 1990s and now appears to be thriving in the North Sea and Baltic Sea. [17]

The number of known living ctenophore species is uncertain since many of those named and formally described have turned out to be identical to species known under other scientific names. Claudia Mills estimates that there about 100 to 150 valid species that are not duplicates, and that at least another 25, mostly deep-sea forms, have been recognized as distinct but not yet analyzed in enough detail to support a formal description and naming. [56]

Early classification Edit

Early writers combined ctenophores with cnidarians into a single phylum called Coelenterata on account of morphological similarities between the two groups. Like cnidarians, the bodies of ctenophores consist of a mass of jelly, with one layer of cells on the outside and another lining the internal cavity. In ctenophores, however, these layers are two cells deep, while those in cnidarians are only a single cell deep. Ctenophores also resemble cnidarians in relying on water flow through the body cavity for both digestion and respiration, as well as in having a decentralized nerve net rather than a brain. Genomic studies have suggested that the neurons of Ctenophora, which differ in many ways from other animal neurons, evolved independently from those of the other animals, [72] and increasing awareness of the differences between the comb jellies and the other coelentarata has persuaded more recent authors to classify the two as separate phyla. The position of the ctenophores in the evolutionary family tree of animals has long been debated, and the majority view at present, based on molecular phylogenetics, is that cnidarians and bilaterians are more closely related to each other than either is to ctenophores.

Modern taxonomy Edit

The traditional classification divides ctenophores into two classes, those with tentacles (Tentaculata) and those without (Nuda). The Nuda contains only one order (Beroida) and family (Beroidae), and two genera, Beroe (several species) and Neis (one species). [56]

The Tentaculata are divided into the following eight orders: [56]

    , egg-shaped animals with long tentacles [20] , with paired thick lobes [20] , flattened animals that live on or near the sea-bed most lack combs as adults, and use their pharynges as suckers to attach themselves to surfaces [20] , with a pair of small lobes round the mouth, but an extended pharynx like that of platyctenids [20] , with short tentacles and a jellyfish-like "umbrella" [20] , ribbon-shaped and the largest ctenophores [20]

Despite their fragile, gelatinous bodies, fossils thought to represent ctenophores – apparently with no tentacles but many more comb-rows than modern forms – have been found in Lagerstätten as far back as the early Cambrian, about 515 million years ago . Nevertheless, a recent molecular phylogenetics analysis concludes that the common ancestor originated approximately 350 million years ago ± 88 million years ago, conflicting with previous estimates which suggests it occurred 66 million years ago after the Cretaceous–Paleogene extinction event. [73]

Fossil record Edit

Because of their soft, gelatinous bodies, ctenophores are extremely rare as fossils, and fossils that have been interpreted as ctenophores have been found only in lagerstätten, places where the environment was exceptionally suited to the preservation of soft tissue. Until the mid-1990s only two specimens good enough for analysis were known, both members of the crown group, from the early Devonian (Emsian) period. Three additional putative species were then found in the Burgess Shale and other Canadian rocks of similar age, about 505 million years ago in the mid-Cambrian period. All three lacked tentacles but had between 24 and 80 comb rows, far more than the 8 typical of living species. They also appear to have had internal organ-like structures unlike anything found in living ctenophores. One of the fossil species first reported in 1996 had a large mouth, apparently surrounded by a folded edge that may have been muscular. [4] Evidence from China a year later suggests that such ctenophores were widespread in the Cambrian, but perhaps very different from modern species – for example one fossil's comb-rows were mounted on prominent vanes. [74]

The Ediacaran Eoandromeda could putatively represent a comb jelly. [2] It has eightfold symmetry, with eight spiral arms resembling the comblike rows of a Ctenophore. If it is indeed a Ctenophore, it places the group close to the origin of the Bilateria. [75] The early Cambrian sessile frond-like fossil Stromatoveris, from China's Chengjiang lagerstätte and dated to about 515 million years ago , is very similar to Vendobionta of the preceding Ediacaran period. De-Gan Shu, Simon Conway Morris et al. found on its branches what they considered rows of cilia, used for filter feeding. They suggested that Stromatoveris was an evolutionary "aunt" of ctenophores, and that ctenophores originated from sessile animals whose descendants became swimmers and changed the cilia from a feeding mechanism to a propulsion system. [76] Other fossils that could support the idea of ctenophores having evolved from sessile forms are Dinomischus and Daihua sanqiong, which also lived on the seafloor, had organic skeletons and cilia-covered tentacles surrounding their mouth, although not all yet agree that these were actually comb jellies. [77]

520 million years old Cambrian fossils also from Chengjiang in China show a now wholly extinct class of ctenophore, named "Scleroctenophora", that had a complex internal skeleton with long spines. [78] The skeleton also supported eight soft-bodied flaps, which could have been used for swimming and possibly feeding. One form, Thaumactena, had a streamlined body resembling that of arrow worms and could have been an agile swimmer. [5]

Relationship to other animal groups Edit

The phylogenetic relationship of ctenophores to the rest of Metazoa is very important to our understanding of the early evolution of animals and the origin of multicellularity. It has been the focus of debate for many years. Ctenophores have been purported to be the sister lineage to the Bilateria, [79] [80] sister to the Cnidaria, [81] [82] [83] [84] sister to Cnidaria, Placozoa, and Bilateria, [85] [86] [87] and sister to all other animals. [9] [88]

Walter Garstang in his book Larval Forms and Other Zoological Verses (Mülleria and the Ctenophore) even expressed a theory that ctenophores were descended from a neotenic Mülleria larva of a polyclad.

A series of studies that looked at the presence and absence of members of gene families and signalling pathways (e.g., homeoboxes, nuclear receptors, the Wnt signaling pathway, and sodium channels) showed evidence congruent with the latter two scenarios, that ctenophores are either sister to Cnidaria, Placozoa, and Bilateria or sister to all other animal phyla. [89] [90] [91] [92] Several more recent studies comparing complete sequenced genomes of ctenophores with other sequenced animal genomes have also supported ctenophores as the sister lineage to all other animals. [93] [26] [94] [95] This position would suggest that neural and muscle cell types either were lost in major animal lineages (e.g., Porifera and Placozoa) or evolved independently in the ctenophore lineage. [93]

Other researchers have argued that the placement of Ctenophora as sister to all other animals is a statistical anomaly caused by the high rate of evolution in ctenophore genomes, and that Porifera (sponges) is the earliest-diverging animal taxon instead. [87] [96] [97] [98] [99] As such, the Ctenophora appear to be a basal diploblast clade. In agreement with the latter point, the analysis of a very large sequence alignment at the metazoan taxonomic scale (1,719 proteins totalizing ca. 400,000 amino acid positions) showed that ctenophores emerge as the second-earliest branching animal lineage, and sponges are sister-group to all other multicellular animals. [8] Also, research on mucin genes, which allow an animal to produce mucus, shows that sponges have never had them while all other animals, including comb jellies, appear to share genes with a common origin. [100]

Yet another study strongly rejects the hypothesis that sponges are the sister group to all other extant animals and establishes the placement of Ctenophora as the sister group to all other animals, and disagreement with the last-mentioned paper is explained by methodological problems in analyses in that work. [101] Neither ctenophores or sponges possess HIF pathways, [102] and are the only known animal phyla that lack any true hox genes. [26]

Relationships within Ctenophora Edit

Since all modern ctenophores except the beroids have cydippid-like larvae, it has widely been assumed that their last common ancestor also resembled cydippids, having an egg-shaped body and a pair of retractable tentacles. Richard Harbison's purely morphological analysis in 1985 concluded that the cydippids are not monophyletic, in other words do not contain all and only the descendants of a single common ancestor that was itself a cydippid. Instead he found that various cydippid families were more similar to members of other ctenophore orders than to other cydippids. He also suggested that the last common ancestor of modern ctenophores was either cydippid-like or beroid-like. [104] A molecular phylogeny analysis in 2001, using 26 species, including 4 recently discovered ones, confirmed that the cydippids are not monophyletic and concluded that the last common ancestor of modern ctenophores was cydippid-like. It also found that the genetic differences between these species were very small – so small that the relationships between the Lobata, Cestida and Thalassocalycida remained uncertain. This suggests that the last common ancestor of modern ctenophores was relatively recent, and perhaps survived the Cretaceous–Paleogene extinction event 65.5 million years ago while other lineages perished. When the analysis was broadened to include representatives of other phyla, it concluded that cnidarians are probably more closely related to bilaterians than either group is to ctenophores but that this diagnosis is uncertain. [103] A clade including Mertensia, Charistephane and Euplokamis may be the sister lineage to all other ctenophores. [105] [101]

Divergence times estimated from molecular data indicated approximately how many million years ago (Mya) the major clades diversified: 350 Mya for Cydippida relative to other Ctenophora, and 260 Mya for Platyctenida relative to Beroida and Lobata. [101]