12 julio, 2024

Coelenterates: what they are, characteristics, habitat, reproduction, examples

What are coelenterates?

The coelenterates (Coelenterata), also known as coelenterates or polyps, group a series of aquatic invertebrate animals, mostly marine. Depending on the classification system, they are considered a phylum or a superphylum.

Within the coelenterates are corals, hydras, jellyfish, anemones, sea pens and some endoparasitic species. Some live in fresh water, as is the case with Chlorohydrabut they are more common in marine environments.

One of the most outstanding characteristics of this group is the presence of stinging cells (nematoblasts), used in defense and in the capture of their prey.

In the case of Ctenophora, there are no stinging cells, but cells that secrete sticky substances to adhere and trap prey (coloblasts).

The groups that have nematoblasts can cause strong skin irritations to bathers in coastal areas. In some cases, such as the «Portuguese frigate» (Physalia physalis), the toxin can cause death.

In general, coelenterates are part of marine ecosystems. Particularly, the coral formations have a great ecological importance, because they contain a high diversity of species. In addition, they form very effective barriers that protect beaches and mangroves from waves.

Characteristics of coelenterates

They are multicellular organisms. Its basic structure is like that of a sack. They have an opening (mouth) that gives access to a single interior cavity (gastrovascular cavity or colonenterum). Said opening is called a stomodium and it fulfills the function of both the mouth and the anus.

– Around the mouth they present a series of 4 to 8 tentacles, which are used to catch and direct food. These are hollow and through their interior they open to the gastrovascular cavity.

– They are diploblastic organisms (the body wall is formed by two layers of cells separated from each other by connective tissue). The ectoderm or ectodermis is the outer layer and the endoderm or endodermis is the inner layer. Between them there is a non-cellular layer, the mesoglea.

– The muscular system is not made up of true muscle cells, but specialized epithelial cells.

– Coelenterates have specialized cells on the surface of the body, called nematoblasts or cnidoblasts. These cells have a capsule called a nematocyst.

– Inside the nematocyst there is a spirally wound filament and a very stinging liquid. The fluid is a toxin called hypnotoxin or actinocongestin.

– Next to the opening or operculum of the nematoblast there is a kind of excitable spinule called a cnidocilum. When something touches the cnidocil, the mechanism is activated and the filament shoots out quickly, digging into the skin of the prey or attacker. In this way, it inoculates the toxin that paralyzes the prey or drives away the aggressor.

– Some of these organisms, especially jellyfish, can cause serious damage to bathers in coastal areas. Its nematoblasts cause severe skin burns. Because of this they are called «aguamalas».

Classification of the coelenterates

The term Coelenterata or coelenterates is controversial. In a broad sense it includes more than 10,000 species.

In classical terms, coelenterates include cnidarians, ctenophores, and placozoans. However, some molecular evidence indicates that this would be a paraphyletic group, since it leaves out animals with bilateral symmetry.

On the other hand, some researchers have provided evidence showing Coelenterata as a monophyletic group (all its elements derive from a single ancestor).

According to these different visions, the group of coelenterates can be treated as a superphylum or a phylum.

Superphylum Coelenterata

Coelenterata would be a superphylum that includes the edges Cnidaria, Ctenophora, and Placozoa.

Cnidaria include anemones, sea pens, colony corals or polyps, jellyfish, and parasites of fish eggs (Myxozoa). They are characterized by presenting cnidocili.

Ctenophora have a structure called a ctenophore. Ctenophores are located on the tentacles and carry specialized cells called colloblasts. These cells secrete a sticky substance that retains the prey when it comes into contact with the tentacle.

The Placozoa are organisms with an extremely simple structure, almost reduced to a colony of cells, forming a flat sheet.

Coelenterata Edge

In other classifications, only groups within cnidarians are considered coelenterates. These make up the phylum Coelenterata, which is generally subdivided into four classes: Anthozoa, Hydrozoa, Schyphozoa, and Myxozoa.


They are found in aquatic ecosystems, mostly marine in tropical areas, although some inhabit freshwater.

They present benthic forms, that is, they inhabit the seabed, such as anemones and corals. And planktonic forms, those that float freely in the water column, as is the case with jellyfish-like forms.

There are pelagic ones (they live offshore, outside the continental shelf), like certain jellyfish, and there are demensal ones (they live in coastal waters), like corals and anemones.


They have alternation of generations. They have a phase of sexual reproduction and another asexual.


Asexual reproduction is by budding. Bumps form on the outer wall. Then cell differentiation occurs forming a mouth surrounded by tentacles. Finally, the bud breaks off and continues to grow until it forms an adult individual.


For sexual reproduction they produce sperm and eggs. On the external surface, transitory organs (gastrulae) are formed that act as testicles and ovaries respectively. In both cases they are protuberances inside which the gametes are formed.

Inside the testicle, the interstitial cells of the ectoderm are transformed into spermatozoa. The spermatozoa come out through the rupture of the wall.

In the ovary, an ectodermal interstitial cell develops into an amoeboid form. It incorporates the rest of the cells present and forms the egg.

The spermatozoa swim to reach the ovary, penetrate and fertilize the egg. The egg is then generated, which develops into an embryo inside a cyst. The cyst detaches and after an incubation period gives rise to a new individual.

In some cases they form a flat, ciliated larva that presents bilateral symmetry (planula larva). This larva swims to the bottom, where it attaches and forms polyps. This polyp in turn reproduces asexually, giving rise to jellyfish that perform sexual reproduction.

adult forms


The polyps are columnar and seated on a base, and can appear solitary (hydra, anemones or actinia) or form colonies (corals and sea feathers).

Polyps have an exoskeleton and an endoskeleton made of calcium carbonate. The mesoglea, or middle layer of the body, is condensed into a more rigid, leathery structure.


Jellyfish are cupuliform, with discal or bulbous shapes. In these, the mesoglea is distended by a gelatin with 99% water.

In some species they alternate the form of a polyp with the form of a medusa. In others only polyps form.

Colonies: coral reefs

Polyps that are arranged in the colony are individually called zooids. The colony is formed by the close anatomical relationship between one zooid and another.

In some cases, all zooids are the same and have the same function, such as red or white coral. In other cases, the zooids are different and perform different functions, as occurs in hydrozoans.

When there is colonial polymorphism, several types of zooids appear: nutrition, reproductive and defenders. There are even floating zooids or pneumatophores in the group of siphonophores.

Colonies grow and expand, requiring specific environmental conditions for their development. Among these we have a water temperature of not less than 20 °C, high solar radiation, not cloudy waters, without excessive agitation.

Depending on the distribution of environmental factors, various types of formations are generated. We have coastal reefs, atolls or coral islands, and coral reefs (such as the great Australian barrier).


They are mainly carnivorous. They feed on small aquatic animals, such as crustaceans, worms, plankton and organic remains, which are dragged by currents and captured thanks to their tentacles.

food capture

They have a nervous system sensitized by simple organic chemicals that are disseminated in the aquatic environment. This allows them to move the prey to their mouths and thus swallow their food.

Some species, such as sea wasps (Chironex fleckeri), detect and move towards the prey.


The food, once swallowed, enters the gastrovascular cavity and is digested there. The waste is expelled through the same cavity through which it entered.

Digestion is both extracellular and intracellular. Extracellular breakdown of food occurs in the colonenterum, and food particles are distributed through the colonenterum to the body, where they undergo intracellular digestion.

Examples of coelenterate species

– Common jellyfish (Aurelia aurita)

– Arctic lion’s mane jellyfish or giant lion’s mane (Cyanea capillata)

– Cannonball Jellyfish (rhopilema esculentum)

– Sea tomato (equine actinia)

– Nomura or giant jellyfish (Nemopilea nomurai)

–Velella (Velella Velella)

– Sea fan (window gorgonian)

– Sea anemone (Entacmaea quadricolor)

– Staghorn coral or deer raven coral (Acropora cervicornis)

– Giant jellyfish (Stygiomedusa gigantea)


Quaglia A. The muscular system of coelenterates, Italian Journal of Zoology.
Shostak, S. Cnidaria (Coelenterates). Encyclopedia of Life Sciences.

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