CNIDARIANS: CHARACTERISTICS, FEEDING, NERVOUS SYSTEM - BIOLOGY - 2025

The cnidarians (Cnidaria) are a phylum of exclusively aquatic organisms. They have characteristic cells called cnids or cnidocytes that give rise to the name of the phyllum.

Currently about 11,000 species are known, among which are some very common such as corals, jellyfish, anemones and gorgonians. Many species form colonies made up of numerous organisms.

Group of Cnidarians. Top left Fire Coral (Millepora sp.), Top right Pulsating soft coral (Xenia sp.) And bottom Giant Caribbean Anemone (Condylactis gigantea). Taken and edited from flickr.com/photos/jveracaripe

Most of the species are marine, but a few have managed to colonize freshwater environments. Some species are benthic and sessile or restricted in movement, others are planktonic. Its size can vary from microscopic to more than 20 meters if the tentacles are included.

characteristics

Cnidarians are diblastic radiated organisms, that is, they develop from two embryonic leaves, ecto and endoderm. Between the ecto and the endoderm these organisms present an acellular mesoglea, or, in some cases, a cellular mesenchyme.

Their level of organization is tissue, they do not present organ systems. They have adhesive or stinging cells called cnids or cnidocytes. The symmetry is basically radial although in some groups it is modified to biradial, tetraradial or of some other type.

The digestive system is a sac-shaped cavity, the gastrovascular or coelenteron cavity, with a single entrance orifice for food and exit for undigested material.

They have tentacles that are normally found in multiples of six or eight. They do not present cephalization. There are two body patterns, polyp and jellyfish.

The polyp is sessile, has a cylindrical shape, with the mouth and tentacles directed upwards. The jellyfish is mobile, bell- or umbrella-shaped, with the mouth and tentacles directed downward.

Many species of cnidarians produce colonies of individual organisms composed of zooids of the jellyfish, polyp, or both. In some species there is an alternation of generation between the polyp phase, which reproduces asexually, and the jellyfish, which reproduces sexually. In other species, only the polyp phase or the jellyfish phase occurs.

Taxonomy

Cnidarians were traditionally located, along with ctenophores, in the phyllum Coelenterata. However, it is now widely accepted that the kinship between these two groups is only apparent. The most recent classification of cnidarians divides them into seven classes:

Anthozoa

Cnidarians known as anemones, corals, and sea feathers. They are only polyps. They can be lonely or colonial. The polyp can present asexual or sexual reproduction, causing new polyps.

Gametes are formed from cells of the gastrodermis. They are exclusively marine. The tentacles are present in multiples of six, or they can be eight.

The gastrovascular cavity is completely divided by septa that originate from the gastrodermis and mesoglea.

Cubozoa

Known as box jellyfish and sea wasps. They only present the jellyfish phase. They have a cubic shape. They present tetrameric division, the centeron is divided into four bags.

The edge is not scalloped and the sub-scallop margin is folded inward to form a veil-like structure, called the velario.

Its sting is very toxic, and can be lethal to humans. Until recently, they were considered an order within the class of Scyphozoans.

Hydrozoa

Known as hydroids or hydromedusas. In most species there is an alternation of generation with an asexual polyp phase with a sexual jellyfish phase. Generally, the polyp phase usually forms colonies of polymorphic individuals.

Jellyfish are veiled and lack clothing and cnidocytes in the gastrovascular cavity. Gonads are always ectodermal in origin. The gastrovascular cavity is not divided by septa.

Scyphozoa

Cnidarians in which the jellyfish phase predominates. They have a small and inconspicuous polyp, but long-lived. The jellyfish does not have a veil, but has clothing and cnidocytes in the gastrovascular cavity.

The gonads are endodermal. The gastrovascular cavity presents an incomplete division, formed by 4 incomplete partitions or septa in interradial position that separate 4 gastric bags.

Myxozoa

Cnidarians of very small size and with a simplified genome. They are a class of microscopic organisms, previously classified as a phylum within the kingdom of the Protists.

Intracellular parasites of almost all animal phyla. Parasitosis occurs with spores with a polar capsule and filaments that anchor the spore to the host.

Recent studies suggested they were related to fungi. However, in 2015 a study revealed that myxozoans are actually cnidarians.

Polypodiozoa

It is a monospecific class of cnidarians parasitic on sturgeons. Molecular studies indicate that they may be related to myxozoans. However, their phylogenetic relationships are not entirely clear, so their classification is temporary.

Staurozoa

Considered until recently as an order (Stauromedusae) within the Scyphozoa. They are small and sessile organisms. They develop directly from a benthic planula larva.

The aboral surface is prolonged in a peduncle with an adhesive disc thanks to which they are fixed to the substrate. They inhabit shallow marine waters at high latitudes.

Nervous system

Cnidarians have a diffuse nervous system, they lack a central nervous system. However, they have integrating areas of neural tissue that could be considered a form of centralization. Neurons are naked and most of them are apolar.

A few neurons are bipolar. In the latter the nerve impulse is transmitted unidirectionally. Neurons are interconnected forming a kind of network called the nerve plexus.

Normally cnidarians have two nerve plexuses, one subepidermal and the other subgastrdermal. The latter may be absent. In bipolar neurons the transmission of the nerve impulse is faster.

In some cases, a nerve plexus of bipolar neurons and another of apolar neurons may be present. Rapid responses are under the control, in these cases, of the bipolar plexus. The slower responses correspond to responses from the apolar plexus.

Sensory structures

Cnidarians lack true sense organs. Polyps lack specialized photoreceptor cells. The sensitivity to light of these organisms is believed to be related to neurons concentrated in the most translucent areas of the body.

Polyps also possess sensory sensory processes that arise from sensory cells. These processes have a mechanoreceptor function.

The Cubozoa and Scyphozoa jellyfish have sensory centers called ropalias. These centers have a pair of chemoreceptor pits, a statocyst, a concentration of epidermal neurons, and eventually an ocellus.

The ropalias are located on the edge of the bell, between a pair of bell lobes (umbrelares). Hydrozoa jellyfish may have statocysts at the umbilicus, undifferentiated sensory cells, and probably chemoreceptors.

Cubozoan jellyfish are the only cnidarians with true eyes that possess retinas, corneas, and lenses.

Location of a ropalio in the jellyfish Tripedalia cystophora. Taken and edited from

Feeding

Most cnidarians are carnivores. To capture their prey, they generally use their tentacles, aided by poisonous cnidocytes called nematocysts.

Medudas

Most jellyfish can also use their oral arms to capture prey. When they use both structures, generally the tentacles are used to paralyze the prey, and the oral arms to direct them to the mouth. Oral arms, however, can also be used to capture food.

Anemones

Anemones of the order Corallimorpharia directly capture their prey using the oral disc as a fishing net to catch small fish and crustaceans.

Coral polyps

Coral polyps secrete strands of mucus that are suspended over the colony. These filaments serve to capture food particles suspended in the water column.

The particles are transported to the mouth by ciliary movements. The food captured in this way is used as a complement to the larger prey captured by the tentacles.

In some species, however, the tentacles are very small and in these cases, the organisms are clearly suspensivores, feeding only on the prey captured by the mucus trap.

Corals

Corals, additionally, take advantage of nutrients produced by zooxanthellae, endosymbiont algae with which they are associated. Some species feed exclusively on these foods and by absorption of nutrient dissolved in the water column.

Polypodiozoa and Myxozoa

Representatives of the Polypodiozoa family are parasites, mainly of sturgeons. The Myxozoa, for their part, are parasites of organisms of almost any phylum of the animal kingdom, and even of some protists.

Digestion

Digestion, in cnidarians, is both intracellular and extracellular. The captured food is directed entirely to the mouth. It then passes into the digestive cavity where the glandular cells of the gastrodermis release enzymes.

The enzymes carry out the extracellular digestion of food in a few hours. Species that possess nematocysts can also inject digestive enzymes into their prey during capture.

The digested material circulates through the digestive cavity for the cells of the gastrodermis to absorb the nutrients. Once the nutrients are absorbed, digestion continues, but in this case, intracellularly.

Intracellular digestion takes place in vacuoles. The undigested remains of food are expelled through the mouth.

Excretion

Cnidarians lack an excretory system, the elimination of nitrogenous waste occurs through the external or internal walls of the body.

Nitrogen is removed in the form of ammonia. The species that have managed to colonize freshwater environments are hyperosmotic with respect to the environment.

Due to this, water tends to penetrate into organisms by simple diffusion. Osmotic control in these species is through the periodic removal of fluid from the gastrovascular cavity.

Reproduction

Cnidarians can reproduce sexually or asexually by different mechanisms. In some groups there is an alternation of generations between a polyp phase of asexual reproduction and a medusa phase of sexual reproduction.

-Hydrozoa

Asexual reproduction

Asexual reproduction in hydrozoans can occur by various means. The most common mechanism of asexual reproduction is budding. The yolk is formed as an evagination from the mother's body.

Then, at the distal end of this extension, the mouth will be formed, which will communicate with the gastrovascular cavity shared with the mother. In the individual forms, the new polyp finishes growing forming tentacles and separates from the mother.

While in colonial forms it remains attached to its progenitor. Polyps can also produce jellyfish buds, or gonophores.

Chains of individuals called cormidia are produced in siphonophores, which can separate and form a new colony. Asexual division can also occur in hydrozoan jellyfish by budding or longitudinal fission.

Sexual reproduction

Hydrozoa sexual reproduction can occur in the polyp phase. In species with reduced or absent jellyfish phase, polyps develop structures called sporozacs.

Sporozacs produce sex gametes. However, it is common for the jellyfish phase, originating in the gonophores, to be in charge of sexual reproduction. In these, the gonads are temporary and are formed by migration of epidermal cells.

Male and female gametes can be released and fertilization occurs in between. In other cases, only the male gametes are released.

The female gametes are retained by the mother and fertilization occurs on or within the female jellyfish. The development of the zygote gives rise to a planula larva that will attach to the substrate and form a polyp.

Sexual and asexual reproduction in Hidrozoa. Taken and edited from

-Scyphozoa

Scyphozoan polyp, or scyphistoma, reproduces asexually by budding (producing new polyps) or by transverse fission. This last type of division is called strobilation and gives rise to young jellyfish called efiras.

Jellyfish reproduce sexually by gametes derived from the gastrodermis. Fertilization can be external or occur in the female's gastric bags. Planula larvae are also produced.

-Cubozoa

The reproduction process of box jellyfish is not well known. Polyps are only known in a few species. Each of these, apparently, transforms and gives rise to a single jellyfish. The box jellyfish reproduce sexually and in some a kind of copulation occurs.

-Anthozoa

Anthozoans only present the polyp phase, they are known as anemones. Asexual reproduction can occur by longitudinal fission, transverse fission, budding of the tentacles, and pedial laceration.

In the latter case, the discopedium spreads and the anemone detaches, leaving pieces of the discopedium that will form new organisms. Parthenogenetic reproduction has also been documented in some Anthozoa.

Sexual reproduction can be by external or internal fertilization. Gametes are formed from cells of the gastrodermis.

-Myxozoa

Very little is known about the reproduction mechanisms of the Myxozoa. It is not yet known if they have sexual reproduction. Asexual reproduction is by formation of uninuclear spores.

-Polypodiozoa

Polypodiozoa cnidarians can reproduce sexually, with both male and female specimens, as well as hermaphrodites. Gametes are of ectodermal origin. It can also reproduce asexually by fission.

-Staurozoa

Staurozoa are pedunculated jellyfish that can reproduce asexually by budding. The adult forms buds that break off to become non-fixed planules that will set and grow into adults. They can also reproduce sexually by external fertilization. They are dioecious.

References

1. RC Brusca, GJ Brusca (2003). Invertebrates. 2nd Edition. Sinauer Associates, Inc.
2. EV Raikova (1973). Life cycle and systematic position of Polypodium hydriforme Ussov (Coelenterata), a cnidarian parasite of the eggs of Acipenseridae. Publications of the Seto Marine Biological Laboratory.
3. Cnidaria. On Wikipedia. Recovered from en.wikipedia.org/wiki/Cnidaria#Classification
4. ES Chang, M. Neuhof, ND Rubinstein, A. Diamant, H. Philippe, D. Huchon, P. Cartwright (2015). Genomic insights into the evolutionary origin of Myxozoa within Cnidaria. PNAS.
5. AC Marques & AG Collins (2004). Cladistic analysis of Medusozoa and cnidarian evolution. Invertebrate Biology