Sea nettles
Sea nettles, Chrysaora quinquecirrha
Scientific classification
Kingdom: Animalia
Phylum: Cnidaria
Hatschek, 1888

Anthozoa - Corals and sea anemones
Cubozoa - Sea wasps or box jellyfish
Hydrozoa - Hydroids, hydra-like animals
Scyphozoa - Jellyfish

Cnidaria (pronounced with a silent c) is a phylum containing some 11,000 species of relatively simple invertebrate animals found exclusively in aquatic, mostly marine, environments. Cniderians include corals, sea anemones, jellyfish, sea pens, sea pansies, and sea wasps, and tiny freshwater hydra.

The name of the phylum comes from cnidocytes, which are specialized cells that carry stinging organelles. The names Coelenterata and Coelentera were formerly applied to the group, but as those names included the Ctenophores (comb jellies, phylum Ctenophora), they have been abandoned. Cnidarians are highly evident in the fossil records, having first appeared in the Precambrian era.

Cnidarians play key ecological roles as well as great benefit to human beings. Consider, for example, the corals. Coral reefs provide a habitat to a rich diversity of animals and even an estimated one million people live on coral islands built up from the skeletal remains of corals. The bright colors of the "flower animals" (corals and sea anemones) provide joy to people observing them in aquariums or through scuba diving, snorkeling, or glass-bottom boats; people can also better comprehend the harmony in nature through observing the mutually beneficial symbiotic relationship between algae and corals. Nonetheless, human activities, such as sewage discharge and destructive aspects of tourism are also endangering many coral reefs.


Classes of Cnidaria

There are four main classes of Cnidaria:

Theoretically, members of Cnidaria have life cycles that alternate between asexual polyps (the body as a vase shaped form), and sexual, free-swimming forms called medusae (singular medusa; the body in a bell-shaped form). The Anthozoa live only as polyps, while Scyphozoa live most of their life cycle as medusa. The Hydrozoa live as polyps, medusae, and species that alternate between the two (Towle 1989). Invertebrates belonging to the class Cubozoa are named for their cube-shaped medusae, which form the dominant part of their life cycle.

Traditionally the hydrozoans were considered to be the most primitive, but evidence now suggests the anthozoans were actually the earliest to diverge. The non-anthozoan classes may be grouped into the subphylum Medusozoa.

The 49th plate from Ernst Haeckel's Kunstformen der Natur, 1904, showing various sea anemones classified as Actiniae

Class Cubozoa encompasses about 20 species, among them are the species Chironex fleckerii and Chiropsalmus quadrigatus, known as sea wasps, which possess a highly potent toxin.

Class Scyphozoa contains about 200 species, commonly known as jellyfish. The name Scyphozoa means "cup animals," reflective of the dominant medusa form (Towle 1989).

Class Anthozoa includes over 6,000 species, including sea anemones and corals such as Scleractinia (stony star corals). Anthozoa means "flower animals," which is descriptive of this class of invertebrates. The medusa stage is not known among this class.

Class Hydrozoa contains about 3,700 mostly marine species, and is a broad spectrum stretching from the tropical fire corals (Milleporidae) to the hydroids (Sertularia), some of which appear in the North Sea. The genus Hydra is made up of freshwater species, and range in size from 1 to 4 cm (Towle 1989). Hydras live independently, but most hydrozoans live in colonies (Towle 1989). Hydrozoa generally display alternation of generations between medusa and polyp forms, although hydras exhibit only the polyp form.

Among the hydrozoa, the order of Siphonophora, which includes the Portuguese Man o' War, deserves special mention. These hydrozoans form colonies that show varying degrees of specialization, so that in extreme cases individuals function essentially as organs of the whole.

A small group of microscopic parasites, the Myxozoa, have been considered to be extremely reduced cnidarians. These attach themselves to their hosts by polar filaments similar to the stinging threads of cnidocysts. Their exact placement within the phylum is uncertain, however, and new studies suggest they may have developed from some other group of animals. Usually they are placed in their own phylum.

Body and behavior

The basic body shape of a cnidarian consists of a sac with a gastrovascular cavity, with a single opening that functions as both mouth and anus. It has radial symmetry, meaning that whichever way it is cut along its central axis (that is, by any plane that passes through its longitudinal axis), the resulting halves would always be mirror images of each other. (Organisms with bilateral symmetry, such as humans, can only be divided into similar halves by one specific plane passing through the longitudinal axis.)

Portuguese Man o' War (Physalia physalis)

The movement of a cnidarian is coordinated by a decentralized nerve net and simple receptors. Several free-swimming Cubozoa and Scyphozoa possess rhopalia, complex sensory structures that can include image-forming eyes with lenses and retinas (Waggoner and Collins 2000), and a gravity-sensing statolith comparable in function to the otolith of the vertebrate inner ear. Tentacles surrounding the mouth contain nematocysts, specialized stinging cells, which they use to catch prey and defend themselves from predators. The ability to sting is what gives cnidarians their name.

The cnidarian is a eumetazoan, possessing true tissues and organs. It is composed of two layers of tissue, known as the ectoderm and endoderm (or gastroderm), with a gelatinous mesoglea in between them containing only scattered cells. Thus, the organisms were long considered to be diploblastic. However, the mesoglea may be homologous with the mesoderm in other animals. That is, new research indicates that cnidaria seem to possess a mesoderm in addition to the ecto- and endoderm, from which the musculature of the medusa develops, among others.

The ectoderm surrounds the cnidarian's "stomach" or "gastrovascular cavity," which is used both to ingest food and excrete waste. It also serves along with the mesogloea as a hydrostatic supporting skeleton. Firm skeletons are only found among polyps, which produce lime for that purpose.

The cnidarian does not possess a true circulatory system. Respiration takes place through diffusion of oxygen directly through their tissues without specialised organs such as tracheae, gills or lungs. The gastrovascular system plays a role in the digestion and dispersion of food and the removal of metabolic waste: it surrounds the gastrovacular cavity as well as its extensions in the tentacles of polyps. Thus, the gastrovacular system serves two separate functions, digestion and transport. Food particles are initially gathered by the feeding muscles of the gastroderm.

The jellyfish Chrysaora melanaster

Cnidarians take their name from a specialized cell, the cnidocyte (nettle cell). Cnidocytes contain cnidae, capsule-like structures capable of everting. The cnidae that sting are called nematocysts. The cnida or nematocyst is secreted by the Golgi apparatus of a cell and is technically not an organelle but "the most complex secretory product known" (Waggoner and Collins 2000). The nematocysts are the cnidarians' main form of offense or defense and function by a chemical or physical trigger that causes the specialized cell to eject a barbed and poisoned hook that can stick into, ensnare, or entangle prey or predators, killing or at least paralyzing its victim. Tentacles surrounding the mouth contain these specialized stinging cells.

Another important cell type is the interstitial cell, pluripotent cells that can transform into other cell types, such as spermatazoa, adenocytes, or nerve cells, though not into epithelial or feeding muscle cells; the latter two can only be produced by cells of the same type. These give many cnidaria an extraordinary capacity for regeneration. In particular, the genus hydra serves as a model for the research of pattern formation processes.

Theoretically, members of Cnidaria have life-cycles that alternate between asexual polyps and sexual, free-swimming medusae. Medusae have a hat or bell-shaped appearance and mostly swim passively with the current. Their tentacles hang freely below their bodies. However, they can actively swim by means of co-ordinated muscle contractions against the water contained in their gastrovacular cavity. Polyps, in contrast, are anchored to the substrate by their basal discs, although a few species can move in curious slow-motion somersaults. By nature, they display their tentacles upwards, away from the substrate. Polyps often live in large colonies.

Within the group Anthozoa, in which the medusal stage is virtually non-existent, the larva, once fusing with the substratum and developing into the polyp stage, grows benthic or sessile, meaning it no longer metamorphosizes into the medusal stage. Among the Scyphozoa and Cubozoa, in which the medusae are the dominant form in the life-cycle, polyps are in turn reduced or absent. Medusae are extremely varied and range in size from a few millimeters to over 30 metres (with tentacles). The Hydrozoa are intermediate, with significant medusoid and polyp forms.

Most cnidaria feed on prey that come into contact with their tentacles. These include the larger of the protists, various worms, crabs, other cnidaria, and even fish. Some groups such as coral live symbiotically with algae, mostly Dinoflagellata but sometimes Chlorophyta. By absorbing the carbon dioxide produced by the cnidarian, utilizing sunlight and releasing the oxygen via photosynthesis, the algae produce energy-rich carbohydrates that the cnidarian uses as its main source of food.


Development of a cnidarian

Asexual reproduction via budding is common among cnidaria, particularly among the Hydrozoa class. Asexual larvae bud laterally from the adult polyps, which develop into polyps themselves. The budding is often incomplete, so that colonies of genetically identical polyps physically connected with each other can form.

However, cnidaria can also reproduce sexually. A characteristic here is the alternation of generations, in which asexually reproducing generations alternate with generations that reproduce sexually, which is otherwise not as common among animals as among plants, fungi, and protists. This particular form of alternation of generations is known as metagenesis.

For this purpose, the adult polyp forms male or female medusae asexually. There are three principal asexual events:

  • Budding is particularly common among Hydrozoa.
  • Strobilation occurs when a medusa forms on the higher (oral) end of the polyp, and is common among Scyphozoa.
  • Finally, complete metamorphosis from polyp to medusa form can also occur.

These then develop to sexual maturity, at which point the male and female gametes are released, which each unite to form a zygote. These develop through cell division into a spherical structure, the blastula, from which the larva (or planula) forms. The larva is flagellate and swims until it encounters a firm substrate, on which it anchors itself and then passes through metamorphosis to the polyp stage.

This process varies significantly between the four classes of cnidaria. Among many Hydrozoa, the medusae remain on the polyps in a reduced form, known as gonophores. A few Hydrozoa, such as the hydra, have no medusa stage whatsoever; instead the polyp itself forms male or female gametes. Within group Anthozoa, the larva, once fusing with the substratum and growing benthic or sessile, no longer metamorphosises into the medusal stage.

Reef formation

Coral in shallow waters. Only here are the lighting conditions suitable for symbiotic algae to photosynthesize.

Cnidaria have great ecological significance through one of their subgroupings, the skeletal coral, which form coral reefs in shallow waters. The endosymbiotic algae are important to this reef formation, both for photosynthesis and for better calcium accumulation. The symbiosis appears, however, not to be entirely voluntary on the part of the algae, as they separate from their coral partners if they can obtain better nutrition elsewhere. This causes the coral to perish. This happens particularly where large amounts of nitrates are introduced, which can be exploited by the algae. The release of untreated sewage into the sea from, for example, newly built hotels and recreational facilities, is responsible for large-scale destruction of coral.

Due to the necessary lighting conditions, coral reefs only appear in warm, clear, shallow waters. Like other animals, such as tube worms and red and green algae, the coral polyps deposit lime (calcium carbonate) from their exoskeleton, which with time can pile up into rock masses. As soon as the lighting becomes insufficient—as is always the case below depths of 60 m—the coral dies off, and the next generation has already been established on their carcasses. In this way, coral reefs can grow upwards as the sea level slowly rises. They always reach up to immediately under the ocean surface.

Coral reefs are an ecosystem rich in species, which through the influence of the tide can have global consequences. They are inhabited by a variety of organisms, including sponges, various annelids, fish, algae, and various protists.

In previous geological eras, numerous rock formations have formed from the limestone deposited by corals, among others. In this way the rich deposits of the Eifel and Berg can be traced back over hundreds of millions of years to the old Devonian coral reefs. More recent are the islands of Bermuda and the Bahamas, as well as many Pacific archipelagos, which date from coral reefs.

Cnidaria as fossils

The phylum has existed for a long time, having been among the Vendian biota of the later Proterozoic period, about 550 million years ago, and cnidaria were among the first recognized animal fossils. Our understanding of fossil groups is varied; while those cnidaria that were formed of soft tissue only remain today in very exceptional cases, the fossil record of, for example, corals is very well known due to the lime remains they left behind.

The first coral reefs date from the early Ordovician of about 500 million years ago, and their form at the time differed significantly from that of corals today, which, following, the mass extinction 240 million years ago at the end of the Permian period, first appeared in the middle of the Triassic period.

Cnidaria and people

A large number of the islands humans inhabit today can be traced back to the carcasses of dead cnidaria. The limestone they left behind is often extracted and commercially exploited. Jewelery has been made from particularly colorful coral since prehistoric times.

On the other hand, humans are regularly killed or permanently disabled by the cnidarian's highly poisonous neurotoxin, particularly on the north coast of the Australian continent. The North Sea is also inhabited by cnidaria that can cause acutely painful skin wounds.

Conversely, the spread of human tourism often has a negative effect on coral. The global death of coral shows that in reef ecology the corals are a key organism, whose death often precedes the extinction of the entire ecosystem. The introduction of nitrate-heavy effluent and cyanide fishing are only some of the human influences that in a short space of time can cause the destruction of wide-ranging habitats.

Another potential danger for coral is rising water temperatures caused by climate change. If water temperatures rise too high, the corals lose the algae with which they live in symbiosis, thus perishing.


This article is partially based on a translation of the corresponding German-language Wikipedia article, retrieved on April 27, 2006.

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