Cnidaria

[nahy-dair-ee-uh]

Cnidaria or Coelenterata, phylum of invertebrate animals comprising the sea anemones, corals, jellyfish, and hydroids. Cnidarians are radially symmetrical (see symmetry, biological). The mouth, located at the center of one end of the body, opens into a gastrovascular cavity, which is used for digestion and distribution of food; an anus is lacking. Cnidarians are further characterized by having a body wall composed of three layers: an outer epidermis, an inner gastrodermis, and a middle mesogloea. Tentacles encircle the mouth and are used in part for food capture. Specialized stinging structures, called nematocysts, are a characteristic of the phylum and are borne in the tentacles and often in other body parts. These contain a coiled fiber that can be extruded suddenly. Some nematocysts contain toxic substances and are defense mechanisms, while others are adhesive, helping to anchor the animal or to entangle prey.

Two body forms and two lifestyles are characteristic of the Cnidaria (see polyp and medusa). The sessile hydroid, or polyp, form is more or less cylindrical, attached to its substratum at its aboral (opposite the mouth) end, with the mouth and surrounding tentacles at the upper, oral, free end. Colonies of hydroids comprise several different types of individuals: some function in feeding, some in defense, and some in reproduction. The motile jellyfish, or medusoid form, is flattened, with the tentacles usually located at the body margin. The medusoid's convex aboral surface is oriented upward, and the concave oral surface is oriented downward.

With few exceptions, the cnidarians are marine. There are over 9,000 known living species; fossil records of cnidarians date back to the Ordovician era. Cnidarians are carnivorous, the major part of their diet consisting of crustaceans. Animals in this phylum have no specialized excretory or respiratory organs but do have a nervous system. Both sexual and asexual reproduction occur. There are three classes of cnidarians.

Class Hydrozoa

The Hydrozoa include solitary or colonial cnidarians, which have a noncellular mesoglea, lack tentacles within the gastrovascular cavity, and have no gullet. As a rule, the hydroid stage predominates in the life cycle, although in some the jellyfish stage is larger. The order Hydroida includes the many small, colonial hydroids so often seen clinging to wharves and submerged objects along the seacoasts everywhere, economically important because they foul surfaces. The order also includes solitary hydroids, some reaching several inches in height. One, in the genus Branchiocerianthus, is said to reach 8 or 9 ft (244-274.5 cm) in length. The common freshwater genus Hydra also belongs to this order, as does the freshwater jellyfish, genus Craspedacusta, and the commonly studied hydroid jellyfish, genus Gonionemus. There are also pelagic hydroid colonies, unusual in having one very large hydroid member, which lives with its mouth downward and its aboral surface upward, like a jellyfish. The aboral end is equipped with a projecting sail. Velella, the purple sailor, is an example. The order Milleporina includes colonial organisms that form a massive, porous exoskeleton, somewhat resembling corals. They are sometimes abundant in tropical seas and may contribute to coral reef formation. The order Siphonophora includes often large, floating colonies made up of members of varying form and function. Typical is Physalia, the Portuguese man-of-war. Its colorful float is a gas-filled member of the colony and attains lengths up to 1 ft (30 cm). Other members of the colony hang downward from the lower surface of the float; some of these have very powerful nematocysts able to cause severe physiological reaction in swimmers coming in contact with them. These organisms are able to kill sizable fish with their tentacles.

Class Scyphozoa

Cnidarians of class Scyphozoa have a predominant jellyfish stage. They are characterized by a cellular mesoglea and tentacles in their gastrovascular cavity. All of the largest jellyfish belong to this class. The common Aurelia aurita is seen in bays and harbors, sometimes in large numbers. It is pallid, unlike some of the more colorful species in the genus Cyanea. Stalked jellyfish, the Stauromedusae, are unusual members of the Scyphozoa; they are found attached to seaweed, especially in cooler marine habitats. The order Rhizostomea includes jellyfish in which the original mouth has closed, and which have many subsidiary mouths found in frilled oral arms. Cassiopaeia is a well-known example, living in warmer, shallow waters, where it is often found lying on the bottom upside down, exposing its green algal symbionts to the sun.

Class Anthozoa

Class Anthozoa includes Cnidaria that have no jellyfish stage. This is the largest class of cnidarians, containing over 6,000 species. A gullet extends for a short distance into the gastrovascular cavity, and septa are present, which increase the surface for digestion and absorption. Anthozoa are flower animals, including a great many beautiful and colorful organisms, e.g., the sea anemone, sea pansy, sea fan, and coral. Anthozoans are colonial or solitary organisms.

Subclass Alcyonaria

Subclass Alcyonaria includes almost universally colonial organisms in which each of the polyps, or hydroid members, has eight feathery tentacles. Most of them produce a skeleton, and many make some contributions to coral reefs. While some are found in temperate seas, they are especially common in subtropical to tropical regions. The organ pipe coral (Tubipora), a soft coral (Alcyonium), the Indo-Pacific blue coral (Heliopora), and the sea pens, which have a stalk extending into the bottom mud or sand, are some typical alcyonarian corals. Horny corals, of the order Gorgonacea, are perhaps the best known. These form branching, upright colonies and have a skeleton that is partly composed of a horny material called gorgonin. These are the sea whips and sea fans so characteristic of shallow tropical waters.

Subclass Zoantharia

The subclass Zoantharia includes both solitary and colonial forms, in which the polyp has more than eight tentacles. The solitary sea anemones belong here, in the order Actiniaria, characterized by the lack of a skeleton. The stony corals so important in forming coral reefs belong to the order Madreporaria; they are especially characterized by their calcium carbonate exoskeleton, marked by many cups for the polyps, each of which contains stony septa dividing the gastrovascular cavity into compartments. The shape of coral skeletons depends on the pattern of growth of the colony. For example, in brain corals the polyps are arranged linearly; in the eyed coral (Oculina) the polyps are separated from each other by spaces, giving the skeleton a pitted appearance. The burrowing anemone, Cerianthus, lives in burrows in the sand and has a greatly elongated body. It is characteristic of the order Ceriantharia.

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cnidarian

or coelenterate

Cnidarian body forms. A cnidarian may display either the sessile polyp form or the free-swimming elipsis

Any of about 9,000 species of mostly marine aquatic invertebrates, constituting the phylum Cnidaria (or Coelenterata), that are unique in possessing specialized stinging cells (cnidocytes) borne on the tentacles. Cnidocytes contain fluid-filled capsules (nematocysts) with a harpoonlike coiled thread used for stinging, paralyzing, and capturing prey. Cnidarians have no well-defined separate respiratory, circulatory, or excretory organs; their tissues, composed of two cell layers, surround a cavity known as a coelenteron (gastrovascular cavity), which is the basic internal organ. Tentacles surrounding the mouth are used to capture and ingest food. Cnidarians are carnivorous, feeding mostly on zooplankton but also on small crustaceans, fish eggs, worms, smaller cnidarians, and even small fish. Cnidarians range in size from nearly microscopic to more than 100 ft (30 m) long and more than a ton (910 kg) in weight. There are two basic body forms: the polyp (e.g., coral) and the medusa (e.g., jellyfish). Seealso hydra; Portuguese man-of-war; sea anemone.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.

Cnidaria

Cnidaria is a phylum containing some 9,000 species of animals found exclusively in aquatic, mostly marine, environments. Despite their early appearance in the evolutionary history of animals and their simple morphology, the modern forms are genetically sophisticated and bio-chemically complex.

The unifying characteristic of the Cnidarians is the presence of cnidocytes (nematocytes), specialized cells that carry organelles called cnidocysts (nematocysts). It is widely accepted that all cnidarians inherited cnidocytes from a single common ancestor. As for the etymology, the word Cnidaria comes from the Greek word "cnidos", which means "stinging nettle". The corals, which are important reef-builders are placed in this phylum, along with sea anemones, jellyfish, sea pens, sea pansies and sea wasps. The name Coelenterata was formerly applied to the group, but as this name included the Ctenophores, it has been abandoned.

Cnidarians are the most primitive eumetazoans, and their divergence from other animals must have occurred in the Precambrian. The first attempt to categorise the Charnia fossils of the Ediacaran period designated them as jellyfish and sea-pens. However, detailed study of the cnidarian growth pattern has discounted this hypothesis.

The basic body shape of a cnidarian consists of a sac containing 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, the resulting halves would always be mirror images of each other. Their movement 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 and a gravity-sensing statocyst comparable in function to the otolith of the vertebrate inner ear. Tentacles surrounding the mouth contain cnidocytes, specialized stinging cells, which they use to catch prey and defend themselves from predators. The ability to sting is what gives cnidarians their name.

There are five main classes of Cnidaria:

Class Anthozoa (anemones, corals, etc.)
Class Scyphozoa (jellyfish)
Class Staurozoa (stalked jellyfish)
Class Cubozoa (box jellies)
Class Hydrozoa (hydroids, hydromedusae and siphonophores: Obelia, Aequorea, Portuguese Man o' War, etc.)

Traditionally the hydrozoans were considered to be the most primitive, but evidence now suggests the anthozoans were actually the earliest to diverge. Sea anemones, sea fans and corals are in this class. The non-anthozoan classes may be grouped into the subphylum Medusozoa. Under this scheme, Anthozoa is also elevated to a subphylum.

Theoretically, members of Cnidaria have life-cycles that alternate between asexual polyps and sexual, free-swimming forms called medusae. In reality there is a vast variation within the life-cycles of cnidarians.

Nutrition

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 methane produced by the sea pansy, utilizing sunlight via photosynthesis and releasing the oxygen, the algae produce energy-rich carbohydrates which the cnidarian uses as its main source of food.

Reproduction

Cnidarians reproduce both sexually and asexually. They reproduce asexually by budding. The bud will eventually fall off the parent organism and become a new polyp. Some cnidarians reproduce sexually by releasing egg and sperm into the water. The eggs will be fertilized by sperm and develop into a larva called a planula. The planula will then develop into a new polyp which will produce a new medusa called ephyra. Medusa body types of cnidarians have both sexual and asexual stages. The stages alternate. Medusae reproduce sexually to produce polyps, which will grow up and reproduce new medusae.

Cnidaria as fossils

The phylum has existed for a long time, having arguably been among the Ediacaran or Vendian biota of the later Proterozoic eon, about 580 to 540 million years ago, and Cnidaria were among the first recognised animal fossils. Our understanding of fossil groups is varied. Whereas those Cnidaria that were formed of soft tissue are only found as fossils in exceptional cases, the fossil record of, for example, corals is very well known due to the calcium carbonate (calcite and aragonite) skeletons they left behind. The first coral reefs date from the early Ordovician of about 500 million years ago. Their form and skeletal mineralogy 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.

Classification

In the past, Cnidaria were classically grouped together with ctenophora as Coelenterata. In view of current research into cladistics, this group is now considered paraphyletic, i.e. it does not include all the descendants of their common ancestor. Despite the outer similarity of the two taxa, such as their radially symmetric bodies, the ctenophora are more likely to be related to the mirror-symmetrical bilateria than cnidaria. For this reason Coelenterata is considered to be an artificial grouping from a cladistic viewpoint.

Cnidaria are further divided into six main classes:

Class Anthozoa (corals) includes about 6,000 species, including sea anemones and corals such as Scleractinia (stony star corals). The medusa stage is not present in this class.
Class Scyphozoa (jellyfish) contains about 200 species, which mostly appear as medusae. Conulariids are thought to belong to this class.
Class Staurozoa (stalked jellyfish) are small sessile jellyfish with a stalk attached to a substrate.
Class Cubozoa (box jellyfish) encompasses about 20 species, which only appear as medusae. Among them are the species Chironex fleckerii and Chiropsalmus quadrigatus, known as sea wasps, which possess a highly potent toxin.
Class Polypodiozoa contains a single species Polypodium hydriforme Ussow, 1885, a parasitic cnidarian in sturgeon oocytes. Recent research shows an evolutionary relationship with Myxozoa.
Class Hydrozoa contains about 3,000 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. Hydrozoa often alternate between asexual polyps and sexual medusae body forms.

Among the Hydrozoa the Order 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.

Finally, the classification of the extinct Conularids is still a matter of contention among taxonomists. Some experts question whether this group should even be included in the animal kingdom.

Obsolete names for groups of cnidarians include Acalephae, which contained Hydrozoa and Scyphozoa, based on the shared character of stinging cells; however, this character is no longer thought to be primitive.

Cnidaria and man

A large number of oceanic islands can be traced back to the skeletal remains of cnidaria. The limestone they left behind is extracted and commercially exploited, particularly in the manufacture of cement. Jewelry has been made from particularly colourful coral since prehistoric times.

Some species of cnidaria are edible and are often used in Eastern Asian cuisine.

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 biology 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 danger for coral is the rising water temperatures caused by climate change: if they rise too high, the corals lose the algae with which they live in symbiosis and perish.