Cnidaria

Cnidaria (IPA: ) is a phylum containing some 11,000 species of apparently simple animals found exclusively in aquatic, mostly marine, environments. On the other hand, their biochemistry and genetic makeup reveal that even the stationary starlet sea anemone is much more complex than it appears at the first glance.

Cnidarians get their name from cnidocytes, which are specialized cells that carry stinging organelles called cnidocysts. As for the etymology, the word Cnidaria comes from the Greek word "cnidos", which means "stinging needle". The corals, which are important reef-builders, belong here, as do the familiar 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 highly evident in the fossil records, having first appeared in the Ediacaran period.

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 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.

There are four main classes of Cnidaria:


 * Class Anthozoa (anemones, corals, etc.)
 * Class Scyphozoa (jellyfish)
 * Class Cubozoa (box jellies)
 * Class Hydrozoa (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, utilising 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 becomes a new polyp. Some cnidarians reproduce sexually by releasing egg and sperm into the water. They (the eggs) will be fertilized by sperm and develop into a new polyp. Medusa body types of cnidarians have both a sexual and asexual stage. The stages alternate. Medusa reproduce sexually to produce polyps, which will grow up and reproduce new medusa.

Cnidaria as fossils
The phylum has existed for a long time, having arguably been among the Ediacaran or Vendian biota of the later Proterozoic period, about 550 million years ago, and cnidaria were among the first recognised 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.

Classification
As mentioned in the introduction, 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 known among 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) is small sessile jellyfish with a stalk and attach to substrate by the stalk.
 * 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 oocyte. Recent research shows 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 display alternation of generations between medusa and polyp forms.

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.

Finally, the extinct Conularids may or may not be members of the Cnidaria.

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
As already mentioned, 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, particularly in the manufacture of cement. Jewelry has been made from particularly colourful coral since prehistoric times.

Some species of cnidaria are edible, and used in especially 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.

Journal articles

 * D. Bridge, B. Schierwater, C. W. Cunningham, R. DeSalle R, L. W. Buss: Mitochondrial DNA structure and the molecular phylogeny of recent cnidaria classes. in: Proceedings of the Academy of Natural Sciences of Philadelphia. Philadelphia USA 89.1992, p. 8750.
 * D. Bridge, C. W. Cunningham, R. DeSalle, L. W. Buss: Class-level relationships in the phylum Cnidaria - Molecular and morphological evidence. in: Molecular biology and evolution. Oxford University Press, Oxford 12.1995, p. 679.
 * D. G. Fautin: Reproduction of Cnidaria. in: Canadian Journal of Zoology. Ottawa Ont. 80.2002, p. 1735. (PDF, online)
 * G. O. Mackie: What's new in cnidarian biology? in: Canadian Journal of Zoology. Ottawa Ont. 80.2002, p. 1649. (PDF, online)
 * P. Schuchert: Phylogenetic analysis of the Cnidaria. in: Zeitschrift für zoologische Systematik und Evolutionsforschung. Paray, Hamburg-Berlin 31.1993, p. 161.
 * G. Kass-Simon, A. A. Scappaticci Jr.: The behavioral and developmental physiology of nematocysts. in: Canadian Journal of Zoology. Ottawa Ont. 80.2002, p.1772. (PDF, online)
 * A. C. Marques, A. G. Collins (2004): Cladistic analysis of Medusozoa and cnidarian evolution. Invertebrate Biology, 123 (1), 23-42.
 * J. Zrzavý (2001): The interrelationships of metazoan parasites: a review of phylum-and higher-level hypotheses from recent morphological and molecular phylogenetic analyses. Folia Parasitologica, 48 (2), 81-103.