Cnidaria

Cnidaria

Sea nettles, Chrysaora quinquecirrha

Scientific classification

Kingdom:	Animalia
Phylum:	Cnidaria Hatschek, 1888

Classes

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

Cnidaria (silent c - pronounced /naɪˡdeɹiə/ from New Latin cnida, fr. Gk κνιδη "nettle") is a phylum containing some 11000 species of relatively simple animals found exclusively in aquatic, mostly marine, environments. Cnidarians get their name from cnidocytes, which are specialized cells that carry stinging organelles. The corals, which are important reef-builders, belong here, as do the familiar sea anemones, jellyfish, sea pens, sea pansies and sea wasps. The names Coelenterata and Coelentera were formerly applied to the group, but as those names included the Ctenophores (comb jellies), they have been abandoned. Cnidarians are highly evident in the fossil records, having first appeared in the Precambrian era.

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, 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 [1], 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 Hydrozoa (Portuguese Man o' War, Obelia, etc.)
Class Scyphozoa (jellyfish)
Class Cubozoa (box jellies)

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.

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.

Body

The cnidarian is a eumetazoan, possessing true tissues and organs. It has radial symmetry, meaning that whichever way it is cut along its central axis, the resulting halves will always be mirror images of each other. 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 are considered to be diploblastic, though the mesoglea may be homologous with the mesoderm in other animals.

The ectoderm surrounds the cnidarian's 'stomach', or gastrovascular cavity which functions as both mouth and anus; it 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.

File:Scyphomedusae Chrysaora melanaster.jpg

The jellyfish Chrysaora melanaster

The movement of cnidaria is controlled by a decentralised net of true nerve cells. Concentrations of nerve cells are found in the mouth area of polyps (the hypostome), on the tentacles and stem (pedunculus), and with jellies a ring of nerves is often found around the screen . A specialised direction of signal transport has in many cases not yet developed, but the connection of nerves over 'gap junctions' does allow a few species to become quickly aroused, with a number of neuropeptides allowing the modulation of arousal. It was long accepted that cnidaria were diploblastic. 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. Up until now taxonomists have classified cnidaria in the order of the most primitive lifeforms, and did not believe they possessed the same mesoderm from which, for example, the musculature and many inner organs of mammals develop. This new scientific evidence appears to warrant a new classification for cnidaria.

Cnidarians take their name from a specialised cell, the cnidocyte (nettle cell). Tentacles surrounding the mouth contain nematocysts, specialized stinging cells. The nematocysts are the cnidarians' main form of offence 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 paralysing its victim.

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.

File:Portuguese man of war.jpg

Portuguese Man o' War (Physalia physalis)

Theoretically, members of Cnidaria have life-cycles that alternate between asexual polyps and sexual, free-swimming forms called 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.

In reality there is a vast variation within the life-cycles of cnidarians. Within the group Anthozoa, 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 metamorphosises into the medusal stage. Among the Scyphozoa and Cubozoa, the medusae are the dominant form in the life-cycle, while the 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.

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 carbon dioxide produced by the cnidarian, 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

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 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 metamorphosises into the medusal stage. Among the Scyphozoa and Cubozoa, the medusae are the dominant form in the life-cycle, while the polyps are in turn reduced or absent.

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 aforementioned endosymbiotic algae are important to this reef formation. 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 tropical 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 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.

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. Jewellery has been made from particularly colourful 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 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.

Classification

As mentioned in the introduction, cnidaria were classicly 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 ctenphora 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 four main classes:

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 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.
Class Scyphozoa (jellyfish) contains about 200 species, which mostly appear as medusae.
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.

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 Conulariida 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.

References
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This article is partially based on a translation of the corresponding German-language Wikipedia article, retrieved on 27th April 2006, which cites the following references:

D. T. Anderson: Invertebrate Zoology. Oxford Univ. Press, Oxford 2001 (2nd ed.), ch. 3, p. 31. ISBN 0195513681
P. Ax: Das System der Metazoa I. Ein Lehrbuch der phylogenetischen Systematik. Gustav Fischer, Stuttgart-Jena 1999. ISBN 3-437-30803-3
R. S. K. Barnes, P. Calow, P. J. W. Olive, D. W. Golding, J. I. Spicer: The invertebrates - a synthesis. Blackwell, Oxford 2001 (3rd ed.), ch. 3.4.2, p. 54. ISBN 0-632-04761-5
R. C. Brusca, G. J. Brusca: Invertebrates. Sinauer Associates, Sunderland Mass 2003 (2. ed.), ch. 8, p. 219. ISBN 0878930973
J. Moore: An Introduction to the Invertebrates. Cambridge Univ. Press, Cambridge 2001, ch. 4, p. 30. ISBN 0521779146
E. E. Ruppert, R. S. Fox, R. P. Barnes: Invertebrate Zoology - A functional evolutionary approach. Brooks-Cole, Belmont 2004, ch. 7, p. 111. ISBN 0030259827
W. Schäfer: Cnidaria, Nesseltiere. in: Rieger, Westheide (ed.): "Spezielle Zoologie. Teil 1. Einzeller und Wirbellose Tiere." Gustav Fischer, Stuttgart-Jena 1997, Spektrum Akademischer Verl., Heidelberg 2004. ISBN 3827414822
B. Werner: 4. Stamm Cnidaria. in: v. Gruner (ed.): Lehrbuch der speziellen Zoologie. Begr. von Kaestner. 2 Bde. Gustav Fischer, Stuttgart-Jena 1954, 1980, 1984, Spektrum Akad. Verl., Heidelberg-Berlin 1993 (5. ed.). ISBN 3-334-60474-8

Scientific literature

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. ISSN 0097-3157
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. ISSN 0737-4038
D. G. Fautin: Reproduction of Cnidaria. in: Canadian Journal of Zoology. Ottawa Ont. 80.2002, p. 1735. (PDF, online) ISSN 0008-4301
G. O. Mackie: What's new in cnidarian biology? in: Canadian Journal of Zoology. Ottawa Ont. 80.2002, p. 1649. (PDF, online) ISSN 0008-4301
P. Schuchert: Phylogenetic analysis of the Cnidaria. in: Zeitschrift für zoologische Systematik und Evolutionsforschung. Paray, Hamburg-Berlin 31.1993, p. 161. ISSN 0044-3808
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) ISSN 0044-3808

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