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:
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.
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.
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.
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.
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:
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.
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.