Cnidaria (corals, jellyfish, and Hydra) are incredibly diverse in form, as evidenced by colonial siphonophores, massive medusae and corals, feathery hydroids, and box jellies with complex eyes. Yet, these diverse animals are all armed with stinging cells called nematocysts. Cnidarians are united based on the presumption that their nematocysts have been inherited from a single common ancestor. The name Cnidaria comes from the Greek word cnidos, which means stinging nettle. Casually touching many cnidarians will make it clear how they got their name when their nematocysts eject barbed threads tipped with poison.
Many thousands of cnidarian species live in the world's oceans, from the tropics to the poles, from the surface to the bottom. Some even burrow. A smaller number of species are found in rivers and freshwater lakes.
There are four major groups of cnidarians:
- Anthozoa, which includes true corals, anemones, and sea pens;
- Cubozoa, the amazing box jellies with complex eyes and potent toxins;
- Hydrozoa, the most diverse group with siphonophores, hydroids, fire corals, and many medusae; and
- Scyphozoa, the true jellyfish.
Many of the very best cnidarian fossils date back to the time when animals first appear in the fossil record, the Vendian. Since then, the fossil record of cnidarians without mineralized skeletons is quite sparse, and restricted to unusual sites with excellent fossil preservation.
On the other hand, cnidarians which possessed hard skeletons, in particular the corals, have left a significant legacy of their existence. While a few mineralized coral-like fossils have turned up in the Cambrian Period, identifiable corals began an evolutionary radiation in the Early Ordovician. These Paleozoic corals included taxa known as tabulate corals, rugose corals, and heliolitid corals. All these forms were wiped out at the end of the Permian Period, in a mass extinction event that claimed something like 95% of all marine invertebrate species.
Scleractinian corals first appear in the Middle Triassic, about 15 milion years after the Permian extinction. They rapidly expanded into ecological niches once dominated by tabulate and rugose corals, and became the dominant hermatypic (reef-building) organisms in shallow tropical marine habitats. Because corals are sensitive to changes in light, temperature, water quality, and salinity, their fossils provide information that can be used to interpret climate and geography of past environments.
Life History and Ecology
Cnidarian ecology is a complex subject indeed, because it is cnidarians, in particular corals, that are the builders of some of the richest and most complex ecosystems on the planet, coral reefs. Other cnidarians are important as predators in the open ocean.
Cnidarians generally occupy two major niches. They may use their cnidocysts to trap prey items. On the other hand, many cnidarians, anthozoans in particular, depend on zooxanthellae, symbiotic dinoflagellates within the tissues, to survive. These single-celled protists carry out photosynthesis within the animal's tissues, and pass on the carbon compounds they fix to their hosts; corals, therefore, are photosynthetic animals in a sense.
Some cnidarians are nearly completely dependent on zooxanthellae; others trap prey but augment their diet with zooxanthellae. While not all corals are dependent on symbionts — some live at great depths where there is never light — colonial, reef-forming corals depend on them; thus, reefs can only exist in shallow water. Notice the white areas on this coral reef exposed at low tide: this loss of symbionts, called bleaching, is deadly to coral reefs.
Cnidarians present particular problems for systematists. Their comparatively simple morphology makes it difficult to compare taxa. The fossil record of soft-bodied cnidarians is very sparse, although the record of corals and other mineralizing cnidarians is excellent. Few molecular phylogenetic analyses so far have examined the Cnidaria in great detail.
Traditionally, it was thought that the Hydrozoa were the most primitive cnidarians. However, a recent cladistic analysis and avaliable molecular data suggest that the Anthozoa, the only group of living cnidarians that completely lacks a medusoid "jellyfish" stage in the life cycle, are in fact most primitive. This seems consistent with the fossil record of the Cnidaria, but further studies will be needed to refine this picture of cnidarian evolution.
The phylum Ctenophora, which includes the "comb jellies," "sea gooseberries," and "Venus's girdles," is not currently considered to be part of the Cnidaria; however, the two are close relatives. The Cnidaria and Ctenophora are grouped together by some workers as the Coelenterata.
More on Morphology
Cnidarians are said to be the simplest organisms at the tissue grade of organization; their cells are organized into true tissues. Cnidarians are essentially bags made of two cell layers. The outer ectoderm, or epidermis, contains the cnidocysts, the stinging cells that are characteristic of the phylum. The inner endoderm, or gastrodermis, lines the gut, which in some cnidarians may be divided up by septa (as in the Anthozoa) or elaborated into branching canals (as in many Scyphozoa. In between epidermis and gastrodermis is the mesoglea, a layer of jellylike substance which contains scattered cells and collagen fibers. The mouth is often, but not always, surrounded by a ring of tentacles.