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Biodiversity Introduction > Porifera > Cnidaria > Polychaeta > Crustacea > Mollusca > Bryozoa > Echinodermata  The Phylum Cnidaria represents an eclectic mix of soft-bodied and sometimes calcified animals united by a common body plan of only two cell layers separated by a jelly-like mesoglea which provides structural support in the water. Cnidarians include corals, seapens, hydroids and hydromedusae, siphonophores, jellyfish, box jellyfish, anemones and zonathids. Many cnidarians can reproduce asexually and sexually. Many also display an “alternation of generations”, a life cycle characterized by an attached polyp phase and a free-swimming larva or medusa phase.
The range of animals included in the Cnidaria encompasses both solitary and colonial animals, with varying degrees of mobility. Most are suspension feeders, but some are top predators. Cold-water coral reefs are themselves comprised of scleractinian corals, but are often associated with diverse assemblages of other cnidarians. Benthic hydroids are ubiquitous elements of coral rubble habitats, while anemones, zonathids and octocorals encrust onto both living and dead coral fragments. Many cnidarians are large enough to support their own microhabitats and diverse assemblages of associated invertebrates. The erect growth forms and often delicate colonies of many cnidarians and the cold-water corals themselves make these animals highly vulnerable to activities such as bottom fishing.
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