Cnidarians (Phylum Cnidaria, formerly known as the coelenterates) are an animal group comprised of a number of familiar marine animals including corals, jellyfish, and anemones.
As a group, cnidarians are characterized by a circular body enclosing a simple gut with a single opening surrounded by tentacles bearing specialized stinging cells called nematocysts used for defense and/or the capture of prey.
The term “coral” is widely used to describe several quite different kinds of coral reef cnidarians belonging to two separate classes of animals (Anthozoans and Hydrozoans) each with differing characteristics, as described below.
Anthozoans (Class Anthozoa) are the most common of coral reef cnidarians; in fact, coral reefs are largely composed of members of this group, which includes hard corals, octocorals, black corals, and anemones. The hard corals, the primary builders of coral reefs, are discussed in some detail elsewhere on our website (see “How Are Coral Reefs Formed“). Here, we introduce the other members of the Class Anthozoa commonly found on coral reefs.
Octocorals (also commonly called soft corals or gorgonians) include such familiar reef creatures as sea fans and sea whips. Octocorals differ from the hard corals in possessing flexible rather than rigid, mineralized skeletons. As they gently sway in the moving seas, the delicate, highly branched colonies of these plankton feeders are often mistaken for plants by novice reef explorers. Octocorals are extremely abundant coral reef cnidarians occupying most reef habitats. Like the sponges, they add both physical and biological diversity to the reef environment.
Black corals are mainly tropical, deep water animals. Because they lack symbiotic algae, they are not restricted by sunlight penetration. Thus, their depth range well exceeds that of the hard corals. The skeletal framework of black coral is highly valued by jewelry makers, they are intensely harvested in some areas. Black coral colonies grow very slowly and some types are among the oldest living organisms on Earth, with individual colonies persisting for thousands of years.
Sea anemones are among the most familiar of coral reef cnidarians and indeed of all benthic marine invertebrates. They are very similar in general body design to hard corals, but are much larger and lack protective external skeletons. Some of the anemones found on coral reefs form close symbiotic relationships with other animals, most notably shrimps, crabs and some fishes. These “guests”, which shelter among the anemone’s tentacles, have developed special chemicals that prevent the host’s nematocysts from stinging them.
Hydrozoans differ from Anthozoans in a number of ways, most notably in having a life cycle that includes a planktonic medusa stage. They are a diverse and widespread group of cnidarians with many fresh water and non-tropical species. One particular group of hydrozoans are true coral reef cnidarians, with distributions restricted to coral reef environments. These are the Hydrocorals, which include two main groups: fire corals, and lace corals.
Fire corals (pictured left) are prominent coral reef cnidarians commonly found both on Indo-Pacific as well as Greater Caribbean reefs. Like hard corals, fire corals possess rigid skeletons made of calcium carbonate and incorporate symbiotic algae into their tissues. The colonies are generally light colored, often tan to yellow, more brittle than those of hard corals, and covered by numerous tiny pores that bring sea water and food to the polyps.
Fire coral colonies take many forms, and may be encrusting, plate-like, or highly branched. The name fire coral is derived from the fearsome sting the nematocysts deliver, probably the single greatest source of animal injury to divers and snorkelers. Fire corals are most common in shallow waters, particularly on lagoon patch reefs and on reef flats or crests.
The related lace corals (pictured right) are not nearly as common as the fire corals. These form complex branching colonies up to 25 cm high, with flattened branches ending in blunt tips. They often exhibit bright colors, particularly violets, reds, oranges, and yellows. Unlike hard corals, the color is imbedded in the limestone skeleton rather than the polyps, and therefore remains after the colony dies. Lace corals range from the Indo-Pacific to the eastern Pacific, but are absent from the Greater Caribbean region.
Readers seeking more information on coral reef cnidarians are referred to our “Additional Resources” section, which contains lists of recommended readings specific to the topic of invertebrate coral reef animals.
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