Corals belong to a group of invertebrate animals called cnidarians (Phylum Cnidaria, formerly known as the coelenterates). This large group includes a number of familiar marine animals that also includes 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 cnidarians belonging to two separate classes (Anthozoa and Hydrozoa), each with differing characteristics as described below. 


Anthozoans (Class Anthozoa) include the most prominent reef-building cnidarians. Modern reefs are mainly formed by – and composed of – members of this group, which are briefly introduced below. 


(popularly known as “hard corals”) are the primary modern reef builders. They occur in great diversity – about 500-600 Caribbean species and ten times that number in the Indo-Pacific region.

These relatively small (about the size of a standard pencil eraser) individual animals mainly grow in large colonies, often many feet in diameter.

Colonies of different species take on many different shapes, although most can be classified as either “branching”, “plate-like”, or “massive” (boulder-like).

The general structure and biology of these animals are discussed in greater detail on another page of our web site.


Seagrass meadows are an important coral reef habitat-type.


(also commonly called “gorgonians”) include such familiar reef creatures as sea fans, sea rods, and sea whips. These colonial animals are often abundant in many reef habitats. particularly in the Caribbean region.

Octocorals differ from scleractinians 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. Like the sponges, they add both physical complexity and biological diversity to reef habitats. 




Black Corals

are mainly tropical, deep water animals. Because they lack symbiotic algae, they are not restricted by sunlight penetration. Thus, their depth range is for all practical purposes unlimited.

Colonies of these animals grow very slowly and some types are among the oldest living organisms on Earth, sometimes persisting for thousands of years.

Their skeletal framework is highly valued by jewelry makers, and they are intensely harvested in some areas. This has led to strict regulation of harvesting in many areas of the world.

coral reef plants


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.

The distribution of one particular group of hydrozoans is restricted to reef environments. These are the Hydrocorals, which include the two main groups discussed below.

Fire Corals

are prominent reef cnidarians commonly found in both the Indo-Pacific and Greater Caribbean regions. They are most common in shallow waters, particularly on lagoon patch reefs and on reef flats or crests.

Their name is derived from the fearsome sting the nematocysts deliver, probably the single greatest source of animal injury to divers and snorkelers.

Like scleractinians, they possess rigid skeletons made of calcium carbonate and incorporate symbiotic algae into their tissues. In many cases these animals make a substantial contribution to building of the underlying reef framework.

Colonies of these hydrozoans take many forms, and may be encrusting, plate-like, or highly branched. The colonies are generally light colored, often tan to yellow, more brittle than those of scleractinians, and covered by numerous tiny pores that bring sea water and food to the polyps.

sea lettuce

Sea lettuce is often found in shallow reef areas

Lace Corals

are not nearly as common as many other cnidarians that form the reef community. The ornate colonies of these animals often exhibit bright colors, particularly violets, reds, oranges, and yellows.

Unlike the scleractinians, the color is imbedded in the limestone skeleton rather than the polyps, and therefore remains after the colony dies.

These hydrozoans typically form complex branching colonies up to 25 cm high, with flattened branches ending in blunt tips. They are indigenous to the Indo-Pacific region, and entirely absent from the Greater Caribbean. 

Beautiful Coral Reef

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