Invertebrates on the Coral Reef

The term 'invertebrate' simply means animals without a backbone. The largest proportion of all the animals on land or under the sea are invertebrates and the total number of marine invertebrates is staggering with possibly over a million species distributed worldwide in all the oceans. Coral reef invertebrates represent a major group and one of the delights of a living reef is the wide range of colourful invertebrates that form a spectacular matrix of multihued corals interspersed with sponges anemones and other decorative, brightly coloured creatures.

Looking first at the non moving or 'sessile' invertebrates the coelenterates rank as the most important group. They include the reef building corals, sea fans and anemones, together with the more mobile jellyfishes. Sponges often accompany the corals and can make up a large proportion of the reef structure especially in the Caribbean. The final significant 'stay put' group are the fan worms beautiful featherduster like creatures that disappear instantly into their tubes when approached.

The most conspicuous and mobile invertebrates are the crustaceans sometimes called the 'insects of the sea'. They include Crabs Lobsters and Shrimps. The slower moving molluscs are represented on the reef by the shellfish and sea slugs but also include the more active Squid, Cuttlefish and Octopi. The final major group of mobile invertebrates are the entirely marine animals that belong to the echinoderms. They are typified on coral reefs by Starfishes and Sea Urchins, but Sea Cucumbers, Featherstars and Sea Lilies are also prominent members of this group.

Crustaceans are like aquatic insects and are described as invertebrates because in place of an internal backbone they have a hard external skeleton. The candy shrimp is a good example. Seafans appear to be more like plants than animals. They are another form of coral and are covered in thousands of tiny polyps, which trap and devour microscopic planktonic animals.

One comes from the Caribbean, where seafans are common on the reefs. Reef building corals such as the elkhorn corals are made up of millions of tiny living coral polyps. These are constantly in the process of growing and multiplying to produce the limestone structures that we call reefs. There are many forms of reef building corals and a well formed reef can provide a spectacular array of shapes. Sponges are very varied in colour and shape and there are as many as nine separate species. Their ability to regenerate and adapt have ensured their survival. They are a prominent feature of reefs all over the world, but are most abundant in the Caribbean, where there are whole reefs consisting mainly of sponges.

The echinoderms are a major invertebrate group found on coral reefs. Starfishes with their five or more arms are familiar members of this group. The Tridacna clam in common with many stony corals plays host to algae in its tissues. Crinoids are a kind of feathery starfish, they have evolved to become nocturnal filterfeeders and trap microscopic zooplankton in their feathery arms. They remain hidden during the day.