A review of macrosymbiosis in the coral reef ecosystem

Abstract

Symbiotic associations are very prevalant in the coral reef ecosystem. Reasons may include the innate high species diversity, the dominance of the substratum by animals, and the complex nutrient recycling which characterize the system. Corals in particular host many symbiont species. They occupy much of the substratum, their branches and folds provide shelter, nematoblasts and enveloping skeletons provide protection, and mucus and rapidly regenerating tissues provide food. Some corals may benefit from the associations. For example, commensal worms and small crustanceans may attack the coral predator Acanthaster planciand drive them away from their hosts. The solitary coral Heteropsammia has gained mobility from an association with a worm, and can thereby exploit soft muddy bottoms which smother most other corals. In the barnacles, seemingly bound by a rigid morphology, is illustrated the great range of symbiotic associations to be found on the reef. Many species exploit the corals for substrata and continue to feed on plankton but one species has taken advantage of the proximity of fast-growing host tissue and become parasitic. Other barnacles have exploited motile species of the benthos and nekton. These gain not only a substratum but mobility, freedom from predators and competitors, feeding currents, and even scraps of food from their hosts. Platylepas ophiophilus. the sea snake barnacle, appears to have been forced into its specialized niche by more efficient competitors. Some have been parasitic; the Ascothoracids and Rhizocephalans are among the most specialized and degenerative of all parasites.