Distribution and Ecology of Animals Associated with Branching Corals (Acropora Spp.) from the Great Barrier Reef, Australia

Head-forming branching corals of the genus Acropora provide discrete live coral habitats which contain a distinct fauna of small shrimp, crabs, and fishes. Associates were collected from 141 colonies of five Acropora species at four localities in eastern Australia. These corals had a mean volume of 4.9 1, and contained an average of 4.5 associate species and 9.1 individuals. Almost all animals collected were obligatory associates of Acropora, that could be placed in six groups of related species. Crabs of the genus Tetralia were present on all 141 corals, Gobiodon fish on 89%, Coralliocaris shrimp on 55%, Cymo crabs on 38%, shrimp of the Philarius/Periclimenes lutescens group on 34%, and Jocaste shrimp on 33% of the corals examined. Twenty species of obligate associate were collected at least five times. The most common species, T. fulva, was found on 76% of the corals, followed by C. superba on 46%, and G. quinquestrigatis on 33%. Examination of stomach contents and feeding behavior showed that the shrimp and crabs feed on coral mucus and, to lesser and varying degrees, zooplankton, other suspended material, and coral tissue. The fish feed on zooplankton and coral tissue. Most associate species generally occurred as single male-female pairs. Distribution data indicate that established pairs exclude conspecifics, obligate associates of their own group, and facultative associates. Discrimination between Acropora species is likely for several species of Gobiodon. Tetralia pairs seem to break up and reform, while pairs of Gobiodon may be stable for long periods. Associate abundance declines with coral depth, seems quite similar over large areas of tile Great Barrier Reef, and in one case changed over time. The evolution of coral association and the factors influencing community composition are discussed.