BY

Forty species of ascidian in nine Families have been recorded from Twin Cays during a ten-year period of study. Twin Cays is the type locality for three of these species. Colony-forming species, notably those in the families Didernnidae, Polycitoridae and Perophoridae, predominate over solitary species which are poorly represented. The distribution in Twin Cays and habitat requirements of these different species are considered and the composition of the fauna is compared with that of other well documented mangrove environments in the Caribbean Basin. It is suggested that several other species of ascidian may colonize Twin Cays in years to come if the environment alters significantly. Notable among these are two species of Ecteinascidia which have breeding populations within a few kilometers distance from Twin Cays