Marine opisthobranch molluscs: Chemistry and ecology in sacoglossans and dorids

Opisthobranchs are marine molluscs apparently unprotected by physical constraints of a shell which is either reduced or completely absent in the adults. Their survival is based on a series of defensive strategies, which involve cryptic behaviour and use of deterrents. During the last twenty years, many studies have been performed to characterize the chemicals isolable from these animals. In this paper we summarize the studies covering two large groups of opistobranch molluscs: sacoglossans and dorids. The main aim is to give to the readers a brief view of the compounds isolated from these molluscs, and, when it is possible, to suggest an ecological role for them. The Order Sacoglossa has been selected as it contains a wide range of morphological types from primitive species with a relatively strong external shell to shell-less types. The chemical studies of these molluscs have been reviewed with the aim of constructing a general scenario based on chemical evidence. Similar reasons prompted us to review the chemical data of the shell-less dorid nudibranchs belonging to the superfamilies Eudoridoidea and Bathydoridoidea. In this case, the review dates from the literature subsequent to Karuso's 1987 review "Chemical Ecology of Nudibranchs". The selection of the two groups, sacoglossans and dorids, is also due to their different, but exclusive feeding habits, green algae for sacoglossans and sponges for dorids. The critical analysis of all these studies illuminates the extraordinary capability of opisthobranch molluscs to create new chemistry through either bioaccumulation of selected metabolites from their dietary sources, bio-transformation of dietary compounds, or de novo bio-synthesis of useful chemicals. However, it is difficult to determine the boundaries of these investigations. In fact, their ecological contributions are relevant and applications useful for man are foreseable.