Marine opisthobranch molluscs: Chemistry and ecology in sacoglossans and dorids

Opisthobranchs are marine molluscs apparently unprotected by physical const raints of a shell which is either reduced or completely absent in the adult s. Their survival is based on a series of defensive strategies, which invol ve cryptic behaviour and use of deterrents. During the last twenty years, m any studies have been performed to characterize the chemicals isolable from these animals. In this paper we summarize the studies covering two large g roups of opistobranch molluscs: sacoglossans and dorids. The main aim is to give to the readers a brief view of the compounds isolated from these moll uscs, 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 morpholo gical types from primitive species with a relatively strong external shell to shell-less types. The chemical studies of these molluscs have been revie wed with the aim of constructing a general scenario based on chemical evide nce. Similar reasons prompted us to review the chemical data of the shell-l ess dorid nudibranchs belonging to the superfamilies Eudoridoidea and Bathy doridoidea. In this case, the review dates from the literature subsequent t o Karuso's 1987 review "Chemical Ecology of Nudibranchs". The selection of the two groups, sacoglossans and dorids, is also due to their different, bu t exclusive feeding habits, green algae for sacoglossans and sponges for do rids. The critical analysis of all these studies illuminates the extraordin ary capability of opisthobranch molluscs to create new chemistry through ei ther bioaccumulation of selected metabolites from their dietary sources, bi o-transformation of dietary compounds, or de novo bio-synthesis of useful c hemicals. However, it is difficult to determine the boundaries of these inv estigations. In fact, their ecological contributions are relevant and appli cations useful for man are foreseable.