Unpalatable compounds in the marine gastropod Dolabella auricularia:: Distribution and effect of diet

Sea hares are a rich source of novel secondary metabolites, most of which are derived from their algal diet, but the natural function(s) of these metabolites are largely unknown. We used field and laboratory assays to measure the palatability of extracts from the tissues, ink, and eggs of Dolabella auricularia. Digestive-gland extracts contained a wide variety of secondary metabolites, including the red algal compound prepacifinol epoxide and its derivative johnstonol, and they were unpalatable to reef fishes. Skin extracts were moderately unpalatable, but our bioassay-guided fractionation led us to (-)-7-dehydrocholesterol, rather than to an algal secondary metabolite. Ink extracts were consistently unpalatable to reef fishes only at high concentrations, suggesting either that ink must be concentrated to deter predators, that unpalatable components of ink rapidly decompose, or that ink has ether functions, Unpalatability of ink was traced to a purple fraction, consistent with the hypothesis that the active compound is aplysioviolin, a known ink constituent modified from a red algal pigment. Egg extracts were moderately unpalatable: however, we could not trace this activity to any algal-derived secondary metabolite. Body-wall extract was highly palatable. Our results suggest that dietary-derived secondary metabolites play a role in chemical defense of D. auricularia via the ink, but are not responsible for unpalatability of skin or eggs. Accumulation of dietary-derived metabolites in the digestive gland may occur to detoxify a chemically rich diet, rather than or in addition to deterring predators.