Chemical defenses in the sea hare Aplysia parvula: importance of diet and sequestration of algal secondary metabolites

Marine algae produce a variety of secondary metabolites that function as he rbivore deterrents. Algal metabolites, however, often fail to deter damage by some herbivores such as mesograzers that both live and feed on their hos t alga. In addition, the degree to which intraspecific chemical variation i n an alga affects a mesograzer's feeding behavior and its ability to deter predators is poorly understood. The red alga Portieria hornemannii contains the secondary metabolites apakaochtodene A and B, which have been shown to vary in concentration among sites on Guam and act as significant deterrent s to fish feeding. On Guam, the sea hare Aplysia parvula preferred and grew best when fed its algal host P. hornemannii. However, high concentrations of P. hornemannii crude extract and the pure compounds apakaochtodene A and B acted as feeding deterrents to A. parvula. Despite differences among sit es in the levels of apakaochtodenes A and B, A. parvula showed no significa nt preference for P, hornemannii from any one location. Aplysia parvula fou nd on P, hornemannii sequestered apakaochtodenes, and both whole animals an d body parts were unpalatable to reef fishes. Sea hares found on the red al ga Acanthophora spicifera, which contains no unpalatable secondary metaboli tes, had no apakaochtodene compounds and were eaten by fishes. This observa tion is consistent with the hypothesis that diet-derived algal metabolites in sea hares play a role in deterring predation.