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

Abstract

Marine algae produce a variety of secondary metabolites that function as herbivore deterrents. Algal metabolites, however, often fail to deter damage by some herbivores such as meso- grazers that both live and feed on their host alga. In addition, the degree to which intraspecific chem- ical variation in 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 apa- kaochtodene A and B, which have been shown to vary in concentration among sites on Guam and act as significant deterrents 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 sites in the levels of apakaochtodenes A and B, A. parvula showed no sig- nificant preference for P. hornemannii from any one location. Aplysia parvula found on P. horneman- nii sequestered apakaochtodenes, and both whole animals and body parts were unpalatable to reef fishes. Sea hares found on the red alga Acanthophora spicifera, which contains no unpalatable sec- ondary metabolites, had no apakaochtodene compounds and were eaten by fishes. This observation is consistent with the hypothesis that diet-derived algal metabolites in sea hares play a role in deter- ring predation.