CHEMICAL DEFENSES IN THE SEA HARE APLYSIA PARVULAAND ITS HOST ALGA PORTIERIA HORNEMANNII AND THEIR EFFECTS ON PREDATION

David W. Ginsburg and Valerie J. Paul
University of Guam Marine Laboratory, UOG Station, Mangilao, Guam,96923 USA.

Marine algae produce a variety of secondary metabolites that functionas herbivore deterrents. Algal metabolites, however, often fail todeter damage by some herbivores such as mesograzers that both live andfeed on their host alga. In addition, the degree to which intraspecificchemical variation in an alga influences a mesograzer's ability to deterherbivores is poorly understood. The red alga Portieria hornemanniicontains the secondary metabolite ochtodene, which has been shown to varyfrom site to site on Guam and to act as a significant deterrent to fishfeeding. On Guam, the sea hare Aplysia parvula preferentiallygrazes on and is only found to live on P. hornemannii. Inlaboratory experiments, sea hares preferentially graze on populations ofP. hornemannii known to have high levels of ochtodene and preferentiallyfeed on the tips of P. hornemannii thalli compared with its bases. Sea hares sequester ochtodene in high concentrations and are deterrentto reef fish predators. However, A. parvula appear to suffer highrates of predation from invertebrate predators when placed in laboratoryaquaria. Lastly, greater levels of ochtodene were found in individual P.hornemannii that had been naturally grazed by the parrotfish Scarusschlegeli as compared with ungrazed algae. Thus, while herbivore-induceddefense in P. hornemannii may protect algal tissues from grazingby generalist reef fishes, it may stimulate feeding by mesograzers suchas A. parvula that use their algal host as food and protectionfrom potential predators.