MEPS 155:115-126 (1997)  -  doi:10.3354/meps155115

The pelagic Sargassum community of the western Atlantic Ocean is species rich, with high densities of predatory fishes and invertebrates living in close association with the floating seaweed. Little, however, is known about predator-prey interactions among Sargassum inhabitants and the factors that might help maintain this species richness. To assess how predators may affect the abundance of sessile Sargassum epiphytes, and how these epiphytes may defend themselves against predators, we examined interactions between the most abundant small predator associated with Sargassum mats, the filefish Monacanthus hispidus, and 4 epiphytic hydroid species. This fish was the only Sargassum-associated predator to significantly consume hydroids in initial assays. When filefish were given a simultaneous choice of all 4 hydroid species, they consumed 40 to 45% of 3 species (Clytia noliformis, Aglaophenia latecarinata, and Tridentata turbinata), but consumed less than 5% of the fourth species, Tridentata marginata. Filefish consistently rejected small portions of T. marginata colonies, but consumed a palatable control food. Bioassay-guided fractionation of T. marginata extract resulted in the isolation of a single deterrent secondary metabolite, tridentatol A. Three additional metabolites (tridentatols B to D) had no effect on filefish feeding. In addition to the defensive role of tridentatol A, the tridentatols (A to D) strongly absorb damaging solar ultraviolet (UV) radiation, and thus may function as sunscreens. To our knowledge, this is the first demonstration of chemical defenses among the pelagic Sargassum fauna, and the first report that a hydroid secondary metabolite deters consumers. Prey chemical defenses are an important factor in maintaining species richness in many predator-rich communities, but despite being chemically defended from predators, T. marginata was far less abundant than any of the other, more palatable, hydroids. The factors that allow high-preference hydroids to persist in such a predator-rich community remain unknown.

Chemical defense · Filefish · Hydroids · Predation · Sargassum · UV-sunscreen