Chemical defenses of three Glossodoris nudibranchs and their dietary Hyrtios sponges

Abstract

Dorid nudibranchs prey upon sponges, tunicates, and bryozoans which generally contain high levels of secondary metabolites. On Guam, the nudibranch Glossodoris pallida was found in Apra Harbor exclusively on a variety of Hyrt~os erecta sponge containing a high concentration of the secondary metabollte scalaradial. Glossodoris hjkeurensis and G. cincta preyed upon a second variety of H. erecta from Cocos Lagoon which did not contaln scalaradial, but rather had high concentrations of heteronemin, a secondary metabolite that was not found in the variety of H. erecta from Apra Harbor. In laboratory tests, G. pallida strongly preferred H. erecta from Apra Harbor while G. hikeurensis and G. cincta preferred H erecta from Cocos Lagoon. H. erecta from Apra Harbor contained 3 major secondary rnetabolites, scalaradial, deoxyscalarin, and scalarin, as well as several minor metabolites. G. pallida sequestered only 2 of these metabolites, scalaradial and deoxyscalarin. H. erecta from Cocos Lagoon contained primarily heteronemin, whereas its predators G. l~ikeurensis and G. cincta contained heteronemin and episcalaradial. Results of feeding deterrence assays with extracts and isolated metabolites differed when tested toward different predators, indicating that these secondary metabolites may be deterrent toward some but not all potential predators Organlc extracts of the Hyrtios sponges were deterrent at half of natural concentrations in some laboratory and field feeding assays while organic extracts of nudibranchs were never deterrent at half of natural concentrations. Heteronemin deterred feeding by fishes in most assays; however, scalaradial did not deter feeding by predatory fishes in most fleld or laboratory assays and was only deterrent at high concentrations in one assay. Minor metabolites or synergistic effects among sponge metabolltes may play a key role in determining feeding deterrence for these sponges. Glossodoris nudibranchs probably excrete or alter some secondary metabolites from their diets.