Interspecific Variation in Palatability Suggests Cospecialization of Antipredator Defenses in Sea Hares

Prey species often deploy different kinds of antipredator defenses, which can interact with each other in ways that are not yet completely understood. Much research into these interactions has utilized gastropod mollusks, usually focusing (in part) on the protective utility of the gastropod shell. This makes the evolutionary reduction of the shell in the opisthobranch gastropods (marine slugs) particularly interesting. This loss of protective function of the shell is associated with the evolution of alternative defenses. Particularly well studied are chemical defenses, especially those using secondary metabolites derived from food resources. As a first step toward understanding interspecific variation in the deployment of multiple defenses, we compared the passive chemical defenses of 3 opisthobranchs (Aplysia californica, Phyllaplysia taylori, and Dolabrifera dolabrifera; hereafter referred to by their genus name). We homogenized the skin and body wall into standardized food pellets, and assayed the palatability of these pellets by measuring their consumption by a common intertidal hermit crab, Pagurus samuelis. This assay revealed significant variation in palatability. Pellets made from the skin and body wall of Phyllaplysia and Dolabrifera had higher palatability, indicating low levels of chemical protection, while pellets from Aplysia showed a 2-fold lesser palatability, indicating greater chemical protection. This phylogenetic variation in palatability is similar to previously reported variation in behavioral sensitization. Although the role of sensitization as a possible antipredator defense is yet poorly understood, these results on palatability raise the possibility that opisthobranchs may cospecialize defenses across species