Evolutionary Perspectives on Chemical Defense in Nudibranch Gastropods (Mollusca: Opisthobranchia)

Nudibranchs are a diverse group of shell-less molluscs (Gastropoda: Opisthobranchia) that inhabit all oceans. With a diet of mostly toxic and chemically well-defended prey, their unique ability to sequester or produce secondary metabolites, or steal venom-containing nematocysts obtained from food make them the focus for examining chemical defenses when mechanical protection from a shell no longer exists. The four nudibranch subgroups – Doridacea, Dendronotida, Arminida and Aeolidida – have varied diets and chemical defense systems, some of which are self-derived and involve biotransformation of chemical compounds. Most nudibranchs spend their lives closely associated with their prey species, requiring chemical cues for settlement and metamorphosis and ultimately enabling them to use their host as both prey and habitat. Largely specialist nudibranch species, such as in genus Phestilla, and their host adaptations and preferences, are excellent subjects for investigating the hypothesis that host use can play a fundamental role in differentiation and speciation. Understanding host preference and prey chemistry could provide insight into the evolution of chemical defense in nudibranch species complexes. This paper reviews the available literature for nudibranch chemical defense mechanisms and the implications for speciation in host-specific predators. Current research in the fields of nudibranch phylogenetics and bioprospecting of their secondary metabolites is also discussed.