Abstract

Opisthobranch gastropods and their marine pulmonate relatives have tended to lose their shells as a consequence of being protected by chemical defense. Metabolites obtained from food have been modified and deployed adaptively.The animals have sometimes evolved the capacity to synthesize metabolites that were originally obtained from food. Some evidence suggests that this capacity has evolved beginning with an initial stage in which only the end product is synthesized, followed by a series of later innovations in which precursors of that end product are added working backward.There is a complex history of changes in what the animals eat and how they utilize metabolites defensively.When a change in feeding habits has deprived the animals of their original defensive metabolites, other compounds are often pressed into service. Among these are polypropionates, which are not biosynthesized by any other eukaryotes.The polypropionates probably exist at low concentration and have some other function in animals that do not use them defensively. There is rigorous and compelling experimental support for the biosynthesis of metabolites by the opisthobranchs themselves. An herbivorous common ancestor has given rise to many herbivorous lineages and to a wide variety of carnivores. Diversification has to some extent corresponded to the taxonomy of the food source, but the animals have often come to exploit unrelated food organisms that share the same metabolites or have a similar texture. The remarkable adaptive radiation of these animals can be explained as a result of their capacity to innovate in how they utilize their food sources and deal with secondary metabolites.