Caenogastropods, members of the largest clade of shelled snails including most familiar marine taxa, are abundant and diverse, and yet surprisingly little is known about their reproduction. In many families, even the basic anatomy has been described for fewer than a handful of species. The literature implies that the general sexual anatomy and sexual behavior do not vary much within a family but for many families this hypothesis remains untested. Available data suggest that aphally, sexual dimorphism, maternal care, and different systems of sex determination have all evolved multiple times in parallel in caenogastropods. Most evolutionary transitions in these features have occurred in non-neogastropods (the taxa formerly included in the Mesogastropoda). Multiple origins of these features provide the ideal system for comparative analyses of the required preconditions for and correlates of evolutionary transitions in sexual strategies. Detailed study of representatives from the numerous families for which scant information is available and more completely resolved phylogenies are necessary to significantly improve our understanding of the evolution of sexual systems in the Caenogastropoda. In addition to basic data on sexual anatomy, behavioral observations are lacking for many groups. What data are available indicate that mate choice and sexual selection are complicated in gastropods and that the costs of reproduction may not be negligible.
