Phylogenetic Evidence of the Re-Evolution of Planktotrophy in Australian Periwinkles

The mode of development of marine invertebrates has major effects on dispersal and population structure. Species without a feeding larval stage (direct developers) are generally more genetically subdivided than species with planktotrophic (swimming and feeding) development. Evidence of the re-evolution of planktotrophy from a direct-developing ancestor has been accumulating within marine invertebrates. However, it remains unclear whether such evolutionary transitions are rare or common. The Bembicium genus (Gastropoda: Littorinidae) provides an additional opportunity to examine the re-evolution of planktotrophy. The genus includes two species with planktotrophic development (Bembicium auratum and Bembicium nanum) and three species with direct development (Bembicium vittatum, Bembicium melanostoma and Bembicium flavescens). Phylogenetic analyses based on sequences of two mitochondrial markers (COI and 16S rRNA) and one nuclear marker (ANT) provide evidence for a switch from direct development back to planktotrophy. Also, pairwise genetic distances between haplotypes reveal that the two planktotrophic species have greater genetic diversity (at the species level) than the direct developers, and the three direct-developing species are genetically structured based on geographical distance. Our study adds to growing evidence for the re-evolution of planktotrophy and suggests that the genetic basis for a switch between modes of development might be simple in marine snails.