In the footsteps of Darwin: dwarf male attachment sites in scalpellid barnacles (Crustacea: Cirripedia: Thoracica) - implications for phylogeny and the evolution of sexual systems

摘要

Since Darwin, the presence of dwarf males in cirripedes has received much attention when trying to understand the evolution of reproductive strategies. The Scalpellidae is a large and monophyletic family of pedunculated barnacles, where the males always attach in confined areas (receptacles) on the rim of the mantle cavity. We used a broad range of microscopic techniques to describe the morphology of the dwarf male receptacle area in 11 species of scalpellids. We correlated this with their diverse habitats and reproductive biology. The purpose was to search for preformations in the receptacle that could assist male settlement and provide the female/hermaphrodite with control over the number eventually to be carried. The species fall into three groups based on their species-specific receptacle morphology and level of preformation. We concluded that small-sized, short-lived scalpellids occupying ephemeral substrata tend to invest more resources in caring for very few males. In contrast, large and long-lived species continuously acquire new males throughout their lifetime without investing any resources in their care. Thus, ecologically different habitats have directed the evolution of different mating systems. We discuss our findings in the light of a recent phylogenetical framework and the evolution of sexual systems.