The evolution of mating systems in barnacles
Abstract
Thoracican barnacles exhibit tremendous diversity among species in reproductive life histories, including hermaphroditism, dioecy and androdioecy. Androdioecy (where populations contain a mix of males and hermaphrodites) is rare among animals, but has been described in more than 30 species of barnacles and appears to have evolved independently at least four times in this clade. Several models have been suggested to explain the evolution and stability of different mating systems in barnacles, including the relatively high frequency of androdioecy. We review this body of theory and compare predictions to existing empirical data on interspecific patterns of sex allocation. Allocation to male and female reproduction may also vary within barnacle species, and these patterns may provide additional insight into the evolutionary forces that select for different mating systems. Theory predicts that for simultaneous hermaphrodites, the optimal allocation to male function should increase with the size of mating groups, due to sperm competition. This prediction is supported by the only previously published study to address this issue. We present new data from another intertidal barnacle (Tetraclita rubescens) that do not support the predicted association between investment in male reproduction and local density of conspecifics. We suggest that this departure from the expected relationship may arise from a reduced importance of sperm competition in this species. Despite over 150. years of interest in barnacle mating systems, many questions remain unanswered. We argue that barnacles provide a superb opportunity to test predictions of sex allocation theory and we offer suggestions for promising areas of future research. © 2010 Elsevier B.V.