Diversity of sexual expressions in barnacles

Abstract

As first discovered by Darwin, barnacles (Cirripedia: Thoracica) show diverse sexual systems including simultaneous hermaphroditism, dioecy (separate sexes), and androdioecy (coexistence of males and hermaphrodites). The males are always much smaller than their hermaphrodite or female mates (hence called dwarf males). Here I review recent theoretical and empirical studies that address causes of the diverse sexual systems in barnacles. The size of mating groups (or more accurately, the average number of matings from other hermaphrodites a female-acting hermaphrodite is expected to accept) is identified as the most important environmental factor responsible for the evolutionary transitions of sexual systems in barnacles. I suggest that barnacles offer both theoretical and empirical model systems for understanding the evolution of sexual systems in animals.