Love the one you're with: proximity determines paternity success in the barnacle Tetraclita rubescens.

A species' mating system sets limits on the strength of sexual selection. Sexual selection is widespread in dioecious species, but is less well documented in hermaphrodites, and may be less important. We used four highly polymorphic microsatellite markers to assign paternity to broods of the hermaphroditic eastern Pacific volcano barnacle Tetraclita rubescens. These data were used to describe the species' mating system and to examine factors affecting male reproductive success. Tetraclita can sire broods over distances of 11.2 cm, but proximity to the sperm recipient had a highly significant effect on the probability of siring success. There was no effect of body size or the mass of male reproductive tissues on siring success. Broods showed relatively low frequencies of multiple paternity; even at high densities, 75% of broods had only one father. High frequencies of single-paternity broods imply either that this species does not compete via sperm displacement, or that sperm displacement is extremely effective, potentially explaining the lack of a positive relationship between male investment and paternity. In addition, there was low variance in siring success among individuals, suggesting a lack of strong sexual selection on male traits. Low variance among sires and the strong effect of proximity are probably driven by the unusual biology of a sessile copulating species.