Multiple mating, paternity, and body size in a simultaneous hermaphrodite, Aplysia californica

Sperm displacement and sperm competition prove difficult tomeasure, but are crucial elements in predicting sex allocationstrategies of sperm-storing hermaphrodites. Body size is predictedto affect sex allocation so that within a population, largeanimals invest a greater proportion of resources in female functionthan do small animals. These mating strategies depend on spermdisplacement abilities and lead to similar levels of paternityacross body sizes despite differences in resource level. Thepresent study investigated mating patterns, multiple paternity,and sperm competition in a field population of a simultaneouslyhermaphroditic sea slug, Aplysia californica (California seahare). Animals mating in the female role were larger than themean for the population, indirectly supporting theoretical predictionsfor increased investment in female function with body size.However, contrary to predictions, animals mating in the malerole were not different in size from the population mean orthe animals they inseminated. Individual tagging revealed thatsea slugs are capable of moving across distances that allowfor the sampling of many potential mates, and that they materepeatedly in both sexual roles. Microsatellite paternity analysisdemonstrated that multiple mating in the field leads to multiplepaternity, and last-sperm donors achieve high levels of paternity.There was no effect of body size on paternity. Further paternitystudies are needed to reveal the mechanisms of sperm precedencepatterns in A. californica.