2003 Multiple mating, paternity and body size in a simultaneous hermaphrodite, Aplysia californica

Sperm displacement and sperm competition prove difficult to measure, but are crucial elements in predicting sex allocation strategies of sperm-storing hermaphrodites. Body size is predicted to affect sex allocation so that within a population, large animals invest a greater proportion of resources in female function than do small animals. These mating strategies depend on sperm displacement abilities and lead to similar levels of paternity across body sizes despite differences in resource level. The present study investigated mating patterns, multiple paternity, and sperm competition in a field population of a simultaneously hermaphroditic sea slug, Aplysia californica (California sea hare). Animals mating in the female role were larger than the mean for the population, indirectly supporting theoretical predictions for increased investment in female function with body size. However, contrary to predictions, animals mating in the male role were not different in size from the population mean or the animals they inseminated. Individual tagging revealed that sea slugs are capable of moving across distances that allow for the sampling of many potential mates, and that they mate repeatedly in both sexual roles. Microsatellite paternity analysis demonstrated that multiple mating in the field leads to multiple paternity, and last-sperm donors achieve high levels of paternity. There was no effect of body size on paternity. Further paternity studies are needed to reveal the mechanisms of sperm precedence patterns in A. californica. Key words: multiple paternity, opisthobranch, sea hare, sex allocation, sperm com-petition. [Behav Ecol 14:554–560 (2003)] Darwin (1874: 260, 613) suggested that traits affectingmating success in one sexual role would not evolve whe