Sexual selection in a simultaneous hermaphrodite with hypodermic insemination: body size, allocation to sexual roles and paternity

Theory predicts that variation in body size within a population of simultaneous hermaphrodites should affect sex allocation, leading to individual differences in mating strategies and increased investment in female function with size. Small animals with fewer resources should invest proportionally more of their resources in male function than large animals, resulting in sperm displacement and paternity patterns that are independent of body size. This study investigated the effect of body size on mating patterns, egg production (an indirect measure of sperm transfer) and paternity in Alderia modesta, a sperm-storing, hermaphroditic sea slug with hypodermic insemination. The relative sizes of two hermaphrodites affected the probability and duration of inseminations; smaller animals inseminated larger mates for longer than vice versa. Sperm transfer began at a smaller size and age than egg production, and estimates of both sperm transfer and egg production increased with body size. Paternity patterns varied widely; in this species, unpredictable sperm precedence patterns may be a consequence of hypodermic insemination and the lack of a well-defined sperm storage organ. Hypodermic injections across all sections of the body successfully transferred sperm and fertilized eggs. The function and consequences of hypodermic insemination are discussed.