Multiple mating affects offspring size in the opisthobranch Chelidonura sandrana

Offspring size can have pervasive effects throughout the life history stages of many marine invertebrates. Although maternal offspring investment is largely determined by the environmental conditions experienced by the mother, egg size might additionally vary in response to the number and quality of previous mating partners. Positive effects of mating multiply with several different males (polyandry) have been confirmed for a variety of species, whereas such investigations are lacking for marine invertebrates. Here we differentiated between the effects of ejaculate amount (repeatedly mated) and ejaculate diversity (polyandry) on maternal offspring investment in the simultaneously hermaphroditic sea slug Chelidonura sandrana. We found that focal “females” mated with four different “males” produced significantly larger egg capsules and larger veligers, while focal “females” mated four times with the same “male” suffered from reduced mid-term fecundity. We found no effect of veliger size on veliger survival. Our results show that female mating patterns are an important addition to understanding the variation in offspring size in internally fertilizing marine invertebrates.