The effects of parasitism on sex allocation of a hermaphroditic acorn barnacle

Sex allocation theory predicts the adaptive allocation of resources to male versus female reproduction in simultaneous hermaphrodites in response to individual characteristics or environmental factors. Because parasites uptake resources from their hosts, their presence could affect the sex allocation of the hosts. We investigated the effects of infestation status and infestation intensity by the rhizocephalan barnacle Boschmaella japonica on reproduction, including sex allocation, of the host intertidal barnacle Chthamalus challengeri. Feeding activity was also examined as a factor related to resource intake. Both male and female reproductive investment decreased with increasing parasite infestation, and the sex allocation of large infested hosts was more male-biased than that of large uninfested hosts. Moreover, in contrast to the model prediction that male investment does not change under resource limitation, male investment decreased in infested hosts whose resources were taken by parasites. This reduction in male investment could be explained by changes in mating group size, since infested hosts have shorter penises and consequently are able to access fewer mating partners.