Sexual competition in an acanthocephalan parasite of fish

Acanthocephalans are polygamous parasites of vertebrates and some species a re known to aggregate in sexual congress to mate. Such a reproductive behav iour could lead to male-male competition for access to females and could ha ve consequences for sexual selection. We dissected 87 gobiid fish, Gobius b ucchichii, harbouring 891 acanthocephalans, Acanthocephaloides propinquus. The parasites were sexed and their body sizes were measured. Testicular vol ume was also evaluated in 82 males in order to establish their phenotypic s exual investment in relation to the estimated sex ratio. We found that para site intensity (i.e. the number of individuals/fish) was not correlated wit h fish size, but that parasite size was significantly related to host size. Our results showed that there was a significant relationship between the m ean female body size and their number within one host. We found that when t he percentage of male parasites in a host increased, presumably increasing male-male competition for access to females, males had a larger testicular volume. We discuss these results in terms of energy allocation, sexual and sperm competition. We conclude that competition for space should be less im portant for males than competition for access to females. Moreover, increas ing testis size should confer advantages to males especially for their repr oductive success when sperm competition occurs.