Abstract
We investigated the influence of Microphallus hoffmanni (Trematoda) on the survival, mating success, and fecundity of its intermediate host, Gammarus aequicauda (Amphipoda). Despite the great size of M. hoffmanni metacercariae, the mean parasite abundance and the ratio of the variance to the mean abundance increased with host size, suggesting that accumulation of this parasite had no significant effect on the survival of its host. Size-assortative pairing was significant and we found no evidence of a parasite-mediated reduction in male competitiveness. Fecundity was reduced in infected females. Our results are in contrast to those obtained with helminths that alter the intermediate host's behaviour to favour their transmission to the definitive host.