ASSORTATIVE PAIRING IN GAMMARUS-INSENSIBILIS (AMPHIPODA) INFECTED BY A TREMATODE PARASITE

We have investigated the influence of Microphallus papillorobustus (Tr ematoda) on the reproductive biology and mating patterns of its interm ediate host Gammarus insensibilis (Amphipoda). Infected Gammarus speci es show altered behaviour which renders them more susceptible to preda tion by Charadriiform birds, the parasite's definitive hosts. In a nat ural population of G. insensibilis, mean parasite intensity was higher for unpaired individuals than for paired individuals. Fecundity was r educed in infected amphipods. Size-assortative pairing was significant , although infected males were found with smaller females compared to uninfected males of the same size. There was also a positive assortati ve pairing by parasitic prevalence. Vertical segregation between infec ted and uninfected individuals, male-male competition for access to un infected females, and female choice may explain assortative mating for prevalence. This study provides the first empirical evidence that par asites can have a direct effect on patterns of mating in gammarids.