Development-related effects of an acanthocephalan parasite on pairing success of its intermediate host

The acanthocephalan parasite, Acanthocephalus dirus, undergoes larval development in the aquatic isopod, Caecidotea intermedius, before completing its life cycle in the intestine of a fish (e.g. sunfish). Inside C. intermedius, the parasite develops through noninfective acanthor and acanthella stages and into the cystacanth stage, which is infective to definitive hosts. Research has shown that individual A. dirus induce phenotypic changes in C. intermedius that increase conspicuousness to definitive hosts, and that the level of modification is most pronounced when the parasite has developed into the cystacanth stage. We examined whether development into the cystacanth stage also correlated with changes in mating behaviour of the host. Using a field survey, we quantified the pattern of development of larval A. dirus to identify specific periods when either infective (cystacanth) or noninfective (acanthor, acanthella) stages dominated. Males infected with cystacanth-stage larvae had lower pairing success than males infected with larvae in the noninfective stages. To identify the behavioural mechanisms underlying this relation (i.e. male response to females, female resistance), we used a field-based experiment to examine the relation between parasite development and mating interactions between males and females. Cystacanth-infected males were less likely to initiate mating attempts with females than were acanthor/acanthella-infected males, and this effect could explain the population-level mating pattern. We discuss the results in the context of both adaptive and nonadaptive mechanisms underlying parasite-mediated changes in mating behaviour.