Strong association between parasitism and phenotypic variation in a supralittoral amphipod

Phenotypic variation is common among conspecifics from the same population, but its causes are not always clear. Parasites may often play a role, as their ability to induce phenotypic changes in their hosts is well established, though not widely acknowledged among marine ecologists. Here, we tested for a possible role of parasites as determinants of marked colour polymorphism, as well as variation in several distinct behavioural traits in the supralittoral amphipod Transorchestia chiliensis (Talitridae). Our results indicate that the juvenile stages of 2 parasites, the acanthocephalan Plagiorhynchus allisonae and a dilepidid cestode, are disproportionately common in certain colour morphs (i.e. green and dark grey in the case of acanthocephalans and green and blue in the case of cestodes) relative to other colour morphs. In addition, amphipods preferring light over dark background substrate were more likely to harbour acanthocephalans, whereas amphipods that either stayed at the surface or did not burrow very deeply harboured more cestodes than those that burrowed deep into the sediment. Our findings also indicate that later developmental stages of P. allisonae are associated with more pronounced host phenotypic variation, suggesting that phenotypic changes escalate in parallel with parasite growth within the host. Furthermore, the 2 parasite species tend not to co-occur in the same individual hosts; thus, their distinct phenotypic effects apply to different subsets of the host population. We suggest that the role of parasitism in inducing and maintaining phenotypic variation within populations of marine invertebrates is probably more important than currently recognised.