Effect of acanthocephalan parasites on the behaviour and coloration of the mud crab Macrophthalmus hirtipes (Brachyura: Ocypodidae)

In the field, the numbers of cystacanths of the parasitic acanthocephalan Profilicollis spp. harboured by crabs are relatively high and correlate with carapace width. In a field experiment, the responses of crabs to the simulated approach of a bird predator (the parasite's definitive host) was not influenced by the number of acanthocephalans they harboured. Crabs that were exposed at the surface of the sediments during receding high tide, however, tended to harbour more parasites than nearby crabs hidden in burrows. An analysis of colour patterns on the carapace of crabs showed that infection levels did not influence carapace pigmentation, and thus did not affect the conspicuousness of a crab relative to the background environment. However, the likelihood of a male crab winning a ritualized fight against a conspecific in the field was associated with its infection level, but in a way that suggests that this finding is a consequence of pathology rather than an adaptation of the parasite to increase its transmission rate. Although only weak evidence was found indicating that Profilicollis manipulates the behaviour or colour of its host to its own benefit, the high infection levels observed suggest that the crab population acts as a major reservoir for larval stages of this parasite that are infective to birds.