Field evidence of the impact of two acanthocephalan parasites on the mortality of three species of New Zealand shore crabs (Brachyura)

The effects of the acanthocephalan parasites Profilicollis antarcticus and P. novaezelandensis on the fecundity and mortality of three species of shore crab (Macrophthalmus hirtipes, Hemigrapsus edwardsi and H. crenulatus) are examined. The number of eggs produced by female crabs was strongly correlated (all P<0.05) with carapace width; parasite load was not a significant determinant of female fecundity. Mortality was inferred from reduction in the mean number of parasites per crab in the largest crab size-classes, indicating that heavily infected individuals are removed from the population. Mortality attributable to the parasites was observed for all three species of crabs, although the effect of parasites varied in both time and space; significant curvilinear regressions between parasite load and crab size-classes were not found in all samples. Crab mortality appears to be influenced by more than the pathological influences exerted by the parasites. Parasite-induced behavioural alterations may cause crab hosts to be more susceptible to predation by definitive hosts. We support this suggestion with three lines of evidence: the lack of parasite effects on fecundity, the weakening of the parasite effect on mortality during the time of year when birds are absent, and previous indications of parasite-mediated alterations in crab burrowing behaviour.