Larval life history, transmission strategies, and the evolution of intermediate host exploitation by complex life-cycle parasites

Abstract

Complex life-cycle parasites use their intermediate hosts both as an energy source and as a vessel for transmission to the next host in the life cycle. Parasites that grow rapidly to a large size may have high fitness (e.g. time spent in uninfective stages is limited), yet those that grow too aggressively may reduce host viability and their own probability of successful transmission. I examined aspects of both the growth and transmission strategy of an acanthocephalan (Acanthocephalus lucii) in its isopod intermediate host. In an experimental infection, the relative rate of larval parasite growth slowed over time, and eventually parasites seemed to reach a threshold biomass sustainable by their hosts. Consequently, late during this infection parasite growth depended on the level of resources a given host could provide. The rapid, unconstrained growth of young A. lucii appeared worse for isopod viability than the slow, constrained growth of larger parasites. Female A. lucii grew larger than males and their size was more strongly related to host size, suggesting they invest more in growth and are consequently more limited by resources. Patterns of sexual dimorphism across acanthocephalan species suggest that sexual selection driving adult dimorphism may promote sexual divergence in larval growth strategies. Isopods infected with A. lucii cystacanths spent less time hiding and had darker abdominal coloration than uninfected isopods. The magnitudes of these two altered traits were not correlated, even when both traits were measured somewhat repeatably from individual hosts. Parasite-induced host alteration seemed to increase over time as parasites grew. Refuge use by infected isopods decreased over eight weeks, and, in general, the altered coloration of infected isopods seemed to increase with parasite growth. An increasing probability of host mortality, as well as a decreasing potential for additional parasite growth, could favor increased parasitic manipulation of host phenotype.