Parasites interact with the cycling of elements such as N and P by responding to environmental nutrient availability or altering host nutrient recycling function. Data on these topics are available for only a few host-parasite species pairs, and they reveal species-specific variation in the interactions between parasites and nutrients. Ecological stoichiometry provides a framework for quantifying the exchange of multiple elements in host-parasite interactions. Measuring the ratios of elements composing parasite tissues and assessing the factors that drive variation among species will be useful for understanding interspecific differences in parasite nutritional demand, the magnitude of resource extraction from hosts, and the functional importance of parasitism to nutrient recycling. Despite growing interest in the ecological stoichiometry of parasitism, few studies have measured the elemental content of parasite tissues, and none have done so for multiple parasite species. In this study, we measured the elemental content (%C, N, and P) and molar ratios (C:N, C:P, N:P) of a diverse assemblage of parasitic helminths, then asked what ecological and evolutionary variables were linked to stoichiometric variation among species.
We sampled 27 macroparasite taxa, spanning 4 phyla, from 16 host species inhabiting freshwater ecosystems in New Jersey. Macroparasites varied widely in their elemental content, exhibiting four-fold variation in %N and five-fold variation in %P. Across all species, parasite body size had a strong negative relationship with %P and positive relationships with N:P and C:P. This pattern supports the growth rate hypothesis, which predicts that smaller taxa require more P to support their relatively higher growth rates. Parasite phylum was also an important determinant of %P, C:P, and N:P. Life cycle stage and infection location were related to variation in %N and %P, respectively. Parasite functional feeding group and trophic level were not related to any stoichiometric variables. Phylogenetic correction for all statistical analyses will be necessary to determine the independence of these patterns from parasite phylogeny. This project is the first to document variation in the organismal stoichiometry of parasites, which will be important to understanding relationships between parasitism and nutrient cycling. Our results provide support for the growth rate hypothesis and identify several phylogenetic and life history variables linked to variation in parasite stoichiometry.