Elemental content of a host-parasite relationship in the threespine stickleback

Parasite infections are ubiquitous and their effects on hosts may play a role in ecosystem processes. Ecological stoichiometry provides a framework to study linkages between consumers and ecosystem process, but the stoichiometric traits of host-parasite associations are rarely quantified. Specifically, whether parasites’ elemental ratios closely resemble those of their host or if infection is related to host stoichiometry remains less known. To answer such questions, we measured the elemental content (%C, %N, and %P) and ratios (C:N, C:P, and N:P) of parasitized and unparasitized Gasterosteus aculeatus (three-spined stickleback) and their cestode parasite, Schistocephalus solidus . Host and parasite elemental content were distinct from each other, and parasites were generally higher in %C and lower in %N and %P. Parasite infections were related to some elemental ratios, specifically C:N, with more intense parasite infections corresponding to hosts with lower C:N ratio. Parasite stoichiometry was independent of their host and there was no relationship between host and parasite stoichiometry. Instead, parasite body mass and parasite density were important drivers of parasite stoichiometry where larger parasites had lower %C, %N, and %P,. Overall, these potential effects of parasite infections on host stoichiometry along with parasites’ distinct elemental compositions suggest parasites may further contribute to how hosts store and cycle nutrients.