Impacts of host phylogeny, feeding styles, and parasite attachment site on isotopic discrimination in helminths infecting coral reef fish hosts.

Stable isotopes of carbon and nitrogen characterize trophic relationships in predator-prey relationships, with clear differences between consumer and diet (discrimination factor, Δ13C, Δ15N). However, parasite-host isotopic relationships remain unclear, with Δ13C and Δ15N remaining incompletely characterized, especially for helminths. In this study, we used stable isotopes to determine discrimination factors for 13 parasite-host pairings of helminths in coral reef fish. Δ15N differences grouped according to phylogeny and attachment site on the hosts: Δ15N was positive for trematodes and nematodes from the digestive tract and varied for cestodes and nematodes from the general cavity. Δ13C showed more complex patterns with no effect of phylogeny or attachment site. A negative relationship was observed between Δ15N and host δ15N value among different host-parasite pairings as well as within 7 out of the 13 parings, indicating that host metabolic processing affects host-parasite discrimination values. In contrast, no relationships were observed for Δ13C. Our results indicate that host phylogeny, attachment site and host stable isotope value drive Δ15N of helminths in coral reef fish while Δ13C is more idiosyncratic. These results call for use of taxon- or species-specific and scaled framework for bulk stable isotopes in the trophic ecology of parasites.