Spatial trends of total mercury (THg) exposure, and the role of intestinal helminths on its distribution within piscivorous mammalian hosts

Abstract

This research project is unique in that it has explored the interface of three broad disciplines: ecology, toxicology, and parasitology. The primary objectives were to determine the role of gastrointestinal (GI) helminths in total mercury (THg) distribution within piscivorous mammalian hosts, and explore the complex interactions that exist within the host GI tract. The project was designed to address these objectives in two pinniped populations: Alaskan ice seals (Phoca largha and Phoca hispida), and California sea lions (Zalophus californianus). Initially, Alaskan gray wolves (Canis lupus) were selected as a reference species for this project, as these animals represent a mammalian definitive host, occupying a top trophic position in a terrestrial food web; nevertheless, preliminary findings demonstrated a subset of these wolves to be subsisting, at least in part, on prey sources of marine origin. Therefore, the project was expanded to include the Alaskan gray wolves as an additional "piscivorous" host for study. At necropsy, host tissues and GI tracts were collected. During GI tract processing, intestinal helminths were removed, weighed, and either saved for identification, or frozen for further analyses. Host tissues (e.g. liver, kidney, cardiac muscle, skeletal muscle), GI lumen contents, and parasites were then analyzed for THg concentrations and stable isotope values (C, N, and S). In Alaskan gray wolves, THg concentrations, and δ13C, δ15N and δ34S isotope values, provided four separate measures supporting the contention that Alaskan gray wolves, with access to marine resources, are relying on piscivory or exploitation of other organisms of marine origin. THg uptake was demonstrated to occur in these animals, and the toxicant-parasite interactions that exist within the GI tract may ultimately affect the host-toxicant interface. The interactions described depend not only on type of parasite and specific toxicant, but also on the complex ecological-like interactions within the host's body. In pinnipeds, parasites were shown to effectively bioaccumulate and/or biomagnify mercury (Hg) within the host GI tract. Within-parasite THg concentrations were not necessarily associated with concentrations in host lumen contents, or host liver and kidney. These data demonstrated that THg distribution in the host is affected by the presence of parasites; consequently, bioavailability of this toxicant to the host may also be affected. A design has been proposed for building an agent-based model, to further explore the interactions described in these studies. This framework will provide a foundation for future work focused on the ecotoxicoparasitology of other related systems.