Abstract

Although parasites are known to alter the physiological responses of their hosts to toxicants, there are relatively few investigations examining the interface of ecotoxicology and parasitology in marine ecosystems. Mature acanthocephalans, cestodes, and some species of nematodes acquire nutrients from the lumen contents within the gastrointestinal (GI) tract of their pinniped host, with concomitant exposure to toxicants in ingesta through passive or active feeding mechanisms. Mercury (Hg) is an example of a toxicant of concern. Upon entering the food web, methylmercury (MeHg) may bioaccumulate and/or biomagnify in fish and upper trophic-level consumers such as marine mammals. Therefore, understanding effect of parasites on Hg bioavailability to the host, and determining the trophic interactions that exist within an individual, can aid in elucidating the effects of dietary exposure to Hg. The purpose of this study was to assess the distribution of Hg within the GI tracts of California sea lions (Zalophus californianus) and to expand our knowledge about host-toxicant-parasite interactions. Total mercury (THg) was quantified in liver and kidney samples from 22 stranded sea lions (15 male, 7 female) and associated gastrointestinal helminths. Stable carbon and nitrogen isotope ratios of host and parasite tissues were measured to assess trophic status. Prevalence of cestodes, acanthocephalans, and nematodes was 73%, 91%, and 91%, respectively. Mean (range) nematode intensity was 61 (2–422), and acanthocephalans was 135.4 (1–909). Because the integrity of individual cestodes was compromised during freezing, cestode intensity could not be determined. Mean THg concentrations in acanthocephalans, cestodes, and nematodes were 371.9 (range: 33.1–2,174.3), 87.5 (range: 46.32–201.5), and 188.8 (range: 40.0–1,135.6) ppb wet weight (ww), respectively. Concentrations were significantly higher in host liver (p < 0.01; range: 1,623–34,946 ppb, ww) relative to other tissues, and were higher in large intestine (LI) acanthocephalans, relative to parasitic groups from other locations or host GI samples. Bioaccumulation factors (BAFs) (the concentration of a toxicant in an organism divided by its concentration in an organism's food) demonstrated THg uptake by parasites. Maximum BAFs were up to 15 times higher in gastric nematodes and LI acanthocephalans relative to other parasites in the small intestine. Parasite nitrogen isotope values ranged from 13.8–17.7‰, potentially suggesting differences in trophic position among parasites. These data confirm that parasites effectively bioaccumulate and/or biomagnify Hg. However, there is great variability in within-parasite THg concentrations, irrespective of concentrations in host lumen contents, liver, or kidney. THg distribution within the host is likely altered by the presence of parasites, and bioavailability of THg to the host may be affected by the parasitic fauna. Further study on the balance of ecotoxicoparasitological interactions within pinniped hosts is, therefore, warranted.