Marine ecosystems are increasingly affected by anthropogenic stressors, including trace elements (TE), highlighting the need for sensitive bioindicators. Fish parasites have proven to be effective bioindicators for TE accumulation, while information on TE levels in parasites of marine mammals remains scarce. The present study combined measurements of TE levels in parasites of fish and mammals. We investigated different developmental stages of the same nematode species, Anisakis simplex, in their different hosts with trophic interactions of larval and adult stages from their respective hosts, the North Sea herring (Clupea harengus) and harbour seals (Phoca vitulina), sampled in the North Sea. Both larval and adult A. simplex accumulated selected TE at higher levels than their hosts, particularly non-essential elements in seals, resulting in high bioconcentration factors. Adult A. simplex exhibited significantly higher concentrations of several essential elements compared to the larval stages, except for Ag, which was accumulated at higher levels in larvae. Stable isotope analysis revealed a consistently lower level of δ15N in both larval and adult A. simplex relative to host tissues. This pattern suggests that adult A. simplex may not primarily feed on host tissues and may instead rely partly on host metabolites, such as ingested food items in the seal stomach. Moreover, elevated δ13C indicates the utilization of distinct carbon sources by the parasites compared to their hosts. These results indicate that TE accumulation in A. simplex is influenced by both developmental stage and host species, supporting their value as stage-specific bioindicators in marine pollution monitoring.
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