Trace element accumulation in parasites of grey seals and harbour seals from the North Sea and Baltic Sea

Environmental pollution is one of the main threats of our time, with anthropogenic processes contributing to elevated inputs of trace elements into marine ecosystems. Trace elements can have toxic effects on marine organisms, thereby underscoring the importance of effective biomonitoring. While parasites have proven to be effective bioindicators in freshwater and marine fish-parasite systems, studies focusing on marine mammals and their associated parasites are limited. Furthermore, earlier studies, mainly on fish, have indicated that parasites may also function as pollutant sinks by reducing element concentrations in infected hosts. This study analyses the accumulation of relevant trace elements in host and parasite tissues of marine mammals, using stranded grey seals (Halichoerus grypus) and harbour seals (Phoca vitulina) from the North Sea and the Baltic Sea. Concentrations of eleven trace elements (Ag, As, Cd, Co, Cu, Fe, Mn, Pb, Sn, Sr and Zn) were quantified in seven host tissues and their helminths (acanthocephalans, cestodes and nematodes) using inductively coupled plasma mass spectrometry (ICP-MS). Parasites, particularly acanthocephalans and cestodes, exhibited higher levels of Co, Mn, Sr and Zn than their hosts, highlighting their potential as bioindicators. Element accumulation varied between parasite taxa, with highest concentrations for most elements found in acanthocephalans or cestodes. Furthermore, regional and infection status-specific as well as species-specific differences were observed in trace element concentrations within host tissues. This study provides the first data on the bioaccumulation potential of helminths from seals and highlights their relevance for future environmental monitoring frameworks.