Far away from home: gastrointestinal parasites trace the journey of a grey seal (Halichoerus grypus) from the Wadden Sea to the Mediterranean Sea

Abstract Grey seals, Halichoerus grypus (GSs), inhabit cold temperate and subarctic waters along the North Atlantic Ocean. Individuals of GS can regularly disperse towards southern areas (ca. 38°N–39°N) but occurrence at lower latitudes is exceptional. On 18 February 2022, a 217-cm-long male of GS was detected in waters off the SW Atlantic coast of Spain (37°N), then entered the western Mediterranean Sea and wandered for 15 days until he died. Here, we use gastrointestinal parasites to investigate the geographical origin of the GS and the length of the journey towards Mediterranean waters. Seven helminth taxa were found, namely, the digeneans Ascocotyle septentrionalis and Cryptocoyle lingua, the nematodes Contracaecum osculatum s.s., Anisakis simplex s.s., and A. pegreffii, and the acanthocephalans Corynosoma sp.1 and C. magdaleni (= strumosum) or C. nortmeri. The parasite composition closely resembles that reported in native harbour seals (Phoca vitulina) of the Wadden Sea (ca. 51°–55°N), from where A. septentrionalis is apparently endemic. Considering that (i) A. septentrionalis cannot be acquired out of the Wadden Sea, and (ii) the lifespan and population structure of the parasites found, we infer that the GS came from waters near the Wadden Sea, and the length of the journey was ca. 1 month, with presumably little ingestion of (parasitized) food. To our knowledge, this is the first study using parasites to unveil the geographical mobility of pinnipeds.