Anisakid biodiversity in two young harbour seals (Phoca vitulina L.) from coastal South-West Norway

Anisakid nematodes are widespread marine parasites with complex life cycles involving invertebrates and fish as intermediate or transport hosts, and marine mammals as definitive hosts. Despite their ecological importance, and the zoonotic potential associated with the larval stages found in fish, recent data on anisakid species diversity in pinnipeds from Norwegian waters remain scarce. In this study, we investigated anisakid infections in two juvenile harbour seals (Phoca vitulina) stranded along the southern coast of Norway. Gastrointestinal nematodes were collected, morphologically classified to the genus level, and subsequently identified to species level through molecular analyses of mitochondrial (mtDNA cox2) and nuclear (rDNA ITS) markers. Five anisakid species were identified: Contracaecum osculatum sp. A (reported here for the first time in harbour seals), C. osculatum (sensu stricto), Phocanema decipiens (s.s.), P. krabbei, and Anisakis simplex (s.s.). The latter species was found in unexpectedly high abundance and in fully developed adult stages in one of the seals. Notably, these adult A. simplex (s.s.) exhibited large body size, in contrast with previous studies reporting either absence or minimal presence of adults in harbour seals. The underlying mechanisms promoting growth and reproductive development of A. simplex (s.s.) in this host species remain unclear, but may involve a combination of host-specific physiological traits, environmental factors, and parasite phenotypic plasticity. Gross pathological examination revealed multiple gastric and intestinal ulcers in the same seal, including seven crateriform lesions consistent with ulcerative gastritis and enteritis, associated with nematode attachment and feeding. These findings expand the current knowledge on anisakid diversity in P. vitulina and provide novel evidence of its role as a definitive host for A. simplex (s.s.) in Norwegian coastal waters. Furthermore, the results suggest that competitive interactions among anisakid species, combined with ecological and physiological host factors, may facilitate the development and maturation of A. simplex (s.s.) in harbour seals. Further studies are warranted to assess the frequency and health implications of such infections in wild pinniped populations.