Nasal mites (Halarachne halichoeri): In vitro survival rate, prevalence and impact on grey seals in the North and Baltic Sea

Abstract Grey seals in German waters harbour a diverse array of parasites, including the endoparasitic nasal mite Halarachne halichoeri , which underwent a long evolutionary parasite-host adaption in the marine environment. Absent from Central Europe for over a century, H. halichoeri was reported in 2014 in grey and harbour seals from the Baltic and North Sea. Despite existing studies on H. halichoeri , knowledge of its prevalence, lifecycle, and ecological implications remains limited. Between 2019 and 2023, 60 stranded grey seals were examined during post-mortem investigations in the frame of the marine mammal health monitoring of Schleswig-Holstein (SH), Germany. Prevalence of parasitic infections was documented and the intensity of infection was assessed semi-quantitatively. Mucosa was inspected for pathological lesions. Parasites were collected and preserved, and live mites were cultivated in vitro under varying conditions. Results revealed a high prevalence of H. halichoeri with 56% (34/60) of the investigated grey seals from the North and Baltic Sea infected. Infection rates were significantly higher in older grey seals compared to young-of-the-year seals, with the Baltic Sea coast showing the highest number of infected grey seals. Survival times for larvae kept in 0.9% sodium chloride solution were up to 103 days, while larvae cultivated in mucus under in vitro conditions lasted only 24 days. For the first time, moulting of H. halichoeri was reported, with larvae progressing through protonymph, deutonymph, and adult stages. Larval and nymphal stages were highly motile compared to sessile adults, suggesting their role in parasite transmission. Histological examinations revealed rhinitis of varying severity, linked to infection intensity. The increasing prevalence of H. halichoeri compared to previous studies may reflect changes in host population dynamics and may indicate altered host interactions driven by broader ecosystem changes.