Trematodes in snail intermediate hosts of a subarctic lake: updated perspectives on molecular diversity

Subarctic freshwater ecosystems have traditionally been considered species-poor in parasites, but recent molecular studies increasingly challenge this view. We present an updated molecular assessment of trematode diversity in molluscan intermediate hosts from Lake Takvatn, northern Norway, more than a decade after the first comprehensive survey (2012–2013) of this system. A total of 2,496 molluscs representing five species were examined in 2024. Molecular analyses of intramolluscan stages revealed 24 trematode species and species-level lineages belonging to seven families. Trematode prevalence in first intermediate hosts reached 12.3% and was higher in Ampullaceana balthica (18.1%) than in Gyraulus acronicus (2.3%), while only a few infections were detected in other molluscan hosts (ten in sphaeriids and one in Valvata sp.). Molluscs also frequently served as second intermediate hosts, particularly for Cotylurus cornutus and Echinoparyphium recurvatum . Despite comparable overall species richness between surveys (both studies reporting 24 taxa), community composition showed apparent turnover, with 15 taxa persisting across surveys, while nine previously recorded taxa were not detected and nine newly recorded lineages were identified, including several potentially new to science. The consistent representation of trematode families between the two surveys indicates long-term structural persistence despite species-level turnover. Ampullaceana balthica remained the main host for trematode transmission, hosting 71% of detected taxa, whereas G. acronicus , previously considered of minor importance, emerged as a significant first and second intermediate host for multiple species/lineages. Our results demonstrate that subarctic lakes can sustain relatively high trematode species richness despite low host diversity, and that trematode assemblages may exhibit temporal changes in composition rather than directional loss of diversity. Host availability, particularly of definitive bird hosts, appears to be a key factor maintaining trematode transmission across high-latitude freshwater ecosystems. These findings highlight the value of snapshot resampling approaches for assessing temporal dynamics of parasite communities in natural systems. • Molecular data revealed 24 trematode taxa parasitising molluscs in a subarctic lake • Decadal comparison shows trematode turnover but stable family structure • Ampullaceana balthica hosted most trematode taxa • Gyraulus acronicus hosts multiple previously undetected lineages • Low-prevalence trematode taxa detected using integrative approach