Are all brachyuran crabs found in the intertidal zone intermediate hosts for digenean parasites?

Digenean trematodes with complex life cycles often use marine crabs as intermediate hosts, but their distribution across crab species is not fully understood. Previous reports of Microphallus similis in edible crabs (Cancer pagurus) relied on morphological identification, leaving potential for misidentification. This study aimed to investigate the prevalence, identity, and host range of digenean parasites in intertidal brachyuran crabs from South Wales, U.K. Over six months, crabs were collected from two rocky shore sites and examined for metacercariae in the hepatopancreas. Parasites were morphologically assessed and identified by sequencing the 28S rDNA region. Metacercariae were found exclusively in juvenile C. pagurus, with ∼ 30 % prevalence and low individual burdens ranging from 1 to 69 cysts. All sequenced parasites were confirmed as M. similis. No infections were detected in Necora puber, Xantho pilipes, or X. hydrophilus. Logistic regression indicated that infection prevalence in C. pagurus varied significantly with month and crab size. This study provides the first molecular confirmation of M. similis in edible crabs from the U.K. and highlights species-specific susceptibility linked to differences in ecology and feeding behaviour. The absence of infection in co-occurring crabs suggests that C. pagurus plays a uniquely important role in the parasite's transmission cycle in intertidal environments.