Site-level variation and molecular identification of dual Microphallus infections in shore crabs (Carcinus maenas).

Digenean trematodes commonly use marine crustaceans as intermediate hosts, yet their distribution and host-specific dynamics remain underexplored in temperate European systems. In this study, metacercarial stages of Microphallus similis and M. primas were identified in the hepatopancreas of Carcinus maenas collected from two distinct locations in South Wales, UK across a 4-month period. M. similis exhibited consistently high prevalence (~ 76%) at both sites, while M. primas occurred only as a co-infection, with lower prevalence (2-9%) and infection intensity. At Oxwich Bay, crab size and coloration were significantly associated with trematode infection, whereas no such associations were observed at Mumbles Head. Phylogenetic analysis confirmed species-level divergence between M. similis and M. primas. Overall parasite burden was significantly higher at Mumbles Head, with larger crabs harbouring more metacercariae. Additionally, metacercariae from Mumbles Head were significantly larger than those from Oxwich Bay, suggesting site-specific variation in parasite development. The higher prevalence, intensity, and cyst size at Mumbles likely reflect habitat and host-community differences between sites, including greater availability of first-intermediate hosts and differences in crab size cohorts and microhabitats that may influence transmission.