Spatial and seasonal prevalence of Hematodinium perezi in Atlantic blue crabs (Callinectes sapidus) from lagoons of the North-Western Adriatic Sea.

The expansion of the Atlantic blue crab, Callinectes sapidus, along the northern Adriatic coast of Italy is threatening local ecosystems and bivalves aquaculture. This invasive species can host the parasitic dinoflagellate Hematodinium perezi, but no records of this infection in blue crabs exist for the northern Adriatic Sea. To investigate the presence and prevalence of H. perezi in C. sapidus in this region, 209 commercial size specimens collected from seven lagoons were surveyed in spring 2024 using a combination of cytological and histological analyses alongside a specific quantitative PCR assay. Hematodinium perezi was detected in four lagoons with all methods, and the molecular findings were further confirmed by Sanger sequencing. In addition, in vitro culture of the dinoflagellate was successfully obtained, allowing the observation of the parasite life stages and their morphology. Presence of H. perezi showed spatial heterogeneity and its prevalence increased from northern to southern lagoons. In a selected lagoon, monthly monitoring was performed from March 2024 to May 2025 on 450 commercial size specimens and revealed substantial temporal variation in infection prevalence. Hematodinium perezi was detected from December to July while between August and November a substantial reduction of the parasite prevalence was observed. The rapid expansion and high local densities of the Atlantic blue crab likely influenced the observed infection patterns.