First Report of Haplosporidium edule Infection in the Olive-Green Cockle (Cerastoderma glaucum) from the Northern Adriatic Sea: Expanding Host Range and Geographic Distribution

Haplosporidium edule is a haplosporidian parasite originally described in the common edible cockle (Cerastoderma edule) along the European Atlantic coast. In this study, we report the first detection of H. edule in the olive-green cockle (Cerastoderma glaucum) from the northern Adriatic Sea, representing both a novel host record and a new geographic occurrence. During a cross-sectional study conducted in May 2019, 90 C. glaucum specimens were collected from three lagoon sites in northeastern Italy. Histological examination of soft tissues revealed haplosporidian developmental stages, including plasmodia, sporoblasts and mature spores, within connective tissues of the mantle, digestive gland, gills and between gonadal tubules in eight individuals from the Goro Lagoon. Molecular characterization based on a fragment of the small subunit ribosomal DNA showed high similarity with the previously published H. edule sequence. Host identification was confirmed through cytochrome c oxidase subunit I barcoding together with morphological and histological analyses. These findings indicate that H. edule has a broader host range than previously recognized. Although prevalence was relatively low, the detection of this parasite in a new host species and geographic area highlights the importance of continued surveillance, particularly in the context of climate change, shellfish translocations and the expansion of aquaculture activities.