First report of Haplosporidium edule infection in the olive green cockle (Cerastoderma glaucum) from the northern Adriatic Sea: expanding host range and geographic distribution.

INTRODUCTION Haplosporidians are a group of small protozoan parasites belonging to the phylum Haplosporidia, widely distributed in aquatic environments (Burreson et al., 2004. Aquat. Living Resour., 17: 499–517). More than 40 species have been described within 4 genera: Haplosporidium, Minchinia, Bonamia and Urosporidium, although their taxonomy is still evolving (Ward et al., 2019. Dis. Aquat. Organ., 133: 57–68). These parasites are typically extracellular and characterized by multinucleate plasmodial stages and spores with a distinctive apical orifice (Reece et al., 2025. Dis. Bivalves Hist. Curr. Perspect., 95–116). Their life cycles remain only partially resolved (Arzul et al., 2015. J. Invertebr. Pathol., 131: 32–42), but epidemiological evidences suggest that haplosporidians may be transmitted directly between hosts, even in non-spore-forming species (Carnegie et al., 2006. J. Eukaryot. Microbiol., 53: 232–245). While some species are associated with severe diseases and mortality events in bivalves, others show low pathogenicity and limited host impact. Among them, Haplosporidium edule has been previously reported in Cerastoderma edule along the Atlantic coasts of Europe (Azevedo et al., 2003. Eur. J. Protistol., 39: 161–167). MATERIALS AND METHODS A cross-sectional study was conducted on 90 specimens of Cerastoderma glaucum collected from three lagoon sites in the northern Adriatic Sea. Thirty viable individuals per site were selected and analysed through histological and molecular methods. Tissue samples (mantle, gills, digestive gland and gonads) were processed for histopathology, while DNA was extracted for PCR amplification targeting the SSU rDNA region of haplosporidians. Positive samples were sequenced and analysed through BLAST comparison and phylogenetic reconstruction to confirm parasite identity. RESULTS Haplosporidian parasites were detected in 8 specimens from the Goro lagoon. Histological observations revealed plasmodia, sporonts and sporoblasts within connective tissues of multiple organs, with infections ranging from light to heavy. Molecular analysis confirmed the parasite as Haplosporidium edule. This finding represents the first report of H. edule in Cerastoderma glaucum and its first detection in the Mediterranean Sea. Despite its wide tissue distribution, infections were not associated with significant lesions or inflammatory responses. CONCLUSIONS This study expands both the host range and geographic distribution of Haplosporidium edule. The low prevalence and absence of marked pathology suggest a currently low pathogenic impact in C. glaucum, possibly reflecting an opportunistic host–parasite relationship. Nevertheless, the emergence of this parasite in a new host and environment highlights the need for ongoing monitoring, particularly considering environmental changes and shellfish translocation. These findings contribute to improving the understanding of haplosporidian ecology, epidemiology and their potential implications for bivalve populations and aquaculture.