First Record of Hepatospora eriocheir Infection in the Chinese Mitten Crab Eriocheir sinensis in the Baltic Sea

Hepatospora eriocheir is a microsporidian parasite of the Chinese mitten crab Eriocheir sinensis, an invasive decapod now widely distributed in European inland and coastal waters. Although the host is common across much of Europe, confirmed European records of H. eriocheir have remained scarce and, until now, have not included the Baltic Sea region. In this study, 15 adult E. sinensis collected from the Vistula Lagoon, southern Baltic Sea, were examined using gross pathological assessment, wet-mount microscopy, histopathology, PCR amplification, Sanger sequencing, and phylogenetic analyses of parasite SSU rRNA and host COI sequences. Hepatopancreatic alterations were observed in several individuals, ranging from pale discolouration and friability to loss of normal tissue organisation. Spores were detected in fresh squash preparations from affected tissue, and histology revealed epithelial disruption, intratubular spore accumulation, and necrotic changes consistent with progressive microsporidian infection. Molecular screening confirmed H. eriocheir in 60.0% of crabs, with positive cases occurring in both females and males. The parasite sequences formed a single, well-supported clade and were highly similar to previously reported H. eriocheir sequences from the United Kingdom and China. Host COI sequences represented three mitochondrial haplotypes, indicating that the infection occurred across more than one host mitochondrial haplotype background. These findings constitute the first record of H. eriocheir in E. sinensis from the Baltic Sea and support the hypothesis that infected crabs reaching the Vistula Lagoon are connected with the wider North Sea invasion system rather than an isolated Baltic lineage.