Lack of Hematodinium microscopic detection in crustaceans at the northern and southern ends of the Wadden Sea and an update of its distribution in Europe

Hematodinium is a parasitic dinoflagellate with a wide distribution along the European coastline. We investigated whether Hematodinium occurs in the Wadden Sea, a large tidal flat system spanning the Netherlands, Germany, and Denmark, where to date no records exist. In total, we investigated eight different species from six sites at the southern (Texel, the Netherlands) and northern (Sylt, Germany) ends of the system. Based on microscopic hemolymph screening, we did not detect Hematodinium in any of the 1252 investigated individuals. An extensive additional literature review revealed 1489 Hematodinium records in Europe from 14 crustacean species, most locally occurring at high prevalence. This makes our finding of lack of Hematodinium infections surprising and suggests that environmental factors such as a lower salinity may limit the distribution of Hematodinium at the investigated locations at both ends of the Wadden Sea. However, whether the entire Wadden Sea represents a distributional gap for Hematodinium remains to be investigated.