Helobdella stagnalis (Hirudinea: Glossiphoniidae), the first facultative mussel-associated leech in Europe

The ecological group of mussel-associated freshwater leeches (Hirudinea) contains facultative and obligate inhabitants of the mantle cavity of freshwater mussels (Unionida) from Asia, Africa, North America, and South America. Here, we report on the record of a mussel-associated leech in Europe and present a list of its freshwater bivalve hosts. The widespread leech species Helobdella stagnalis (Glossiphoniidae) was recovered as a facultative inhabitant of the mantle cavity of the native Anodonta anatina, Unio pictorum, and U. tumidus, and the non-native Sinanodonta woodiana (Unionidae). Globally, the freshwater mussel-associated leech assemblage contains 17 species, including four Helobdella representatives: H. lineata and H. modesta in North America, H. adiastola in South America, and H. stagnalis in Europe.