Invading Europe: the tropical aquatic worm Branchiodrilus hortensis (Stephenson, 1910) (Clitellata, Naididae) extends its range.
We report the occurrence of non-indigenous Branchiodrilus hortensis (Stephenson, 1910) in Serbia and its rapid spread in Southeast Europe. These new findings were observed at locations about 1000 Rkm downstream from the nearest known record in the Danube (in July 2008) in the middle part of the river section in the Slovak- Hungarian part. Branchiodrilus hortensis was registered for the first time in Serbia in September 2018 during routine monitoring of the Danube River in the lower part of the river section. Since then, we have continuously found this species in the Serbian part of the Danube, and it has spread to the main tributaries and canals of the Danube basin. These new records are a valuable contribution to the knowledge of this species' distribution. Recent findings and the native distribution of this worm suggest that its introduction in the Danube was probably human-mediated, and it has successfully established its niche in the new environment. At this phase we cannot predict the possible effects of its presence on aquatic ecosystems and therefore further monitoring of its distribution and population dynamics is necessary.