New invader in the Polish Baltic Sea Proper: phylogeny and global distribution of Calyptospadix cerulea Clarke, 1882 (Cnidaria, Hydrozoa)

Calyptospadix cerulea Clarke, 1882 is a colonial athecate hydrozoan known for forming dense biofouling communities, having broad environmental tolerance and global yet taxonomically obscured distribution. Here, we confirm for the first time its presence in the Polish part of the Baltic Sea (Gulf of Gdańsk), marking a significant range expansion into the Baltic Proper. We support our morphology-based identification, with the first molecular data for C. cerulea , allowing its phylogenetic placement within a clade alongside Bimeria vestita Wright, 1859 and Cordylophora caspia (Pallas, 1771), therefore suggesting reassignment to the family Cordylophoridae. We also reviewed historical occurrence data of C. cerulea , spanning nearly 150 years of research, to provide up-to-date description of its distribution range. In addition, our in situ observations suggest that C. cerulea plays an important role in providing secondary substrate for number of species in benthic environments of the southern Baltic Sea. As C. cerulea is likely well-suited to the Baltic’s variable brackish conditions, its presence raises concerns about potential ecological impacts on native fouling communities and industrial infrastructure. Given its ecological plasticity and expanding range, we emphasize the need for continued monitoring and further research into its population dynamics, ecological interactions, and potential impacts.