the Gulf of Trieste. Usually, only adult individuals have warts on the body, although they were present on the observed individuals. Most probably, individuals of M. leidyi were released with ballast water originating from the Black Sea, as there is a direct connection between the Port of Koper and various Black Sea ports (David and Perkovic, 2004; Alexandrov, 2004; Matishov et al., 2005; David et al., 2007).