The round goby can be identified by the black spot on the back of the anterior dorsal fin, which is absent in other gobies. The base colour of the round goby is usually light, but there are darker spots or patches on the sides and back. During the spawning season, the male is almost black. The round goby can grow up to 25 cm in length.
The round goby is native to the Black Sea and the Caspian Sea regions, from where it has spread to new areas in the ballast waters of ships. The first sighting of the round goby in the Baltic Sea was made in 1990 in Gdansk Bay in Poland. In Finland, it was first found in 2005, then only in 2009, but today it is widely found in the Gulf of Finland and the Archipelago Sea in some places in high densities. The round goby is an invasive alien species, and a round goby caught must not be returned to the water.
