The round goby (Neogobius melanostomus) is a fish. Defined as a euryhaline bottom-dwelling goby of the family Gobiidae, it is native to Central Eurasia, including the Black Sea and the Caspian Sea. Round gobies have established large non-native populations in the Baltic Sea, several major Eurasian rivers, and the North American Great Lakes. Characteristics Round gobies are small, soft-bodied fish characterized by a distinctive black spot on the first dorsal fin. The eyes are large and protrude slightly from the top of the head and, like most gobies, the pelvic fins are fused to form a single disc (shaped like a suction cup) on the belly. Round gobies range in length from , with a maximum size of . They weigh between , their weight increasing with age. Male round gobies are larger than females. Juvenile round gobies (less than one year old) are grey. Upon maturation, round gobies become mottled with grey, black, brown, and olive green markings. Adult male round gobies turn inky black during the spawning season and develop swollen cheeks. Male and female round gobies are easily differentiated by the shape of their urogenital papilla, which in males is white to grey and long and pointed and in females is brown, short, and blunt-tipped. Distribution and habitat Round gobies are widespread in the Sea of Marmara and in the rivers of its basin and can also be found in the Black Sea and the Sea of Azov, along all coasts and fresh waters of their basins as well as in the coastal lakes and lagoons. They are also found in the rivers of Crimea and the Caucasus (Mezib, Pshada, Vulan, Kodori, and Çoruh) and in the Caspian Sea, represented by subspecies Neogobius melanostomus affinis. Since 1990, the round goby has been registered as introduced in the North American Great Lakes, in parts of Europe, and in the Baltic Sea as an invasive species. The first catch in North America was documented by Jude et al 1992 and Crossman et al 1992, caught by an angler in Sarnia, Ontario, fishing the St. Clair River on June 28, 1990. The studies of Jude, Crossman, together with Jude et al 1995 found a range of sizes between in the St. Clair. Round gobies are also rapidly expanding into tributaries of the Great Lakes in North America and were recently discovered in at least one of the Finger Lakes in