Round gobies are small, soft-bodied fish, characterized by a distinctive black spot on the first dorsal fin. Their eyes are large and protrude slightly from the top of the head and, like most gobies, round gobies have pelvic fins that are fused to form a single disc on the belly of the fish shaped like a suction cup. Round gobies range in length from 4 to 10 inches (maximum of 9.7 inches, and in weight from 0.176 ounces to 2.816 ounces, increasing as they 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 gray, 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 through the shape of their urogenital papilla, which is white to grey, long and pointed in males, and brown, short and blunt-tipped in females.