Moccasin, a poisonous snake related to the copperhead and rattlesnake. It is found in swamps, lakes, and rivers in North America, Mexico, and South America. It is a stout-bodied snake with a wide head and pits beneath its eyes. It feeds on frogs, fish, small snakes, and birds. The water moccasin, also called water adder and cottonmouth (because of its white mouth), is found in the southeastern United States and from southern Illinois west to Texas. It is brown, olive or black above with lighter underparts. There are dark crossbands on the back. It reaches a length of six feet (1.8 m). When alarmed, unlike other venomous snakes, it opens its mouth, showing its fangs. The bite of the water moccasin can be fatal to humans.
The water moccasin is Agkistrodon piscivorus of the pit viper subfamily, Crotalinae, of the viper family, Viperidae.The water moccasin is a North American pit viper that lives in swamps and bayous.