While some snakes spend time in water, sea snakes live there permanently. However, instead of gills, they have a single lung, and must surface to breathe about once an hour. Valves keep their nostrils, which sit on top of their snouts, closed the rest of the time. These snakes also absorb oxygen through their skin, and have small, flattened heads, and paddle-like tails to aid with swimming. Most species live in warm waters in the Indian and Pacific Oceans. Sea snakes have very potent venom and release small amounts when biting fish and other prey.