sea snake, name for any of the venomous marine snakes of the family Hydrophidae, found in tropical waters of the Indian and Pacific oceans. The sea snake's body is flattened laterally and its oarlike tail is used as a scull. A specialized lung and nostrils with valves enable it to remain submerged for periods of up to 8 hr. Most species are dark above and lighter below, or ringed with black and grayish green. They feed on small fish and are preyed upon by sea birds, sharks, and larger fish. Their potent venom quickly immobilizes their prey; however, they are not aggressive and rarely strike at humans when caught. Most sea snakes are completely marine and lack the enlarged ventral scales that enable land snakes to grip the ground. These snakes bear live young at sea. Most inhabit the shallow waters of the Indonesian region, but the common sea snake, Pelamis platurus, ranges from Madagascar E to Mexico and is sometimes found swarming by the thousands in open ocean. It is black or dark brown above and yellow below and grows up to 3 ft (90 cm) long. A few sea snake species leave the water to lay eggs on coral reefs. These snakes have ventral plates like those of land snakes. Sea snakes are classified in the phylum Chordata, subphylum Vertebrata, class Reptilia, order Squamata, family Hydrophidae.