The Yellow-bellied sea snake is a venomous snake found in tropical oceanic waters around the world except for the Atlantic Ocean. It is one of the most widely distributed snakes in the world. As the name implies, this snake has a distinctive bicolor pattern with a yellow underbelly and brown back, making it easily distinguishable from other sea snake species. Colors of the snake are variable, but most often distinctly bicolored, black above, yellow or brown below; ventrally, there may be a series of black spots or bars on the yellow or brown background, or the yellow may extend dorsally so there is only a narrow middorsal black stripe, or a series of black crossbars. Yellow-bellied sea snakes are fully adapted to living their whole lives at sea. Adaptations to aquatic life include the reduced ventral scale size, laterally compressed body and paddle-tail for swimming, valved nostrils and palatine seal for excluding seawater, and cutaneous gas exchange for prolonging dive times. These snakes can uptake up to 33% of their oxygen requirements through the skin while diving and swimming at the surface of the water. Sea snakes also have a special salt gland located in the lower jaw that was formerly believed to filter out salt from the surrounding seawater but has been found not to be used for that purpose, as sea snakes drink fresh water only.
Yellow-bellied sea snakes have an extensive distribution covering the entire tropical Indo-Pacific, as well as extending to Costa Rica, Southern California, and northern Peru. It is the only sea snake to have reached the Hawaiian Islands. The species is the most commonly beached sea snake on the coast of Southwest Australia, including records at beaches near metropolitan areas. These sea snakes have also been reported in colder waters such as the coasts of southern California, Tasmania, and New Zealand. On the African Atlantic coast, they have been reported to occur in the Agulhas Current, along the coasts of South Africa and Namibia. Yellow-bellied sea snakes are completely pelagic and are often observed on oceanic drift lines, using surface currents and storms to move around the ocean. Their distribution appears to be largely determined by favorable water temperatures, oceanic currents and the recent formation of land bridges that have blocked farther dispersal. The favored habitat for hunting and reproduction includes free-floating mats of sea kelp occurring in the Indian Ocean.
Yellow-bellied sea snakes live in the open ocean; on land, they move poorly due to their smaller belly scales that form a ventral keel. These sea snakes are sometimes observed in large aggregations of thousands on the surface of the water in oceanic drift lines, which has been proposed as a strategy to catch prey. They hunt by floating on the surface of the water to attract pelagic fish that are seeking shelter; prey is captured via a backward swimming motion and rapid lunge of the jaws. The ability to swim backward is an unusual and distinguishing characteristic of this species. It is suggested that these snakes find their prey by sensing the vibration generated by fish movement. They hunt during the day and spend nights on the ocean bottom. Yellow-bellied sea snakes can stay underwater up to 3 hours sometimes rising to the surface to breathe.
In warmer seas, Yellow-bellied sea snakes breed year-round. They are ovoviviparous and females give birth to 2-6 live young. The gestation period lasts around 6 months. Newborn baby snakes measure around 220-250 mm in total length and are able to feed on their first day of life.
There are no major threats to Yellow-bellied sea snakes at present. However, they do suffer from pollution, bycatch and from being trapped by ghost fishing nets.
According to IUCN, the Yellow-bellied sea snake is fairly common and widespread throughout its range but no overall population estimate is available. Currently, this species is classified as Least Concern (LC) on the IUCN Red List and its numbers today are stable.