About Hydrophis platurus (Linnaeus, 1766)
Hydrophis platurus, commonly the yellow-bellied sea snake, has a distinctive bicolor pattern with a yellow underbelly and brown back, which makes it easy to tell apart from other sea snake species. Like many other sea snake species, yellow-bellied sea snakes are fully adapted to spend their entire lives at sea, mating, feeding, and giving birth to live young in an ovoviviparous reproductive process. Adaptations for aquatic life include reduced ventral scale size, a laterally compressed body, and a paddle-shaped tail for swimming; valved nostrils and a palatine seal to keep seawater out; and cutaneous gas exchange to extend how long the snake can dive. This species can meet up to 33% of its oxygen requirements through its skin while diving and swimming at the water surface. Sea snakes have a special salt gland located in the lower jaw; this gland was formerly thought to filter salt from surrounding seawater, but research has found it is not used for this purpose, as sea snakes only drink fresh water. For terminology related to snake scales, see snake scales. Morphologically, this snake has a compressed body, with the posterior section less than half the diameter of the neck. Its body scales are juxtaposed and subquadrangular in shape, arranged in 23–47 rows around the thickest part of the body. Ventral scales number 264–406, are very small, and if distinct, divided by a latitudinal groove; they are usually indistinguishable from adjacent body scales. The head is narrow, with an elongated snout. Head shields are entire, nostrils are positioned superiorly, and nasal shields contact one another. The prefrontal scale contacts the second upper labial. There are one or two preoculars, two or three postoculars, two or three small anterior temporals, and seven or eight upper labials. Four or five upper labials sit below the eye, but are separated from the eye border by a subocular. Coloration is variable, but most often the snake is distinctly bicolored: black above, yellow or brown below, with a sharp line dividing dorsal and ventral colors. On the yellow or brown ventral background, there may be a series of black spots or bars. Alternatively, the yellow color may extend onto the dorsal side, leaving only a narrow middorsal black stripe, or a series of black crossbars. Maximum total length is 720 mm (28 in) for males and 880 mm (35 in) for females. Maximum tail length is 80 mm (3.1 in) for males and 90 mm (3.5 in) for females. The yellow-bellied sea snake is one of the most widely distributed snakes in the world. It is completely pelagic, and is often observed on oceanic drift lines, using surface currents and storms to move across the ocean. Its distribution is largely determined by favorable water temperatures, oceanic currents, and recently formed land bridges that block further dispersal. It has an extensive distribution covering the entire tropical Indo-Pacific, and extends to Costa Rica, southern California, and northern Peru. It is the only sea snake that has reached the Hawaiian Islands. Its preferred habitat for hunting and reproduction includes free-floating mats of sea kelp found in the Indian Ocean. It is the most commonly beached sea snake on the coast of Southwest Australia, including records at beaches near metropolitan areas. It has also been reported from Australia's Christmas Island and Cocos (Keeling) Islands. Long-term survival for this species requires a minimum water temperature of 16–18 °C (61–64 °F). Even so, the species has been reported in colder Pacific waters, including the coasts of southern California, Mexico, Tasmania, and New Zealand. New Zealand would otherwise have no native snakes if not for infrequent strandings of yellow-bellied sea snakes and banded sea kraits. Despite being made up of wayward individuals, these occurrences make the yellow-bellied sea snake the most commonly seen snake (and most commonly seen sea snake) in New Zealand. As a result, the species is considered native (indigenous) to New Zealand and is protected under the Wildlife Act 1953. These occurrences in colder water are believed to be linked to El Niño and other severe weather events, which may create unusually strong new ocean currents that transport the snakes far off their normal course. In October 2015, a beached yellow-bellied sea snake was reported and photographed in Ventura County, California, well outside the species' normal range, for the first time in 30 years. A few months later, in January 2016, a stranded individual was found washed up on the north end of Coronado Beach (better known as Dog Beach) in Coronado, California, just south of Naval Air Station North Island (NASNI). The specimen was later transported to the Scripps Institute of Oceanography in La Jolla for examination. The yellow-bellied sea snake is the only sea snake that has been found in the Atlantic Ocean, though this only occurs in limited circumstances. Its presence in the Atlantic is not considered part of its native range, but rather the result of dispersal from its native Pacific range. It has been found in all countries along Africa's eastern coast and all eastern African islands, including Djibouti, Eritrea, Kenya, Madagascar, Mauritius, Mayotte, Mozambique, Réunion, Seychelles, Somalia, South Africa, and Tanzania. On Africa's Atlantic coast, it has been reported in the Benguela Current, with specimens found along the coasts of South Africa and Namibia. It has also been found four separate times in the Colombian Caribbean, making it the only sea snake recorded in the Caribbean Sea. However, these occurrences are believed to result from human activity, such as ship discharge, intentional release, or passage through the Panama Canal, and the species is not considered native to the Caribbean. The Isthmus of Panama, the land bridge between North and South America that formed between roughly 10 million and 3 million years ago, acts as a dispersal barrier that prevents the species from entering the Caribbean from the Pacific. The man-made Panama Canal is presumed to not allow crossing because it is filled with fresh water. Due to the species' wide distribution and relative lack of dispersal barriers, it has long been assumed that individuals from different localities belong to a single breeding population, meaning there is high gene flow between groups. However, a study using haplotype networks on two populations from Costa Rica found shallow genetic population structure that aligns with variation in color patterns: brown and yellow in Golfo de Papagayo, and completely yellow in Golfo Dulce.
