The Yellow-bellied Sea Snake has the distinction of being the most widely ranging snake in the world as well as the most aquatic, never having to set scale on land or sea floor its entire pelagic life. The Yellow-bellied Sea Snake is a moderately-built snake with an elongated head distinct from the body. The upper half of the body is black to dark blueish-brown in color and sharply delineated from the yellowish lower half. The tail is paddle-shaped and yellow with dark spots or bars. Body scales are small, smooth and hexagonal in shape; the head scales are large and regular. The large eye has a blueish-black iris. Average total length of around 1 meter. The total length of the largest measured specimen was 1,130 millimeters (unsexed). Based on a population in the Gulf of Panama in the eastern Pacific Ocean, the mean snout-vent length and mean total length of females is significantly greater than males. This species is unlikely to be confused with any other sea snake, due to its highly unique appearance. Yellow-bellied Sea Snakes are widespread in the tropical parts of the Pacific and Indian Oceans between the 18-20 Cº isotherms. Currents occasionally carry the snakes into temperate waters, but these are almost certainly far from their breeding and feeding waters. The residency status of Yellow-bellied Sea Snakes along the New South Wales coast is unclear. The vast majority of sightings have been of specimens in poor condition, most likely carried down passively by currents from warmer waters. However, the observation in earlier times of individuals in Port Jackson and gravid females in Botany Bay suggests that they may be or at least might have been resident. In the wild, the Yellow-bellied Sea Snake eats only fish. It hunts by stealthily approaching its prey or by waiting motionless at the surface and ambushing fish that come to shelter underneath it (small fish are often attracted to inanimate objects such as floating debris). With its mouth agape the snake makes a rapid sideways swipe to snare any fish that comes too close. This snake can even ambush small fish behind its head by smoothly swimming backwards so that the prey then comes within range of its mouth. Yellow-bellied Sea Snakes swim by lateral undulation of the body, and can move both forwards and backwards. They are capable of bursts of speed of up to 1 meter per second when diving, fleeing and feeding. When swimming rapidly, they sometime carry their head out of water. On land however the snakes are unable to stay upright and move effectively because their compressed shape makes them roll onto their side.
In the open ocean, Yellow-bellied Sea Snakes often occur in large numbers in association with long lines of debris. These “slicks” form in calm seas and consist variously of debris, foam and scum brought together by converging water currents. In some areas, such as the Gulf of Panama in the eastern Pacific Ocean, the slicks can vary in width from 11-300 meters and stretch for many kilometers. Several thousand Yellow-bellied Sea Snakes may be associated with a single slick. It is not clear whether the snakes actively swim to the slicks or whether they are carried into them passively. Snakes in these slicks have been observed feeding; however mating behavior in these large aggregations has not been recorded. Being a pelagic species the Yellow-bellied Sea Snake has limited access to hard objects, such as coral to rub against when the skin is due to be shed. Instead the snake uses a knotting behavior whereby it coils and twists upon itself sometimes for hours on end, to loosen the old skin. The skin is shed frequently and in captivity may be sloughed as often as every 2-3 weeks. The knotting behavior also helps to detach organisms such as algae and barnacles that adhere to the skin. Breeding probably occurs throughout the year in warmer seas but may be restricted to the warmer months in cooler waters. In Australia, females have been found washed onto Sydney beaches in winter (June-July). In the southwest Indian Ocean, females with small developing embryos have been found in late winter and females with near-term embryos have been found in early spring and mid-autumn. All sea snakes are ovoviviparous (development of eggs that remain within the mothers body up until they hatch or are about to hatch.). The young are born alive in the water where they live out their entire life cycle. In some species, the young are quite large, sometimes up to half as long as their mother. Females reach sexual maturity at a snout-vent length of at least 623 millimeters. From observations in captivity, gestation has been inferred to last at least 5 months. The female gives birth to between 2 and 6 young, measuring around 250 millimeters in total length. The young are born with substantial fat-bodies, nevertheless they will feed on their first day of life. Unlike most other species of sea snake, the Yellow-bellied Sea Snake does not seem to have many predators. In places where the snakes occur in large numbers together with potential predators (large fish, seabirds and marine mammals), no attempts at predation have been observed. The bright coloration of this species serves as a warning, not only that the snake is highly venomous, but also unpleasant and possibly even toxic to ingest. Most people are only likely to encounter a Yellow-bellied Sea Snake if a sick or injured animal drifts ashore. Although these specimens are usually in poor condition, they still pose a risk if they are picked up or wash against a person in the surf. If roughly handled this species is likely to bite. The fangs are quite short (1.5 millimeters) and only a small dose of venom is usually injected, however this venom is highly toxic and contains potent neurotoxins and myotoxins. Symptoms of envenomation include muscle pain and stiffness, drooping eyelids, drowsiness and vomiting and a serious bite can lead to total paralysis and death.