Yellow-bellied Sea-Snake

Reptiles of Ecuador | Serpentes | Elapidae | Hydrophis | Hydrophis platurus

English common names: Yellow-bellied Sea-Snake, Yellowbelly Sea-Snake, Pelagic Sea-Snake.

Spanish common names: Serpiente marina de vientre amarillo, serpiente marina barriga amarilla, serpiente marina pelágica, serpiente de mar.

Recognition: ♂♂ 72 cm ♀♀ 114.3 cm. In Ecuador, the Yellow-bellied Sea-Snake (Hydrophis platurus) is the only snake having a paddle-shaped tail adapted to swimming and a sharply demarcated bicolored longitudinal yellow and dark-blue to black pattern.

Natural history: Extremely rare throughout much of its range, including Ecuador, but extremely common in certain feeding and breeding areas. Hydrophis platurus is mostly diurnal,1 but also nocturnal,2,3 and is the world's most pelagic snake, living its entire life cycle at sea.4 Yellow-bellied Sea-Snakes are usually found drifting 1–20 km offshore4 in areas having mean monthly water temperatures of 28–32°C.5 Here, thousands of snakes (up to 12 snakes per m2)6 may congregate where lines of debris 3–6 m wide4 and several kilometers long are maintained by convergence of oceanic currents.7,8

Occasionally, individuals of Hydrophis platurus enter mangrove swamps9 and even tidal waters of rivers up to 5 km from the river mouth.10 Some individuals are washed ashore during rough weather.11,12 Once on the beach, Yellow-bellied Sea-Snakes are comparatively helpless, and soon die from heat or exhaustion.11 Despite being entirely marine, individuals of H. platurus can survive in freshwater for at least nine months.13 They do not drink undiluted seawater14, but freshwater that accumulates on the sea surface during heavy rainfall.14 As a result, Pelagic Sea-Snakes in regions of seasonal drought can spend much of their life dehydrated.15

Special thanks to Ellen Smith, our official protector of the Yellow-bellied Sea-Snake, for symbolically adopting this species and helping bring the Reptiles of Galápagos project to life.

At sea, Yellow-bellied Sea-Snakes swim at a speed of 0.3–7.1 km/h, and do so either forward or backward by sideward undulations aided by the laterally compressed body and tail.11,16 Individuals of Hydrophis platurus spend the majority (51–99.9%) of the time diving1 to an average maximum depth of 15 m, although they can dive down to 50 m.1 Pelagic Sea-Snakes can survive submerged for up to 24 h, but single dives last for up to about four hours.1,8 During dives, individuals of H. platurus are able to satisfy 33% of their total oxygen requirement and release 94% of their carbon dioxide by means of gas exchange across their semi-permeable skin.17 Dives are thought to help the snakes regulate their body temperature,8 escape predators,8 avoid sea surface turbulence,18 and possibly detect surface drift lines to join.19 When not diving, the sea snakes float motionless on the surface,20 basking, ambush foraging on pelagic fishes, and drifting passively with surface currents along with floating debris.8,21

When the water temperature is optimal (>25°C),22 the sea snakes can feed. They are specialist float-and-wait predators of small or juvenile surface dwelling fish that seek refuge under the debris, or under the snakes themselves.23–27 Since Yellow-bellied Sea-Snakes are presumed to have poor eyesight,26 they probably use small tactile mechanosensory organs located around their mouth28 to locate fish by the vibrations they produce while moving.29 They seize prey with a laterally directed strike23 and consume it usually without envenomating it.30

Hydrophis platurus is, however, extremely venomous,7 and its neurotoxic venom immobilizes, and is lethal to, fish.4 Despite having the most potent venom (LD50 0.055 mg/kg) among Ecuadorian snakes,31 reports of deaths following bites of H. platurus are rare.7 This is probably related to the snakes' reluctance to bite when in water,12,32 their small (~2 mm in length)33 fangs, and their low average venom yield (0.87–2.8 mg),34–35 which is lower than the estimated 5.9 mg needed to kill a 72 kg person.35 However, human deaths have been reported.7,32,36 Critically envenomated victims die from muscle paralysis that leads to respiratory failure33 within a period of 12–48 h.32,37

In addition to being venomous, individuals of H. platurus are also believed to be poisonous or at least distasteful.30 They have a “warning” coloration and few confirmed predators (pelicans,38 hawks,39 and storks40). They are entirely avoided by native fish,30 picked up and dropped by sea birds,41–43 and regurgitated if taken by marine fish30 and mammals.44 Stranded snakes are scavenged by crabs.45

In areas where mean monthly water temperatures exceed 25°C, Hydrophis platurus breeds throughout the year.5,46 Copulation takes place in water.4 After a gestation period of 6–8 months,4 females “give birth” (the eggs hatch within the mother) in the water to 1–10 young4,11 that are 22–26 cm in total length at birth.4 Females may surround and protect newborns during their first days of life.47

In some populations, as many as 18.9% of the individuals of H. platurus, especially large ones, are colonized by epibionts such as shrimps, crabs, snails, barnacles, hydroids, bryozoans, tunicates, and fish.48,49 As a response, Yellow-bellied Sea-Snakes engage in spontaneous "knotting,"50 a behavior that presumably helps eliminate this fauna as well as help the snake shed (every 9 to 43 days)22 in a substrate-less environment. Snakes of this species are host to a variety of internal parasitic worms.11,51 Captive individuals of the Yellow-bellied Sea-Snake have lived up to 3.5 years.52

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Conservation: Least Concern.53 Hydrophis platurus is listed in this category because it is the world's most widely distributed snake species,54 it has presumed stable populations, and is not facing major immediate threats of extinction.53 Instead, with the recent arrival of the species in the Atlantic,55 Hydrophis platurus has the potential to increase its range.56 Minor but confirmed threats to the species include oil spills57 and bycatch in fisheries.53 Another threat faced by the Yellow-bellied Sea-Snake is climate change. Since the species relies on freshwater from rainfall, it could potentially be affected by changing rainfall patterns associated with climate change.

Distribution: The entire tropical Indian and Pacific oceans.7,21 Sightings of Yellow-bellied Sea-Snakes in the Atlantic55 may be the result of snakes passing through the Panama Canal. Records of Hydrophis platurus along the coast of mainland Ecuador and Galápagos are thought to be the result of waif dispersal.5

Distribution of Hydrophis platurus in mainland Ecuador Distribution of Hydrophis platurus in Galápagos

Etymology: The generic name Hydrophis, which comes from the Greek words hydros (meaning “water”) and ophis (meaning “serpent”),58 refers to the aquatic habits of this genus of snakes. The specific epithet platurus, which comes from the Greek words platys (meaning “flat”) and oura (meaning “tail”),58 refers to the distinctive paddle-shaped tail of the species.

See it in the wild: Pelagic Sea-Snakes cannot be expected to be seen reliably along the coast of mainland Ecuador, since individuals arrive as a result of waif dispersal and are recorded no more than once every few months.

FAQ

Do sea snakes breathe? Yes, much like terrestrial snakes do. Although in some species, like in Hydrophis platurus, the snakes can satisfy 33% of their total oxygen requirement and release 94% of their carbon dioxide by means of gas exchange across their skin.17

Do sea snakes camouflage? Most sea snakes, including the Yellow-bellied Sea-Snake, do the opposite. They have bright colors that act as a warning to potential predators.

Do sea snakes have gills? They don’t, but they can still obtain oxygen and release carbon dioxide while submerged.17

What happens when a Yellow-bellied Sea-Snake bites you? The consequences of being bitten by a Yellow-bellied Sea-Snake varies depending on, among other things, the amount of venom injected and the physical condition of the victim. Sometimes, there may be no noticeable effects at all. On rare occasions, critically envenomated victims may die from muscle paralysis that leads to respiratory failure.33

Where do sea snakes live? The majority of sea snake species are found throughout the Indian Ocean and western Pacific Ocean. Only the Yellow-bellied Sea-Snake occurs in the eastern Pacific and, recently, also in the Caribbean.

Why are sea snakes so venomous? Sea snakes are extremely venomous in order to be 100% efficient in quickly immobilizing fish in the water without risking the prey escaping or struggling.

Author: Alejandro ArteagaaAffiliation: Khamai Foundation, Quito, Ecuador.

Academic reviewers: Harvey Lillywhite and Coleman Sheehy.

Photographers: Sebastián Di Doménico and Alejandro ArteagaaAffiliation: Tropical Herping (TH), Quito, Ecuador.

How to cite? Arteaga A (2020) Hydrophis platurus. In: Arteaga A, Bustamante L, Vieira J (Eds) Reptiles of Ecuador: Life in the middle of the world. Available from: www.reptilesofecuador.com

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