Sea Snakes: Venom, Characteristics, Behavior

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20120519-sea snake tTimor-Dive_Atauro_.JPG Sea snakes are found throughout the tropical western Pacific and Indian oceans, and are particularly numerous around Australia. The Philippines and Indonesia. Even though some species have some of the world's most toxic venoms, they are not aggressive and rarely present a threat to humans or cause human injuries or fatalities. Generally they do not bite unless they are handled. Often they don't even release venom when they bite. They are not found in the Atlantic Ocean or the Caribbean Sea.

Sea snakes, or coral reef snakes, are elapid snakes (of snakes characterized by their permanently erect fangs at the front of the mouth) that inhabit marine environments for most or all of their lives. They encompass two subfamilies — Hydrophiinae and Laticaudinae. Hydrophiinae also includes Australasian terrestrial snakes, Laticaudinae only includes the sea kraits (Laticauda), of which three species are found exclusively in freshwater..Most sea snakes are venomous. [Source: Wikipedia]

If these three freshwater species are excluded, there are 69 species of sea snakes divided between seven genera. They tend to have laterally flattened bodies that enable them to swim more efficiently and give them an eel-like appearance. They generally feed on fish, particularly eels. Some feed on fish eggs. Some dive deep to search for prey. Most can not survive on land. Some tie their bodies into slip knots which "they work down their length from head to tail so that one fold, as it pass over another, runs off parasites" attached to the snake's skin. Some can become quite large. One giant sea krait was 3.6 meters (11.6 feet) long. /=\

Sea snakes have been utilized by humans for food and their body parts have been sources of medicines and valuable materials. Their venoms have been used for medicine and in drug research. Sea kraits skins have been harvested leather and sold in the Phillipines since 1930. The Japanese commercially importing sea krait skins them from the Philippines and exporting them to Europe as "Japanese sea snake leather". In the Ryukyu Islands of Japan and some other Asian countries the eggs and meat of sea kraits are consumed as food. The snakes are also an important part of various coral reef ecosystem and are sought out by divers. They are sometimes captured when they appear on land. Reduction of coral reef and mangrove habitats, pollution of coastal areas, and overfishing and depletion due to commercial fishing bycatch affect many species of sea snakes. /=\

Websites and Resources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Fishbase ; Encyclopedia of Life ; Smithsonian Oceans Portal ; Woods Hole Oceanographic Institute ; Cousteau Society ; Monterey Bay Aquarium ; MarineBio

Sea Snake Characteristics

Japan Sea Snake (Ornate Sea Snake Hydrophis ornatus)

Sea snakes are reptiles. Most sea snakes are able to respire through their skin. This is unusual for reptiles because their skin is thick and scaly. Even so they have to come to the surface to breath. Other adaptions that sea snakes have made for marine life include nostrils that can be closed with a valve arrangement and paddle-shaped tails that improve their swimming ability. They lack the ventral scales that help terrestrial snakes move.

As is true with most reptiles, sea snakes are cold blooded (ectothermic, use heat from the environment and adapt their behavior to regulate body temperature), heterothermic (having a body temperature that fluctuates with the surrounding environment) and have bilateral symmetry (both sides of the animal are the same). Pelagic sea snakes can stay underwater for between 1.5 and 3.5 hours and are capable of cutaneous breathing — removing oxygen from the water and releasing carbon dioxide. The snake has a salt gland under its tongue, which secretes salt taken in from the water.

Sea snakes belong to the Family Hydrophiidae. They differ in appearance from other snakes in that they have an oar-like tail and laterally compressed bodies to aid in swimming. Sea snakes are air breathers and must surface to breathe. A specialized lung and nostrils with valves enable sea snakes to remain submerged for periods of up to 8 hours. Most sea snakes are completely marine and lack the enlarged ventral scales that enable land snakes to grip the ground. Once ashore, these ocean-going snakes are helpless, and cannot crawl. Generally, sea snakes are not aggressive. They are not thought to strike humans unless provoked, nor do they typically actively pursue swimming prey. However, there are species that may bite if they are stepped on or handled roughly. [Source: U.S. Army, Center for Health Promotion and Preventive Medicine (Usachppm) Entomological Sciences Program, Aberdeen Proving Ground, Maryland]

Sea Snake Senses, Reproduction and Lifespan

Seasnakes communicate with vision, touch and chemicals usually detected by smelling, pheromones (chemicals released into air or water that are detected by and responded to by other animals of the same species) and vibrations and sense using vision, touch, vibrations and chemicals usually detected with smell. Among many species of sea snake, vision is important. Sensing water vibrations probably plays a key role in finding prey, avoiding predators and other purposes in their marine environment. Like terrestrial snakes, many sea snakes have a forked tongue have a forked tongue and vomeronasal organ that senses chemical "odors" supplied by the tongue. One sea snake species has photoreceptors in its tail, to ensure that when it hides in a cave, it gets its whole body inside. [Source: Animal Diversity Web (ADW) /=]

Banded sea snake

Some sea snakes are viviparous (give birth to live young) and some are oviparous (lay eggs). Other are ovoviviparous (eggs are hatched within the body of the parent). Sea snakes, as is the case with most snakes and lizards, are believed to be polygynandrous (promiscuous), with both males and females having multiple partners. Most sea snakes — and, reptiles, in general — produce highly precocial young that can take care of themselves pretty right after they are born.

According to Animal Diversity Web: It has been suggested that sea snakes generally exhibit a relatively high rate of mortality especially in their young. In captivity, sea snakes have proven to be difficult to maintain and often refuse food, become anorexic, and die in a short time due to variety of known and unknown causes. In at least one case study, some of the known causes of death in captive , they. Therefore, the population dynamics of some species of Laticauda may be affected by drought and global climate change. [Source: Eric Wright, Animal Diversity Web (ADW) /=]

Sea Snakes on Land

Many sea snakes reside mostly in water but spend time on land and their morphological characteristics reflect. Many lay eggs and digest food on land and have have things like cylindrical body shape and ventral scales for crawling and climbing on land which terrestrial snakes have. However, they hunt and catch prey in the ocean and have certain aquatic adaptations for life in the water including valvular nostrils, salt glands, and a laterally compressed, paddle-like tail. Sea snakes have many special adaptations for diving including a saccular lung allowing them to dive to depths up to 60 meters (197 feet) in search of food.

Sea kraits (Laticauda) oviparous (lay eggs) and return to land to lay their eggs. Their clutch size often varies in accordance with geographical location. For example, clutch size of yellow-lipped sea kraits has been reported as 4 to 10 in Fiji and 14 to 20 in New Caledonia. Gestation periods have been difficult to document because of asynchronous breeding in many populations./=\ [Source: Eric Wright, Animal Diversity Web (ADW) /=]

Females lay eggs on the shore but it is unclear if they return to the sea or stay on shore to care for their eggs. It has been documented that females tend to spend more time on land than males do, but hypotheses of whether this is due to parental investment or a different, unrelated cause has not been tested.

Some species aggregate on land.. Some researchers have hypothesized that rainfall and the availability of freshwater may be a limiting factor in the populations of some sea snake species, many of which drink fresh water or very dilute brackish water to counteract the dehydration they experience on land and in salt water and to maintain a proper water balance.

Sea Snake Venom

All sea snakes have fixed fangs and potent venom. Stoke’s sea snakes have fangs that are capable of penetrating a wetsuit. Some species of sea snakes have venom that is several times more toxic than the cobra’s. Fortunately, only small amounts of venom are usually injected, so fatalities are rare. The most serious bites involve multiple serrated-edged lacerations which may result in death from respiratory, heart, or kidney failure. The venom of sea snakes is painless and only small amounts of venom are usually injected. Fatalities are rare. The more serious bites involve multiple serrated-edged lacerations that produce muscle stiffness, difficulties in speaking and swallowing, flu-like symptoms and muscular paralysis. Antivenin is available which can neutralize the effects of the venom of most species of cobras. No species specific antivenins are produced for sea snakes, Asiatic coral snakes or Asian lance-headed

Sea snake venom is very toxic but a very small amount of toxin is injected with a bite. Sea snakes have difficulty penetrating wet suits. Their almost painless bites can cause a variety of muscle pains and paralysis. If enough venom is injected the victim’s legs go numb after a couple hours, his eyes and his jaw locks, and later it the worst cases he goes into convulsions and dies of respiratory failure. For many species there is an antivenin.

Powerful toxins (lethal dose): 1) anthrax (0.0002); 2) geographic cone shell (0.004); 3) textrodoxotine in the blue ring octopus and puffer fish (0.008); 4) inland taipan snake (0.025); 5) eastern brown snake (0.036); 6) Dubois’s sea snake (0.044); 7) coastal taipan snake (0.105); 8) beaked sea snake (0.113); 9) western tiger snake (0.194); 10) mainland tiger snake (0.214); 11) common death adder (0.500). Lethal doses is defined as the amount in milligrams needed to kill 50 percent of the animals tested.

In a National Geographic article from the 1960s, the writer spent some time in the Great Barrier Reef catching and studying sea snakes. One time he was bitten by a snake but his quarter inch thick west suit protected him. Another time a member of his group was bitten on his un-gloved hand but snake didn't have a chance to clamp down on him so he too was unhurt.

Sea Snakes Food and Predators

Some sea kraits feed almost exclusively on eels, including conger eels, snake eels and moray eels, and likely play a role in controlling eel population in the coral reef ecosystems they inhabit. Examinations of sea snake stomachs also indicated they feed on various kinds of reef fish and bottom-dwelling fish.

Known predators of sea kraits includes sea eagles (Haliastur indus and Haliaetus leucogaster) and sharks, especially tiger sharks (Galeocerda cuvieri). One a portunid crab was observed attacking and feeding of on a Yellow-lipped sea krait. Anti-predator adaptations include seeking cover in crevices or amidst plants on land to digest their food after foraging. They do this at least in part because their swimming ability is drastically impaired after a meal, leaving them vulnerable particularly to sharks.

The facts that sea kraits are highly venomous also offers protection, sometimes in unique and innovative ways. The tail of some sea kraits, if rotated, looks like a second head. For example, according to Animal Diversity Web, “the head and tail of the yellow-lipped sea krait is very similar. In this way the sea krait can trick a predator into thinking it has two dangerous, venomous heads and therefore serve as a preventive, mimetic adaptation. This is especially important because they spend much time probing crevices for food, leaving them exposed to attack from behind. /=\

Dolphin Feast on Venomous Sea Snakes

In the video described above, according to NBC News, one intrepid animal, dolphin Z, was seen on video hunting and devouring eight yellow-bellied sea snakes in one day, apparently without suffering any ill effects, although they’re known to be extremely venomous and have made other marine mammals vomit. The researchers said: “It is notable that on one day, dolphin Z preyed on 8 yellow bellied sea snakes. The dolphin clicked as it approached the snake and then sucked it in with a bit more head jerking as the flopping snake tail disappeared and the dolphin made a long squeal.” Scientists think the snakes were juveniles that had not developed strong venom, and that wild dolphins may be taught by other members of their group to avoid them. [Source: Tom Metcalfe, NBC News, August 18, 2022]

According to Business Insider: Dolphins have never been documented eating sea snakes, only playing with them. The attacks puzzled scientists, since consuming venomous snakes can be dangerous. In one video, the dolphin catches a snake and swims around with it for a while, jerking its head repeatedly to swallow the prey. Then it emits a high-pitched "victory squeal," according to the study. "The dolphin clicked as it approached the snake and then sucked it in with a bit more head jerking as the flopping snake tail disappeared and the dolphin made a long squeal," the study authors wrote. [Source: Morgan McFall-Johnsen, Business Insider, August 20, 2022]

At first the researchers didn't believe their eyes. They searched for other fish that might look like a sea snake on camera, but they found no other explanation. "I've read that other large vertebrates rarely prey on the yellow-bellied sea snake. There are reports of leopard seals eating and then regurgitating them. This snake does have the potential to cause neurotoxicity after ingestion and its venom is considered fairly dangerous," Dr. Barb Linnehan, director of medicine at the National Marine Mammal Foundation, said in a statement emailed to Insider.

Image Sources: Wikimedia Commons; YouTube, Animal Diversity Web, NOAA

Text Sources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated March 2023

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