The yellow-bellied sea snake is beautiful, but extremely deadly. Despite the toxicity of its venom, which contains neurotoxins and myotoxins, there aren’t any reported human deaths. This is primarily because this species of sea snake is only encountered when a sick/weak snake drifts ashore.
The yellow-bellied sea snake is a pelagic snake, which means that it can live on the shore or out at sea. It’s 3 feet long and thin, with a black/dark-brown top side and a yellow belly. Its venom is highly toxic, but it’s not aggressive and is rarely encountered by people.
It’s bright colors serve as a clear warning to predators that it has a deadly bite. According to the Australian Museum, it also has a foul taste and ‘may’ even be toxic if ingested. Not surprisingly, it’s rarely predated upon.
Yellow-Bellied Sea Snake Facts
Their scientific name, Hydrophis platurus, is the perfect descriptive. ‘Platurus’ refers to their flat tail, which is an adaptation to swimming at sea. Hydrophis comes from the Greek, and means ‘water snake’.
What does a yellow-bellied sea snake look like? The first thing you’ll notice is that they have distinct yellow bellies. They’re a two-tone pattern, with black on top and yellow on their belly (their ventral side). Towards the end of their body, their tail is spotted almost like a yellow and brown cow.
In some locations, the snake looks different. The snake from Costa Rica, for example, doesn’t just have a yellow belly. According to a news report by Sci News, it has a yellow dorsal side too, so it’s yellow all over.
This is a result of divergence, where an isolated population changes in relation to their ‘parent’ population over time. This is the basis of evolutionary change. The Costa Rican snake is a distinctive new subspecies.
Unfortunately, it’s not clear how long these snakes live in the wild, according to the Royal Society. Their study into the range of the yellow-bellied sea snake stated that the scientists were ‘unable to include life-history traits in our simulation, i.e. survival rate, age at maturity, reproductive frequency, lifespan, etc.’
They went on to estimate their lifespan at 10 years, although ’10 years may slightly overestimate the lifespan of H. platurus.’ One of a very small number that has been kept in captivity only lived for 2 years. But this is most likely the result of inadequate care, since we don’t know what requirements they need to be kept in care.
In the wild, sea snakes have lifespans of between 5-10 years. Again, scientists aren’t certain as to their exact lifespan.
Size and Weight
Yellow-bellied sea snakes are quite short. On average, males are up to 28 inches long, whereas females are 35 inches long. It’s normal for snakes for the female to be longer and heavier than the male (sexual dimorphism). The older the snake, and the warmer the water, the longer the snake. Like all snakes, yellow-bellied sea snakes never stop growing until they die.
They are quite thin. They’re not heavy either given their length. Lighter male snakes are 90-110g. Heavier female snakes are between 130 and 140g.
The Costa Rican subspecies is shorter and lighter than the nominative species. It’s up to 10 inches shorter, and half as light.
It is unique among snakes—not only do they live out at sea, but they can live their whole lives at sea. They’ve also made some useful adaptations to help them live in the ocean.
Where They Live
The yellow-bellied sea snake is a ‘pelagic’ sea snake. This means that it’s comfortable in deep ocean ranges, far away from shore. They’re the only snake species that can live far away from shore.
They’re a member of a family of snakes, all of which are sea snakes, that all live out to sea. But all other species prefer living around the shore.
At the same time, they do also live around coasts too. They prefer waters between 65 and 70 degrees. They’re fully adapted to living at sea.
They mate, eat, give birth and die under the sea, practically never coming near land. They can breathe up to 33% of their oxygen through their skin, underwater.
They live at the water’s surface, rather than deep down in the ocean. They’re frequently found on smooth-water drift lines, which are ocean movements in a particular direction without large and obvious waves.
This is the perfect environment for them to hunt, as many fish follow these drift lines too. Many fish are attracted to shadow-casting objects that sit on the water, and the snake uses this to their advantage by drawing prey to themselves.
Where They’re Found
They have the widest geographic range of any snake. You can find them on the Pacific coast of Mexico, from Baja California, all the way down to Colombia and the Galapagos Islands. Across the sea, you can also find them in Hawaii and other Pacific islands further south, like Fiji and Polynesia.
Further west still from the U.S., you can find these snakes all the way around the Philippines, Malaysia, and Indonesia. They also inhabit the northern and western shores of Australia, the south of Japan, and the coast of China too.
Head even further and you can see them around India, Sri Lanka, Iran, the Arabian Peninsula and the entire eastern coast of Africa. In total, they populate almost 80 degrees of latitude and 260 degrees of longitude. That’s the biggest geographic range of any snake species.
Are They Endangered?
Because they have one of the widest ranges, they’re nowhere close to being endangered. The IUCN, the organization that assesses whether a species is endangered or not, haven’t looked at yellow-bellied sea snakes.
However, many threats do exist. Ocean acidification is an ongoing process, which is continually making the ocean, particularly around the shores, more acidic.
The most obvious example of how this is damaging the oceans is through damaging coral reefs. Coral reefs in southeast Asia and Australia have already been decimated, and these are an important habitat for many sea snakes, including this species.
Plastic waste in the ocean is threatening the habitat of all animals in the ocean. It degrades into smaller and smaller pieces, which seeps its way into everything in the ocean. Plants, small animals, and larger animals too.
Yellow-bellied sea snakes have a special adaptation that helps them to breathe underwater. Sea snakes have a lung adaptation.
Almost all snakes only have one lung. In sea snakes, this lung has been extended, and divided in two. The upper half of the lung is a regular lung, which draws in air, and allows oxygen to enter the bloodstream.
The lower half of the lung is a tool they can use to stay underwater for longer. It’s essentially a balloon that they can fill with oxygen-rich air, that they can draw from for as long as they need to. This can help them stay underwater for hours on end.
Besides that, yellow-bellied sea snakes are ovoviviparous. This means that the eggs are formed, live, and then hatch within the mother. When the snakes are born, they’re born live. This is necessary for snakes that live out to sea, because eggs and the snakes inside them need oxygen.
Inside their mother, they get oxygen from her, whereas eggs at the bottom of the ocean wouldn’t. When the snakes are born, they can head to the water’s surface to breathe.
Their scientific name also refers to their flat tail. This is another adaptation that they have. It helps them swim quicker, almost like a scuba diver’s flipper. Every snake has a tail section, which is the section below their vent/cloaca.
There are no organs, just bone, and muscle, and the tail is vital to the movement of any snake. Sea snakes use their tail, like a fish uses theirs, moving it from side to side to move forward.
Reproduction and Breeding
Yellow-bellied sea snakes reproduce and give birth out at sea. Before breeding season, they make their way to warmer waters, over 70 degrees. This is where they feel most comfortable and least vulnerable. Where water is this warm, they can mate at any time of year, but where it’s cooler, they have to wait until spring and summer until they get started.
Having mated, they will gestate her eggs for six months. She doesn’t lay her eggs once they’re formed. She keeps them inside herself until the snakes are ready to live and hunt for themselves. When they’re ready, they’ll hatch within her. She’ll make her way to a tidal pool, where she’ll give birth.
Once she gives birth to the snakes, she leaves them alone completely. The majority of snakes are the same.
What Do Sea Snakes Drink?
No animal can drink sea water (apart from a few species of sea birds, like albatrosses). Yellow-bellied sea snakes definitely can’t. So, how do they survive without having fresh water to drink?
Well, they spend most of their time dehydrated. They can spend 7 months without drinking. They conserve the water they do have, and get some hydration from food that they eat. To get more water, they wait until it rains heavily. Then, they drink small amounts of fresh water on the surface.
Of course, these heavy rains are few and far between. That’s why they’ve adapted to their environment, managing to go without water for exceptionally long times.
Over these periods, their body weight drops as a result of all the water they lose, by over 5%. That might sound like a lot, but it actually isn’t. We can lose the same amount—more in fact—but the difference is that we can lose it rapidly over a warm day, whereas they take seven months or more.
Bite and Venom Information
Yellow-bellied sea snakes have exceptionally potent venom. They are one of the most deadly snakes in America.
Let’s find out just how toxic their venom is, whether they frequently bite or even kill people, and what you can do if you’re bitten by one.
Are They Aggressive?
It’s quite rare indeed for people to encounter this snake. The majority of them live out to sea, where there isn’t anybody to encounter them. When people do come across one, that person is normally working on a fishing trawler, and the snake was accidentally caught in their net. When this happens, the snake is naturally quite aggressive.
That being said, sea snakes aren’t an aggressive kind of snake. If you were scuba diving, for example, and one showed up, then it would try and get away from you.
Compared to them, you’re a big, large, and unpredictable animal. They’re more afraid of you than you are of them, so the first thing that any sea snake will do is try and get away from you.
If the snake is cornered, it will be aggressive. If it feels like it can’t escape, and that it has to stand its ground to survive, then it will. That’s when most bites happen, both with sea snakes and almost all other species. They can bite humans underwater.
Toxicity is measured in LD50. This is a measure of how toxic a substance is. If you take a population of 100 mice or rats, the LD50 is the amount of venom injected into each subject to kill 50% of the entire test group.
Many factors affect how potent venom is, including whether the venom is injected into a vein directly or just under the skin. But when comparing like to like, you can figure out which snake’s venom is the most toxic.
The yellow-bellied sea snake’s venom is highly toxic. All you need is 0.067 mg/kg to kill a mouse. Now, the exact amount required to kill a mouse isn’t the same as the amount necessary to kill a person. But pro-rata, if all factors were the same, you would need 4.5mg to kill a fully-grown man. That’s about half a teaspoon.
What Does Sea Snake Venom Do?
The venom is sea snakes, and elapid snakes generally, is usually neurotoxic. A study in the Journal of Pharmacy and Pharmacology found that yellow-bellied sea snake’s venom is a neurotoxin and myotoxin. It contains a toxin called Pelamis toxin A. The scientists found that it has many effects:
- They begin by stimulating the respiratory system, but this quickly turns to paralysis.
- Just after this effect, the subject’s blood pressure then drops dramatically.
- The nervous system stops working as it should. The twitch response, for example, disappears (like when a doctor hits you on the knee with a small hammer to make your leg jump—this stops working).
They found that pelamis toxin a is the primary effective ‘ingredient’ in the yellow-bellied sea snake’s venom. It works by preventing the nervous system from working.
The nervous system works through electrical impulses. The brain sends an electrical signal along a wire (the nerve) to a different part of the body—say, the lungs. Then, receptors in the lung pick up on these signals, and act accordingly.
The venom attaches itself to these receptors. When the receptors are entirely covered by the venom, they can’t pick up signals from the brain anymore.
That’s why you stop breathing. Your brain is trying to tell your lungs to breathe, but they can’t pick up the signal.
Can Sea Snake Venom Kill You?
So, sea snake venom can kill you. Not only that, but the venom yield per bite is 1.0 to 4.0mg. This means that almost anybody of normal size would die if they were bitten by a yellow-bellied sea snake.
However, sea snakes are known not to bite with venom (dry bites). They need to save it to kill small prey—the fish that they hunt.
If they were to use all of their venom to kill you, that might protect them for the time being, but they wouldn’t be able to hunt and eat without any venom left. They often choose not to use their venom when they bite (only a quarter of the time).
Besides that, there’s something else that prevents them from killing people they come across. Sea snakes are proteroglyphous, which means that their fangs are quite short, and are fixed in place at the front of their jaw.
Given that their fangs are so short, and their mouths are small, they struggle to bite people they find. This is because they are often wearing scuba suits or similar. Also, it’s because their arms and legs are too big and round for them to get their mouth around. Also, they’d much rather try and get away from you than stand their ground and fight.