Lifeform of the week: Banded sea kraits are swimming, tree-climbing, venomous beauties

The venomous banded sea krait has impressive adaptions for life on land and at sea. But don’t ask me about its mating habits.

Normally, I am not a fan of swimming snakes – sea snakes and their freshwater kin. Snakes on land are interesting, but their presence in bodies of water strikes me as ominous. I envision them gliding beneath the water’s surface, waiting to graze my unsuspecting ankles as they scurry out of the way, since as we all know they’re more afraid of you than you are of them.

Via CraigD

It’s one of the reasons I don’t care for night swimming. But Laticauda colubrina, commonly called the banded sea krait or the yellow-lipped sea krait, is a stunningly beautiful (if venomous) reptile. Sporting stylish black bands* along its silvery body as it ripples beneath the surface along a coral reef, the snake looks like it just slithered out of a Tim Burton remake of Finding Nemo.

Surf and Turf

Sea snakes evolved not from fish but from terrestrial snakes that elected to re-enter the ocean. While true sea snakes eventually gave up living on land altogether, Laticauda colubrina still spends a good deal of its time out of the water, up to 50% according to some estimates. The snakes hunt in ocean waters but emerge onto land in order to digest their food, shed their skins, mate, and lay eggs.† Banded sea kraits share several adaptations with other sea snakes, including nostrils with valves that can open to take in air while above water and a flattened end on their tails that serves as a paddle, enabling faster swimming. However, unlike many fully aquatic sea snakes, they have also retained ventral scales that allow them to move efficiently while on land. They’ve even been known to climb trees.

Home sweet home. Image Credit: Nick Hobgood

Where They Live

When species of venomous land-dwelling snakes decided to give aquatic living another shot some five million years ago, the idea did not make its way to the western hemisphere. Sea snakes are found only in the Pacific and Indian Oceans.‡ Banded sea kraits are common in parts of Southeast Asia and the Pacific Islands.

Due to the demands of their amphibious lifestyle, they do not inhabit the open sea. Rather, they tend to cluster around small islands and near coral reefs. They are exactly the kind of animal you’re likely to run into while snorkeling off the coast of Singapore.

What They Eat

Banded sea kraits are finicky eaters – feeding specialists if you prefer the more scientific term. They subsist on a diet of mostly conger eels.§ The kraits search for their preferred food in the nooks and crannies of coral reefs. When they successfully locate their prey, they bite, injecting a venom of potent neurotoxins that rapidly reduce the victim’s ability to swim and breathe, making it possible for them to subdue and swallow the eel. If you find the mental image of a snake gulping down something that looks suspiciously like another snake disgusting, I won’t argue with you. After consuming an eel, a sea krait’s body is visibly bloated and its swimming speed significantly reduced. Realizing that they are now themselves vulnerable to predators, the kraits lumber onto land and find a safe shelter in which to digest their meal.

Ladies and Gentlemen

Sexual dimorphism in banded sea kraits is limited to size. Females are larger than males, and larger females are more desirable to male suitors.** Size difference among males, however, does not affect their reproductive success, as males do not compete against each other for females. One study of mating groups of banded sea kraits found that about half the groups involved two or more males attempting to woo a single female. However, only one male actively courts the female. The others merely hang around like platonic friends, opportunistically waiting for the female to be in the mood while boyfriend isn’t paying attention. After much lying about, someone finally manages to copulate, a process that is graphic enough to be relegated to the footnotes section.†† As mentioned, females lay their eggs on land. Clutch size varies by geographic region. Depending on where you spot the snake, you can expect it to deposit anywhere between 4 and 20 eggs.

Paddle-shaped tail in the forground, head in the background. Image Credit: Jens Petersen

Two Heads Are Better Than One

Despite being equipped with insanely powerful venom, the banded sea krait still has to watch out for predators. Eagles and sharks have been known to get the better of them. While the snakes’ striking visual pattern can serve as a warning of their venomous ways to would-be predators, another anti-predator adaptation may also exist in the species’ flattened paddle-like tail. Due to similarity in markings, the tail, when turned on its side, strongly resembles the banded sea krait’s head. It has been suggested that the snake uses this resemblance to trick predators into thinking they are looking at a fanged and vigilant head rather than a benign tail. This is potentially advantageous for a snake that forages by sticking its head into coral reefs, impeding its ability to keep an eye on its surroundings.

Can They Hurt You?

Over 60 known species of sea snakes reside in the Pacific and Indian Oceans, and unfortunately all of them are venomous. In one bite, a banded sea krait can deliver roughly ten times the venom needed to kill an average-sized human. But such events seldom occur. Banded sea kraits do not acknowledge our species as either prey or predator. Mostly they just ignore us. This is easily seen in nature documentaries in which divers with cameras swim inches away from the snakes while the animals poke their heads into coral reefs in search of something more interesting. Fishermen are occasionally unlucky enough to encounter perturbed sea snakes while sifting through their nets, but no documented human fatalities have been attributed to Laticauda colubrina. Still, you can add banded sea kraits to your list of good reasons not to go sticking your hands into coral reefs.

* In snake patterns, “striped” refers to longways markings (parallel to the body like pin-striped pants), whereas “banded” indicates crossways markings (perpendicular to the body, like Harry Potter’s scarf).

† Laticauda is the only genus of sea snake that lays eggs. Other non-amphibious sea snakes give birth to live young in the water.

‡ An interesting consequence of this is that animals who serve as potential prey for sea snakes occupying these oceans have evolved a tolerance to snake venom that their cousins in the Atlantic and Caribbean lack.

§ While banded sea kraits prey on conger eels, they inadvertently help out another kind of eel: the banded snake eel, which has evolved to look like the dangerously-venomous krait in order to deter predators.

** “Largeness” is quantified as body length rather than relative body mass, so you can think of male sea kraits as favoring tall girls rather than chubby ones.

†† Like most snakes, including sea snakes, males of the Laticauda colubrina species have two penises sheathed in the lower portion of their bodies. Snake sex consists of the insertion of one “hemipenis” into the female’s cloaca, a body opening used for both excretion and reproduction. For the banded sea krait, this takes place on land, not underwater in the coral reef. Keep in mind that I didn’t invent this system – I am merely relaying it to you.

World-class snake robotics from Scandinavia

For some blenny fish species, looks can be deceiving

Alex Reshanov

MORE ARTICLES