Snakes on the Reef! They Won’t Bother You. Probably.

banded sea krait
A banded sea krait heads for the reef after surfacing for air. 

IF THE IDEA OF MEETING UP WITH SEA SNAKES ON A DIVE UNSETTLES YOU, here are two important facts about sea snakes and sea kraits: 1) They’re armed with highly deadly venom, and 2) They’re remarkably unaggressive.

That being said, here’s another important thing about these two groups of sea snakes: Don’t mess with them.

All sea snakes are venomous snake-like reptiles, but in anatomy and lifestyle, there are two different kinds. So, what’s the difference between true sea snakes and sea kraits?

VENOMOUS BUT NOT AGGRESSIVE

People can be bitten and even killed by sea snakes but it’s highly unusual. By and large, you have to get in their faces and really annoy them for them to react. In the fall of 2018, Australia experienced its first and only sea snake fatality in 80 years, and that was from a highly disturbed snake entangled in a trawler’s fishing net. The young man it bit was a crew member on the boat, trying to untangle the net.

While sea snakes are descended from terrestrial cobras – snakes that do have highly deadly bites – their sea snake cousins are generally docile and unaggressive. They have small fangs and they mostly don’t inject venom when they bite.

Some snakes use their venom to paralyze prey, but for many it seems to be a defensive weapon. Fishermen getting nipped by sea snakes is not unknown, but statistically, only three percent of sea snake bites are fatal.

yellow-bellied sea snake
Yellow-bellied sea snakes (Pelamis platurus) are the only pelagic sea snakes, and possibly the most abundant species of reptile in the world. 

KRAITS IF BY LAND, TRUES IF BY SEA

There are 69 known species of sea snakes, from banded sea kraits (Laticauda colubrina) that can devour whole conger eels to turtlehead sea snakes (Emydocephalus annulatus) that scrape  the eggs from fish nests they find on rocks and coral.

Most of these marine serpents are “true sea” snakes in subfamily Hydrophiinae, descended from land-based Australian cobras.  A dozen species are the sea kraits, in subfamily Laticaudinae, descended from Asian cobras. White with black stripes encircling their bodies, banded sea kraits are the sea snakes most likely to be encountered by divers on coral reefs.

VIVIA LA DIFFERENCE!

What’s the difference between kraits and true sea snakes? First, true sea snakes live their entire lives in the sea, mating, giving birth to live offspring, lapping up fresh water and digesting their prey without going on land. Kraits are semiaquatic, going onto land to mate and lay eggs, drink water and digest prey (which can take weeks).

Another significant difference: True sea snakes are ovoviviparous, giving live birth from fertilized eggs the female carries within her body. Sea kraits are oviparous, mating on land and laying their eggs in holes and rock crevices.

Banded sea kraits (Laticauda colubrina) are the sea snakes most likely to be seen by divers on the reef. Fortunately, they’re not aggressive. 

SNAKES ON A REEF

As air-breathing reptiles that need to return to the surface periodically for air, sea snakes primarily live and hunt mostly in the 100 feet/30 meter range. This means that most are shallow-water denizens, likely to be found hanging around reefs, hunting edibles day and night.

Most stalk prey like small fishes, crustaceans, small octopuses and squid, sticking their snouts into holes and crevices hoping to flush out something delicious. Sea kraits are famous for hunting and devouring eels, sometimes swallowing them whole. Sometimes, hopeful fishes follow them around, hoping they can grab those somethings when they dart out the other end of a hole.

BREATHS OF FRESH AIR. OR NOT.

The need to stay shallow is especially true for sea kraits, which apparently need to surface for air every 30 minutes or so.

On the other hand, true sea snakes have developed a trick. They can also breathe through their skin while submerged, absorbing up to 33 percent of the oxygen they require and throwing off most of their carbon dioxide waste. An ability to expel nitrogen may also explain why they don’t experience the bends.

With enlarged left lungs that aid them with buoyancy, some true sea snakes can remain submerged for as long as eight hours. And, a journal article recently reported sighting of two sea snakes off the northern Australia coast moving along the sea bottom at the record depth of some 800 feet/240 meters.

STRICTLY INDO/PACIFIC

Encounters with sea snakes actually aren’t that common and they’re probably less studied than other marine denizens. One possible reason: They fall in the gap between marine biologists and the herpetologists who study snakes on land. Each specialty tends to leave them to the other to focus on them.

And, they don’t get attention in the Tropical Atlantic/Caribbean basin, where they don’t exist. For reasons of timing and geohistory, sea snakes spread throughout the Indo-Pacific basin, but never beyond it.

39 An olive sea snake (Aipysurus laevis), a commonly encountered true sea snake. 

It’s surmised that they originated in the Pacific sometime after the oceanic gap between the Americas was closed by the rise of the Central American land bridge several million years ago. Unable to survive frigid temperatures at the tips of Africa and South America, they’ve never made it into the Atlantic. Also closed off to them are other possible avenues. The Red Sea – a small, enclosed body – is too salty for their survival, keeping them from passing through the Suez Canal into the Mediterranean. The Panama Canal is not salty enough.

And while most sea snakes are shallow reef and estuary denizens, only one, the yellow-bellied sea snake (Pelamis platurus), is truly pelagic, riding the currents to crisscross the Pacific and inhabiting a range stretching from East Africa to the west coast of South America. It’s possibly the most abundant reptile on the planet.

SHAPES & SCALES

Anatomically, both sea snakes and sea kraits have evolved flattened, paddle-like tails that enable speedy ocean swimming. Many true sea snakes also have flattened bodies that improve swimming, as well. But sea kraits have cylindrical bodies ending in their paddle-tails.

The difference is important due to their lifestyles. Due to their keel-like bodies, the ventral scales on most true sea snakes are reduced in size and don’t overlap, which would leave them unable to crawl on land, if they tried to do so.  Sea kraits’ cylindrical bodies don’t have this problem; their underbody scales support their movements on land.

Additionally, true sea snakes have developed a design that places their nostrils on the tops of their snouts, facilitating drawing in air at the surface. Sea kraits have retained terrestrial snakes’ arrangement of lateral nostrils.

Depending on species, true sea snakes range in size from 3.3 feet/1 meter to 5 feet/1.5 meters. Sea kraits are tend to be shorter – often 3.3 feet/1 meter for adults.

FINDING WATER

All sea snakes need to be able to drink fresh water. Sea kraits have no difficulty accessing it, assuming the land they frequent has it. Sea-bound true sea snakes face a bigger challenge – they can’t access a handy spring or stream. They have glands under their tongues that enable them to excrete salt that accumulates in their systems. It was assumed that this enabled them to extract fresh water from seawater.

However, new research has shown that true sea snakes drink recently fallen rainwater to rehydrate at sea. For a time after it’s fallen, rainwater forms a freshwater “lens” atop seawater before becoming mixed with it.

PRINCIPAL SOURCES: Marine Biology, Fourth Edition, Peter Castro, Michael Huber;  Indo-Pacific Coral Reef Field Guide, Gerald Allen, Roger Steene;  Venomous Sea Snake Facts (Hydrophiinae and Laticaudinae),” ThoughtCo.com; Why Fatal Sea Snake Bites Are Unusual,Forbes Magazine; “Banded Sea Krait,” Oceana.corg; “A Turtle or a Sea Snake,? Smithsonian Ocean; How sea snakes, surrounded by salt water, quench their thirst,” National Geographic; “Sea snake bite causes first death in Australia in more than 80 years,” Australian Broadcasting Company; DEADLY SEA SNAKES LURE RESEARCHERS,” New York Times; Sea Snake,” Encyclopedia Britannica; “Sea Snake,” “Sea Krait,” Wikipedia.com.

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