Laticauda colubrina

Description

The head of a banded sea krait is black, with lateral nostrils and an undivided rostral scale. The upper lip and snout are characteristically colored yellow, and the yellow color extends backward on each side of the head above the eye to the temporal scales.

The body of the snake is subcylindrical, and is taller than it is wide. It has an upper surface that is typically a shade of blueish gray, while the belly is yellowish, with wide ventral scales that stretch from a third to more than half of the width of the body. Black rings of about uniform width are present throughout the length of the snake, but the rings narrow or are interrupted at the belly. The midbody is covered with 21 to 25 longitudinal rows of imbricate (overlapping) dorsal scales. The tail of the snake is paddle-shaped and adapted to swimming.

On average, the total length of a male is 875 mm (34.4 in) long, with a 130 mm (5.1 in) long tail. Females are significantly larger, with an average total length of 1,420 mm (56 in) and a tail length of 145 mm (5.7 in).

Geographic range and habitat

The banded sea krait is widespread throughout the eastern Indian Ocean and Western Pacific. It can be found from the eastern coast of India, along the coast of the Bay of Bengal in Bangladesh, Myanmar and other parts of Southeast Asia, to the Malay Archipelago and to some parts of southern China, Taiwan, and the Ryukyu Islands of Japan. The species is also common on Fiji and other Pacific islands within its range. Vagrant individuals have been recorded in Australia, New Caledonia, and New Zealand.

Venom

The venom of this elapid, Laticauda culubrina, is a very powerful neurotoxic protein, with a subcutaneous LD₅₀ in mice of 0.45 mg/kg body weight. The venom is an α-neurotoxin that disrupts synapses by competing with acetylcholine for receptors on the postsynaptic membrane, similar to erabutoxins and α-bungarotoxins. In mice, lethal venom doses caused lethargy, flaccid paralysis, and convulsions in quick succession before death. Dogs injected with lethal doses produced symptoms consistent with fatal hypertension and cyanosis observed in human sea snake bite victims.

Some varieties of eels, which are a primary food source for banded sea kraits, may have coevolved resistance to banded sea krait venom. Gymnothorax moray eels taken from the Caribbean, where the banded sea krait is not present, died after injection with doses as small as 0.1 mg/kg body weight, but Gymnothorax individuals taken from New Guinea, where banded sea kraits are endemic, were able to tolerate doses as large as 75 mg/kg without severe injury.

Behavior

Banded sea kraits are semiaquatic. Juveniles stay in water and on adjacent coast, but adults are able to move further inland and spend half their time on land and half in the ocean. Adult males are more terrestrially active during mating and hunt in shallower water, requiring more terrestrial locomotive ability. On the other hand, adult females are less active on land during mating and hunt in deeper water, requiring more aquatic locomotive ability. Because males are smaller, they crawl and swim faster than females.

Body adaptations, especially a paddle-like tail, help banded sea kraits to swim. These adaptations are also found in more distantly related sea snakes (Hydrophiinae) because of convergent evolution. However, because of the differences in motion between crawling and swimming, these same adaptations impede the snake's terrestrial motion. On dry land, a banded sea krait can still move, but, on average, at only slightly more than a fifth of its swimming speed. In contrast, most other non-Laticauda sea snakes are virtually stranded on dry land.

When hunting, banded sea kraits frequently head into deep water far from land, but return to land in order to digest, shed skin, and reproduce. Individual banded sea kraits return to their specific home islands, exhibiting philopatry. One study found that when banded sea kraits on Fijian islands were relocated to different islands 5.3 km away, all recaptured individuals were found on their home islands in an average of 30.7 days.

Banded sea kraits collected near the tip of Borneo had heavy tick infections.

Hunting and diet

Hunting is often performed alone, but L. colubrina may also do so in large numbers in the company of hunting parties of giant trevally and goatfish. This cooperative hunting technique is similar to that of the moray eel, with the banded sea kraits flushing out prey from narrow crevices and holes, and the trevally and goatfish feeding on fleeing prey.

While probing crevices with their head, banded sea kraits are unable to observe approaching predators and can be vulnerable. The snakes can deter predators, such as larger fish, sharks, and birds, by fooling them into thinking that their tail is their head, because the color and movement of the tail is similar to that of the snake's head. For example, the lateral aspect of the tail corresponds to the dorsal view of the head.

Banded sea kraits primarily feed on varieties of eels, but also eat small fish. Males and females exhibit sexual dimorphism in hunting behavior, as adult females, which are significantly larger than males, prefer to hunt in deeper water for larger conger eels, while adult males hunt in shallower water for smaller moray eels. In addition, females hunt for only one prey item per foraging bout, while males often hunt for multiple items. After hunting, the banded sea kraits return to land in order to digest their prey.

Courtship and reproduction

The banded sea krait is oviparious, meaning it lays eggs that develop outside of the body.

Each year during the warmer months of September through December, male banded sea kraits gather on land and in the water around gently sloping areas at high tide. Males prefer to mate with larger females because they produce larger and more offspring.

When a male detects a female, it chases the female and begins courtship. Females are larger and slower than males, and many males will escort and intertwine around a single female. The males then align their bodies with the female and rhythmically contract; the resulting mass of snakes can remain nearly motionless for several days. After courtship, the snakes copulate for about an average of two hours.

The female banded sea kraits then lay as many as 10 eggs per clutch. The eggs are deposited in crevices where they remain until hatching. These eggs are very rarely found in the wild; only two nests have been definitively reported throughout the entire range of the species.

Interaction with humans

Because banded sea kraits spend much of their time on land, they are often encountered by humans. They are frequently found in the water intake and exhaust pipes of boats. Banded sea kraits are also attracted to light and can be distracted by artificial sources of light, including hotels and other buildings, on coasts.

There are fewer recorded bites from this species compared to other venomous species such as cobras and vipers as it is less aggressive and tends to avoid humans. If they do bite, it is usually in self-defense when accidentally grabbed. Most sea snake bites occur when fishermen attempt to untangle the snakes from their fishing nets.

In the Philippines, banded sea kraits are caught for their skin and meat; the meat is smoked and exported for use in Japanese cuisine. The smoked meat of a related Laticauda species, the black-banded sea krait, is used in Okinawan cuisine to make irabu-jiru (Japanese: イラブー汁, irabu soup).