Reptiles and Amphibians of Thailand

Thailand's Snakes, Lizards, Turtles, and Frogs

Crab-eating Water Snake

Fordonia leucobalia

This species is found in Thailand, but not within Bangkok itself
Crab-eating Water Snake White-bellied Mangrove Snake Fordonia leucobalia งูปลาหัวเทา bangladesh
Crab-eating Water Snake in Bangladesh (© Scott Trageser)
Other color forms of Crab-eating Water Snakes in Australia

English name: Crab-eating Water Snake (aka “Crab-eating Snake”, “White-bellied Mangrove Snake”)
Scientific name: Fordonia leucobalia
Thai name: งูปลาหัวเทา (Ngu Pla Hua Tao)

Description: To 90cm long. A strong, cylindrical body with head indistinct from neck. In south and southeast Asia it is a dull grey to black with some black spotting, but in Australia and New Guinea the species is incredibly variable with numerous variations of piebald black and white or some shade of yellow, red, or orange with various black markings or black-outlined white blotches. Belly is white. Tail is quite short.

Relevant scale counts: Smooth scales in 23-27 midbody scale rows. Loreal scale is usually absent, nasal scales are completely separated by an internasal, and the second pair of chin shields is in contact.

Similar Species: Glossy Marsh Snake has a slightly more slender, weaker build and just 17 scale rows at midbody. Loreal scale is present and the second pair of chin shields is separated by small scales.
Yellow-bellied Water Snake has a more distinct head, often (though not always) has a distinctly yellow belly, 19 midbody scale rows, has a loreal scale and has nasal scales in contact, and is only found in fresh-water environments.

Range: In coastal regions of northern Australia, Indonesia, and the Philippines across Vietnam and southeast Asia west to Bangladesh and western India.

Habitat: This species is found in mangrove mudflats. During the day it hides in burrows in the intertidal region, often around mud lobster mounds.

Place in the ecosystem: Feeds primarily on crabs, though it will also eat other crustaceans such as mud lobsters and shrimp. May be eaten by larger snakes, monitor lizards, wading birds, civets, and large fish.

Danger to humans: The Crab-eating Water Snake is rear-fanged but its venom is not a threat to humans.

Conservation status and threats: It is widespread, very common in some places, and there are no known threats to most of its populations. It is considered endangered in Singapore.

Interesting facts: The Crab-eating Water Snake is one of just 4-5 snakes in the world that are known to dismember their prey rather than swallowing them whole. Crab-eating Water Snakes will attack a crab by striking with its mouth closed to pin the crab to the ground with its chin and then wrapping its body around it, potentially injecting it with venom from its rear fangs. The especially strong head and robust fangs of the snake allow it to control a hard-shelled crustacean whereas the vast majority of snakes prefer softer prey. When the crab’s movements have slowed, the snake will press the crab into the sand with its body while it pulls the legs off with its mouth and swallows them one at a time. The body may be left behind, or swallowed if it is small enough. Crab-eating Water Snakes sometimes use a similar technique to eat large mud lobsters, pulling off the tail and leaving the body behind.

Interestingly, another local mudflats resident, the Glossy Marsh Snake, also feeds on crabs. However, it is unable to attack the crabs when they are fully shelled, instead hunting for crabs that have recently shed their shells and preying on them during the short vulnerable period when they are soft, allowing them to swallow the crab whole. As a result of this difference in prey items, Glossy Marsh Snakes have somewhat less musculature than the Crab-eating Water Snake despite otherwise looking quite similar.

References:
Wild Singapore: Crab-eating Water Snake
The Biodiversity of Singapore: Crab-eating Water Snake
Atlas of Living Australia: White-bellied Mangrove Snake
IUCN Redlist: Fordonia leucobalia
An update on crustacean-eating snakes
A Checklist and key to the Homolopsid snakes
Evolution in the Mud

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