English name: Brackish Frog (aka: “Gulf Coast Frog”, formerly “Crab-eating Frog” or “Mangrove Frog”)
Scientific name: Fejervarya moodiei, formerly Fejervarya cancrivora
Thai name: Kob nam kroi, Kob nam kem
Description: Up to 6.8 cm long. A medium-sized frog with a deep, narrow head and characteristic folds and warts across the back and head. The snout is pointed and beak-like. Body brown or greenish-brown to gray with some darker marks. Both the lips and legs are barred. Color varies from brown, or greenish-brown, to gray, with irregular darker bars scattered on the lips and hindlimbs. The belly is white and can have some dark markings.
Tadpoles up to 40 mm. Body is oval, with a short tail less than twice body length. Color is dark with dark spots.
Call: A fast call reminiscent of someone gargling their throat
Similar Species: Asian Grass Frog has a shallower head and a less pointed snout. It is not found in brackish waters.
Chinese Edible Frog is larger with heavier body and legs. It is not found in brackish waters.
Habitat: One of the rare amphibians that can tolerate water with some salt, the Brackish Frog is found on mangrove mudflats, estuaries, and coastal swamps. It will move into manmade water sources as well such as roadside ditches and rice paddies. Its range is generally limited to coastal areas. Tadpoles develop in rain puddles above the hide tide line.
Place in the ecosystem: Eats mostly crabs and shrimp when in brackish waters, but switches to insects and smaller frogs when in fresh water.
Danger to humans: Brackish Frogs are not dangerous to humans.
Conservation status and threats: The Brackish Frog is harvested for consumption in some areas. Its mangrove forests are also threatened by development.
The IUCN report for Crab-eating Frogs (Fejervarya cancrivora) in 2004 reported that their numbers were increasing and they faced no conservation threats. However, that was before the species was split, with Crab-eating Frogs found in Malaysia and Indonesia, while Brackish Frogs are found in Thailand, Cambodia, Vietnam, China, and west to Burma, Bangladesh, and India. In trying to obtain photographs of the species it was much more difficult to find recent photos for Brackish Frog populations, so I cannot speak on their conservation status.
Interesting facts: Along with its close relative the Crab-eating Frog, the Brackish Frog is the only amphibian that can continuously live in salty water. It does this by raising the urea levels in its own blood plasma until so that the ion concentrations become higher than the surrounding environment, so it does not lose fluids to the salt water as other frogs would. It is the only amphibian in the world known to do this. They are also unique in having skin membranes that can more efficiently move excess salt out of the body, such that even tadpoles can survive in at least 50% salt water.
Amphibian Species of the World: Fejervarya moodiei
Thai National Parks: Brackish Frog
IUCN Red List: Fejervarya cancrivora
Amphibia Web: Fejervarya cancrivora
Ask Nature: Plasma maintains salt balance
Ask Nature: Membranes maintain salt balance