Amphibians
Crab-eating Frog
Fejervarya cancrivora · also called crab-eating frog, mangrove frog
Salt water kills almost every frog on Earth within hours, drawing the water straight out of their thin, permeable skin. The crab-eating frog is one of the rare exceptions, tolerating brackish mangrove swamps and even brief exposure to nearly full-strength seawater by deliberately raising the salt concentration inside its own blood to match the water around it.
Overview
The crab-eating frog, Fejervarya cancrivora, lives in mangrove forests, brackish estuaries, and coastal wetlands across South and Southeast Asia, hunting small crabs and other invertebrates that give the species its name.
Built to handle the sea
By raising urea and other solutes in its blood, the frog resists the water loss that salt water would normally cause, and remarkably, even its tadpoles tolerate brackish conditions — letting the species complete its entire life cycle in water that would be lethal to almost any other amphibian.
Status
Least Concern — its rare salt tolerance lets it exploit coastal habitat few other frogs can use, keeping it widespread and secure despite localised mangrove loss to coastal development.
Crab-eating Frog: common questions
Why is salt water normally so dangerous for frogs?
Most frogs have thin, highly permeable skin that plays a major role in breathing and absorbing water, but this same permeability makes them extremely vulnerable to salt water, since the higher salt concentration outside the body draws water out of the frog's tissues through osmosis, quickly leading to dangerous dehydration. This sensitivity is a major reason why the vast majority of amphibian species are restricted to fresh water and cannot survive for any meaningful length of time in brackish or marine environments.
How does the crab-eating frog manage to survive in salty water?
The species raises the concentration of urea and other solutes within its own blood and tissues, increasing its internal osmotic concentration so that it more closely matches, or at least more effectively resists, the surrounding salt water, which significantly reduces the rate at which water is drawn out of its body. This physiological strategy, along with some behavioural avoidance of the very highest salinity conditions, allows the crab-eating frog to tolerate brackish water and even brief exposure to water approaching full seawater strength far better than almost any other amphibian.
Can crab-eating frog tadpoles also survive in salt water?
Yes, remarkably, the tadpoles of this species also show a meaningful degree of salinity tolerance, which is unusual since tadpoles are generally even more vulnerable to osmotic stress than adult frogs due to their more delicate, highly permeable skin and gills. This tadpole-level tolerance allows the crab-eating frog to complete its entire life cycle, from egg to tadpole to adult, directly within brackish mangrove pools and tidal creek habitats that would be lethal to the developing young of most other frog species.
Does the crab-eating frog only eat crabs?
No, while crabs are a notable and name-giving part of its diet, taking advantage of the abundant small crabs found in its mangrove and coastal wetland habitat, the crab-eating frog also eats a range of other available invertebrates, including various insects, when the opportunity arises. Its willingness and physical ability to tackle tougher-shelled prey like small crabs, however, does distinguish it somewhat from many other frog species that rely more exclusively on softer-bodied insects and similar invertebrates.