Saltwater frogs exhibit water conservation to exploit their environment
Research carried out on crab-eating frogs (Fejervarya cancrivora) in Indonesia by Operation Wallacea scientists has found that these saltwater amphibians have some skin resistance to water loss that allows them to reduce desiccation during the critical period of acclimation from land to sea. This is in contrast to the norm for semi-aquatic species, which typically exhibit no skin resistance to water loss.
Thin permeable skin is both the cornerstone and the millstone of amphibian evolution. On the one hand skin provides a useful site for gas exchange; on the other, free water movement across skin confines amphibians to freshwater. Indeed, of the 6,500 recognised amphibians, only the crab-eating frog can enter the sea.
The discovery presented an ideal opportunity to investigate water conservation in perhaps the world’s most unique amphibian
Unlike other amphibians, which rapidly dehydrate in seawater, crab-eating frogs absorb urea across their urinary bladder in order to eliminate the osmotic gap between body fluids and seawater. Seawater acclimation requires several days, leaving the frogs with two equally disagreeable options — they can take up salt and lose water to the sea, or face desiccation on land.
Investigating water resistance
In 2010 we discovered crab-eating frogs on Hoga Island, Indonesia. The discovery presented an ideal opportunity to investigate water conservation in perhaps the world’s most unique amphibian. Using gravimetric wind tunnel methods, we tested the frog’s ability to resist water loss across skin.
High resistance values are common for arboreal and desert fossorial frogs, which may have values up to 300 sec/cm*. However, semi-aquatic species, like the crab-eating frog, typically exhibit no skin resistance, so that water evaporates at rates equal to free water surface (ie 0 sec/cm). Surprisingly, we found a skin resistance value of 0.27 with a standard error of ± 0.06 sec/cm. While this value seems small, the relationship between vapour density, water loss, and skin resistance means that modest increases in resistance may dramatically reduce overall skin evaporation rates.
Skin resistance is likely necessary for successfully exploiting saltwater environments. During acclimation, crab-eating frogs must limit seawater exposure, and the ability to resist desiccation allows for extended land-sea transition times as urea concentrations increase. By entering marine environments, crab-eating frogs escape traditional freshwater competitors and predators and gain access to new prey sources — an ingenious and unprecedented physiological feat for an amphibian.
* Resistance values are measured in the unusual units of sec cm-1