Saltwater frogs exhibit water conservation to exploit their environment

Crab-eating frogs main image

Vive la Résistance! First crab-eating frog captured from Hoga Island, Indonesia. Frogs were captured from habitats ranging in salinity from freshwater (0%) to hypersaline (mangrove pools, 40%)

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

Crab-eating frog in gravimetric wind tunnel

A crab-eating frog in a gravimetric wind tunnel resistance trial at the Hoga Island Research Laboratory. The frog has been anaesthetised and placed in the typical water conservation posture

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.

Successful adaptation

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

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4 Responses to “Saltwater frogs exhibit water conservation to exploit their environment”

  1. this work has already been publicated in a scientific journal?

    • Wayne A. Bennett Reply May 2011 at 11:47 am

      An expanded article with full results and methods is currently in press with the Journal of Herpetology. We expect that it will be in print the end of this year or the beginning of next year.

  2. Thanks for the answer. I’m interested in this topic because I started my doctoral studies, and as part of the doctoral project I will analyze the thermal and osmotic sensibility of the locomotor performance in populations of the Thoropa miliaris complex (Anura: Cycloramphidae). The populations are located in coastal and away from the coast at the Atlantic Forest.

  3. Why do they look like normal frogs

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