AsianScientist (Oct. 15, 2012) – Scientists in Singapore have discovered that soft-shelled turtles excrete waste urea through gill-like projections in their mouths.
Chinese soft-shelled turtles (Pelodiscus sinensis), like all turtles, have lungs and breathe air while leading an aquatic lifestyle. They are happy to sit on the bottom of swamps and snorkeling at the surface to breathe.
Scientists from the National University of Singapore were therefore puzzled by why these turtles occasionally submerged their heads in water even when they were on land. While the turtles do not have gills, they have gill-like projections in their mouths.
The team led by Dr. Yuen K. Ip suspected that the turtles, like certain types of fish, might be excreting waste nitrogen as urea through their gills. To test this, they bought turtles from a market in Singapore’s Chinatown and observed them in water for six days.
The researchers found that the turtles only excreted six percent of their urea production through the kidneys. The remaining urea was being excreted through their mouths, at a rate 50 times higher than the excretion rate through their hind ends. This is why the turtles would periodically dip their heads into a puddle, sometimes for up to 100 minutes, and swish water through their mouths.
Publishing their results in the Journal of Experimental Biology, the researchers also found that the turtles express in their mouths a gene for a urea transporter. Using the chemical phloretin to deactivate the urea transporter prevented the turtles from excreting urea through their mouths.
So, why do Chinese soft-shelled turtles go to such great lengths to excrete urea through their mouths when most other animals do it through their kidneys? Ip and his colleagues suspect the salty environment where the turtles live have an important part to play in this biological oddity.
Animals that excrete urea have to drink a lot of water, and the turtles live in an environment surrounded by salty water. This is a problem because these reptiles cannot excrete the salts.
“Since the buccopharyngeal [mouth and throat] urea excretion route involves only rinsing the mouth with ambient water, the problems associated with drinking brackish water… can be avoided,” they explain.
The turtles’ ability to excrete waste through their mouths, while unpalatable to us humans, has been key to their success in inhabiting a marine environment, the authors say.
The article can be found at: Ip YK et al. (2012) The Chinese soft-shelled turtle, Pelodiscus sinensis, excretes urea mainly through the mouth instead of the kidney.
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Source: The Company of Biologists.
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