A team of biologists led by Dr Yuen Ip of the National University of Singapore has discovered that Chinese soft-shelled turtles, Pelodiscus sinensis, effectively urinate through mouth.

Chinese soft-shelled turtles are exquisitely adapted to their aquatic lifestyle, sitting contentedly on the bottom of brackish muddy swamps or snorkelling at the surface to breath.

They even immerse their heads in puddles when their swampy homes dry up. Why do these air-breathing turtles submerge their heads when they mainly depend on their lungs to breathe and are unlikely to breathe in water?

Given that some fish excrete waste nitrogen as urea and expel the urea through their gills, the team wondered whether the turtles were plunging their heads into water to excrete waste urea through their mouths, where they have strange gill-like projections. The study appears online in the Journal of Experimental Biology.

The team measured the amount of urea that passed into the turtles’ urine and found that only 6% of the total urea that the animals produced was excreted through the kidneys. Removing the turtles from the water and providing them with a puddle to dip their heads into, the researchers noticed that the turtles submerged their heads occasionally and could remain underwater for periods lasting up to 100 min.

They also calculated the excretion rate of urea through the mouth by measuring the amount of urea that accumulated in the water and found that it was as much as 50 times higher than the excretion rate through the cloaca. And when the team injected urea into the turtles and measured their blood- and saliva-urea levels, they realized that the saliva-urea levels were 250 times greater than in the blood. The turtles were dipping their heads into water to excrete urea through their mouths.

Knowing this, the team reasoned that the animals must produce a specialized class of protein transporters in their mouths to expel the waste and, as these transporters can be deactivated by phloretin, the team decided to test the effect of phloretin on the turtle’s ability to excrete urea. When the turtles were supplied with phloretin in their puddle of water, they were unable to excrete urea from their mouths when they submerged their head. And when the team analyzed the turtles’ cDNA, they found that the animals carried a gene that was very similar to urea transporters found in other animals. Finally, they checked to see if the turtles express this gene in their mouths and found evidence of the mRNA that is necessary to produce the essential urea transporter, allowing the reptiles to excrete urea waste through the mouth.

According to the team, Chinese soft-shelled turtles excrete urea through their mouths when most other creatures do it through their kidneys because they have something to do with their salty environment.

Explaining that animals that excrete urea have to drink a lot, they point out that this is a problem when the only water available is salty – especially for reptiles that 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,” the biologists explained.

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Bibliographic information: Ip, Y. K. et al. 2012. The Chinese soft-shelled turtle, Pelodiscus sinensis, excretes urea mainly through the mouth instead of the kidney. J. Exp. Biol. 215, 3723-3733; doi: 10.1242/jeb.068916