The Chinese soft-shelled turtle may not be winning any beauty contests, io9 describes it as looking “like someone glued the snout of a pig onto the face of a fish, with the texture of a scrotum”, but it has an amazing (and slightly disgusting) feature: it can pee through its mouth.
Yuen Ip from the National University of Singapore noticed that when the turtle emerges from water, or is stranded on land during dry spells, it will plunge its head into puddles. While submerged, it rhythmically expands and contracts its mouth. Ip found that the turtle gets rid of most of its urea through its mouth rather than its kidneys, via gill-like studs in its mouth. It can breathe and urinate through the same structures.
Ip found that just 6 percent of the urea that the turtles produces was ejected via their kidneys. The rest came out through the mouth. When Ip gave the turtles a chance to dip their heads in water, he found that they can sit there sucking, swilling, and spitting out the liquid for up to 100 minutes. This oral route gets rid of urea between 15 and 50 times faster than the kidneys.
Tiny studs in the turtle’s mouth, which behave like gills to supplement the animal’s lungs when it stays underwater for extended periods, are also covered in urea transporters — proteins usually found in the kidneys which flush urea out of cells. The level of urea in the turtle’s saliva is a “phenomenal” 250 times greater than in its blood.
Ip thinks the answer to the turtle’s bizarre behaviour (which is also seen in fish) lies in its saltwater habitat: to urinate through the normal kidney route, the turtle would need to constantly drink more water to make up for what it lost. As reptile kidneys are notoriously poor at getting rid of unwanted salts this would soon lead to toxic levels of salt build-up in the turtle’s body.
The Chinese soft-shell has evolved to deal with this problem through oral urination. It doesn’t need to drink anything. It just gargles some surrounding water and spits out its waste.