October 11, 2012
Chinese Soft-Shelled Turtles Release Waste Through Their Mouth
Lee Rannals for redOrbit.com — Your Universe Online
"Necessary? Is it necessary for me to drink my own urine? No, but I do it anyway because it's sterile and I like the taste," says Rip Torn's character, Patches O'Houlihan, in the hit comedic movie Dodgeball: A True Underdog Story.
Some fish are known to excrete waste through their gills, and after observing how these turtles immerse their heads in puddles while in swampy homes, the team decided to dig a little deeper into the matter.
Y. K. Ip from the National University of Singapore and colleagues purchased turtles from a local China Town wet market and immersed them in water for six days.
They measured the amount of urea that passed into the turtles' urine and found that only six percent of the total urea the animals produced was excreted through the kidneys.
After removing the turtles from the water, and providing them with a puddle to dip their heads into, the team noticed the turtles submerged their heads on occasion. They were able to calculate the excretion rate of urea through the mouth by measuring the amount of urea they accumulated in the water and found that it was as much as 50 times higher than the excretion rate through the cloaca.
Once the team injected urea into the turtles and measured their blood- and saliva-urea levels, they found the levels were 250 times greater in the saliva than the blood. This indicates that the turtles are dipping their heads into the water to excrete urea through their mouths.
After discovering this phenomena, the team determined the turtles must produce a specialized class of protein transporters in their mouths to expel the waste. Upon determining this, 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.
The team found the animals carried a gene that was similar to urea transporters found in other animals. They also 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.
Ip and his colleagues suspect the reason for the turtles needing to drop their heads into the mud in order to go number one is due to their salty environment. They said animals that excrete urea have to drink a lot, which is a problem when the only water available is salty.
"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 said in the journal.