Mystery of glass frog transparency solved, researchers say

Dec. 23 (UPI) -- Researchers say they have solved the mystery of how glass frogs become mostly transparent while they sleep. The discovery may lead to innovations in how to understand blood clotting.

Researchers Jesse Delia, of the Museum of Natural History in New York, and Carlos Taboada, of Duke University, teamed up to solve the mystery after Delia witnessed a glass frog sleeping in Panama.

The species is the only known terrestrial animal that can maintain transparency on the inside and outside.

"Using photoacoustic imaging to track red blood cells in vivo, we show that resting glass frogs increase transparency two- to threefold by removing [about] 89% of their red blood cells from circulation and packing them within their liver," according to the study published online Thursday in Science.

The researchers beamed lasers at the frogs to track the movements of individual blood cells and found that they store their red blood cells in their liver, while flooding the rest of their circulatory system with plasma as they sleep.

When they wake, the red blood cells quickly flow back into their circulatory system. The glass frog's liver also is coated with a film of crystals that block out the redness of the blood inside.

"They somehow pack most of the red blood cells in the liver, so they're removed from the blood plasma. They're still circulating plasma ... but they do it somehow without triggering a massive clot," Delia told BBC.

It is not understood how the frogs are able to survive for long periods with most of their red blood cells stored in their liver. It also is not understood how the frogs are able to store so much blood in their livers without causing a catastrophic clot.

The fact that the frogs can store so many red blood cells without suffering from clots could offer insight into blood clotting in humans, the researchers said.

"Glass frogs' ability to regulate the location, density and packing of red blood cells without clotting offers insight in metabolic, hemodynamic and blood-clot research," the study said.