Masters of camouflage: Glassfrogs hide red blood cells in their liver to become transparent

Glassfrogs make themselves transparent while they rest by taking red blood cells from circulation and concealing them in their livers. A multi-disciplinary team of biologists and biomedical engineers has shown how these frogs make themselves see-through in research that appears December 23 in the journal Science.

It's easy to miss a glassfrog in its natural environment. The northern glassfrog, Hyalinobatrachium fleischmanni, measures no more than a few centimeters, and they are most active at night, when their green skin helps them blend in with the surrounding leaves and foliage.

But these amphibians become true masters of camouflage during the day when they're asleep.

When glassfrogs are resting, their muscles and skin become transparent, and their bones, eyes and internal organs are all that's visible. These frogs sleep on the bottoms of large leaves, and when they're transparent, they can perfectly match the colors of the vegetation.

Many animals in the sea can change the color of their skin or become completely transparent, but it's a far less common skillset on land. One reason transparency is so difficult to achieve is because of red blood cells in the circulatory system. Red blood cells are adept at absorbing green light, which is the color of light usually reflected by plants and other vegetation. In return, these oxygen-rich cells reflect red light, making blood—and by extension the circulatory system—highly visible, especially against a bright green leaf.

Glassfrogs are some of the only land-based vertebrates that can achieve transparency. They conducted additional imaging tests on the animals, proving via optical models that the animals were able to achieve transparency because they were pushing red blood cells out of their vessels.

For a see-through animal, its biology was shockingly challenging to decipher. The research drew on the expertise of biologists and biomedical engineers. 

During the experiments conducted in the lab, the results were startlingly clear: When the frogs were asleep, they removed nearly 90 percent of their circulating red blood cells and stored them in their liver. In further tests, the team also saw that red blood cells flowed out of the liver and circulated when the frogs were active, and then re-aggregated in the liver while the frogs were recovering.

Carlos Taboada et al, Glassfrogs conceal blood in their liver to maintain transparency, Science (2022). DOI: 10.1126/science.abl6620. www.science.org/doi/10.1126/science.abl6620