Glass frogs hide red blood cells in their liver to become transparent

Glass frogs use transparency as a camouflage strategy when they sleep. A group of researchers discovered that to do this, these batrachians accumulate red blood cells in the liver, which contains reflective crystals of guanine.

This finding, which is described in a study by the American Museum of Natural History and Duke University (USA), also opens avenues for research into the prevention of blood clots.

Living in the American tropics, glass frogs are nocturnal amphibians that spend the day sleeping upside down on translucent leaves the same colour as their backs, a common camouflage tactic. The skin under the animal becomes translucent allowing you to see its bones and organs.

"There are more than 150 known species of glass frogs in the world and we are just beginning to understand some of the incredible ways in which they interact with their environment," explained Jesse Delia, from the museum and co-author of the study.

The work published in Science could lead to new avenues of research related to blood clots, which frogs somehow avoid as they pack and unpack about 90 per cent of their red blood cells daily in their livers.

Transparency is a common form of camouflage among animals that live in water, but it is rare on land, and in vertebrates, it is difficult because their circulatory system is filled with red blood cells that interact with light.

Glass frogs overcome that challenge by "hiding the red blood cells in plain sight" by "nearly pausing their respiratory system during the day, even at high temperatures," explained Carlos Taboada, a study co-author at Duke University.

The researchers used a technique called photoacoustic imaging, which uses light to induce the propagation of sound waves from red blood cells, making it possible to map the location of cells inside sleeping frogs without disturbing them, since transparency is altered by light activity, stress, anaesthesia and death.

The study focused on a specific species of glass frog, Hyalinobatrachium Fleischmann, which at rest increases its transparency two to threefold by removing almost 90% of its red blood cells from circulation and packaging them in the liver, which contains reflective crystals of guanine.

When they need to be active again, they return red blood cells to the blood, giving them the ability to move; at that time, the light absorption of these cells breaks the transparency.

In most vertebrates, red blood cell aggregation can cause potentially dangerous clots in veins and arteries, but this is not the case in frogs, raising a number of important questions for biological and medical researchers.

This is the first in a series of studies documenting the physiology of vertebrate transparency, and the researchers hope it will "spur biomedical work to translate the extreme physiology of these frogs into novel targets for human health and medicine" Delia said.

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