Transparent glassfrogs “hide” red blood cells in their livers and “disappear” at night

Side-by-side comparison of a glassfrog taken with a flash while it slept and when it was active, illustrating the difference in the amount of red blood cells delivered to the body’s tissues. Photo by Jesse Delia.

Glassfrogs “disappear” as they go to sleep. The frog’s bright green back blends into the background as it rests on a lush leaf, and its reddish underside quickly becoming clear.

A study released in Science explains how the northern glassfrog (Hyalinobatrachium fleischmanni) accomplishes this trick by hiding away over 90% of its circulating red blood cells within its liver. The results explain how one of the only land creatures with a see-through body conceals its blood.

Researchers have suggested that understanding why these clots never develop may have significance for human diseases.

Northern glassfrogs rarely reach a size greater than 1 inch (2.54 centimeters) in length and spend most of their adult lives sitting on leaves in the forest canopy of Central and South America, well above the swiftly running streams where they lay their eggs. Even while the frogs are awake, an observer may see red blood pulsing through their veins since their bellies are transparent. However, the frogs’ transparent bellies have long captivated biologists because of how well they protect the species from predators. Red blood cells must be concealed if you really want to be invisible according to Sönke Johnsen, a professor of biology at Duke University in North Carolina.  Somehow, these glassfrogs are filtering out their red blood cells and putting them into their livers, where they should clump together and cause a clot; however, it does not.

Johnsen and coworkers followed individual red blood cells as they traveled throughout the bodies of glassfrogs to get insight into this phenomenon. By shining a bright light onto the frog’s body, scientists recorded the sound waves emitted when the light strikes hemoglobin, the protein in red blood cells that delivers oxygen and gives blood its unique hue. This technique is called photoacoustic microscopy.

Even with a transparent animal, it can be difficult to observe exactly what is going on inside, the researchers added. Unlike light, sound is able to penetrate living tissue.

Once scientists had this method perfected, understanding what causes a glassfrog to turn transparent was as easy as continually disturbing the hapless creatures. The researchers let the frog sleep, poke it a couple of times, and then let it sleep some more. The  research showed that glassfrogs remove 89% of their red blood cells from circulation and store them in their livers. Without hemoglobin in their blood, they become nearly invisible to the naked eye because their skin reflects so little light.

Johnsen and his team believe that more research into this phenomenon will help them better understand human coagulation diseases and guide their work on developing new anticoagulants.

Whether it’s large clots in strokes that cause severe damage or small clots in the peripheral that cause so much suffering, the human body is always at this razor edge between clotting too little and too much. The knowledge about  frog clotting could be applicable to human coagulation because of the similarities between the two processes.

Yet many aspects of this procedure, such as how they manage to live with such low levels of circulating hemoglobin during sleep, remain a mystery. Johnsen and colleagues will need to identify how glassfrogs manipulate their blood before the amphibians can be used as a resource in therapeutic research.

The act these frogs are performing is similar to that of a human taking all of his or her blood and storing it in a lunch bag inside of the body, as stated by Johnsen. This begs the question, how do glassfrogs accomplish this feat? The fact that we have no idea is itself fascinating.


Taboada, C., Delia, J., Chen, M., Ma, C., Peng, X., Zhu, X., Jiang, L., Vu, T., Zhou, Q., Yao, J., O’Connell, L., & Johnsen, S. (2022). Glassfrogs conceal blood in their liver to maintain transparency. Science (New York, N.Y.)378 (6626), 1315–1320.