Glass frogs, which live in the American tropics, are nocturnal amphibians that spend their days sleeping upside down on translucent leaves that match the color of their backs – a common camouflage technique. However, their stomachs reveal a surprising feature: translucent skin and muscles that reveal their bones and internal organs, giving the glass frog its common name.

In vertebrates, achieving transparency is difficult because their circulatory systems are packed with red blood cells (RBCs) that strongly attenuate light. Studies have shown that toothfish and eel larvae become transparent by not producing hemoglobin and red blood cells. But glass frogs use an alternative strategy, according to the new study’s findings.

A new study documented how glass frogs overcome this challenge by hiding these cells from view. The study, led by scientists from the American Museum of Natural History and Duke University, shows that glass frogs perform their “vanishing actions” by storing nearly all red blood cells in their uniquely reflective livers.

Carlos Taboada, co-lead author of the study from Duke University, said: “Glassfrogs overcome this challenge by hiding red blood cells from view. They almost pause their breathing during the day, even at high temperatures.”

Using the photoacoustic imaging technique – which uses light to induce sound wave propagation of red blood cells – scientists mapped the location of the cells in sleeping frogs without restraint, contrast agents, sacrifice or surgical manipulation.

Hyalinobatrachium Fleischmann, a species of glass frog, was the subject of the study. They found that resting glass frogs increase transparency two to threefold by removing nearly 90 percent of their red blood cells from circulation and depositing them in their liver, which contains reflective guanine crystals. The red blood cells are recirculated when the frogs need to become active again, allowing them to move. At this point, the light absorption of the red blood cells causes the transparency to be broken.

Red blood cell aggregation can lead to life-threatening blood clots in veins and arteries in most vertebrates. However, the absence of coagulation in glass frogs poses some fundamental problems for biological and medical experts. This ability of glass frogs provides insight into metabolic, hemodynamic, and blood clot studies.

Jour reference:

  1. Carlos Taboada et al. Glass frogs hide blood in their livers to maintain transparency. Science. DOI: DOI: 10.1126/science.abl6620