Glassfrogs, which live in the American tropics, are nocturnal amphibians who spend their days sleeping upside down on transparent leaves that match the color of their backs—a common camouflage technique. However, their stomachs reveal a startling feature: translucent skin and muscle that makes their bones and internal organs apparent, giving the glassfrog its common name.
In vertebrates, attaining transparency is difficult because their circulatory system is full of red blood cells (RBCs) that strongly attenuate light. Studies have shown that ice fish and larval eels achieve transparency by not producing hemoglobin and red blood cells. But glassfrogs use an alternative strategy, according to the new study’s findings.
A new study documented how glassfrogs overcome this challenge by concealing these cells from view. The study, led by scientists at the American Museum of Natural History and Duke University, shows that glassfrogs perform their “disappearing acts” by stowing away nearly all red blood cells into their uniquely reflective livers.
Carlos Taboada, the study’s co-lead author from Duke University, said, “Glassfrogs overcome this challenge by hiding red blood cells from view. They almost pause their respiratory system during the day, even at high temperatures.”
Using the photoacoustic imaging technique- which uses light to induce sound-wave propagation from red blood cells- scientists mapped the location of the cells within sleeping frogs without restraint, contrast agents, sacrifice, or surgical manipulation.
Hyalinobatrachium Fleischmann, a particular species of glassfrog was the subject of the study’s focus. They found that resting glassfrogs increase transparency two- to threefold by removing nearly 90 percent of their red blood cells from circulation and packing them within their liver, which contains reflective guanine crystals. The red blood cells are reintroduced into circulation whenever the frogs need to become active again, giving them the capacity to move. At this point, light absorption from the red blood cells causes transparency to be broken.
Red blood cell aggregation can result in life-threatening blood clots in veins and arteries in most vertebrates. However, the absence of clotting in glassfrogs poses several fundamental problems for biological and medical experts. This ability of glassfrogs offers insight in metabolic, hemodynamic, and blood-clot research.