Glass frogs — known for their highly transparent abdomens and muscles — execute their “vanishing behavior” by storing almost all of their red blood cells into their unique light-reflecting livers, new research shows.The study, led by scientists from the American Museum of Natural History and Duke University, will be published Friday in the journal scienceThe work may open up new avenues of research related to blood clots, which frogs somehow avoid as they pack and unpack about 90 percent of their red blood cells each day into their livers.
“There are more than 150 known species of glass frogs in the world, but we’re really just beginning to understand some of the truly incredible ways they interact with their environment,” said co-lead author Jesse Delia, a Gerstner postdoctoral fellow. In the herpetology department of the museum.
Glass frogs in tropical America are nocturnal amphibians that sleep upside down during the day on translucent leaves that match the color of their backs—a common camouflage tactic. Their bellies, however, reveal something surprising: translucent skin and muscles that allow their bones and organs to be seen, hence the glass frog’s name. Recent research has shown that this adaptation masks the frogs’ silhouette on leafy perches, making them harder for predators to spot.
Transparency is a common form of camouflage for animals that live in water, but is rare on land. In vertebrates, achieving transparency is difficult because their circulatory system is filled with red blood cells that interact with light. Studies have shown that icefish and eel larvae achieve transparency by not producing hemoglobin and red blood cells. But according to the results of the new study, glass frogs use another tactic.
“Glass frogs overcome this challenge by essentially hiding red blood cells,” said study co-lead author Carlos Taboada from Duke University. “They nearly stop their respiratory system during the day, even in the heat.”
At Duke University, the researchers used a technique called photoacoustic imaging, which uses light to induce the propagation of sound waves in red blood cells. This allowed the researchers to map the location of cells in sleeping frogs without restraint, contrast agents, sacrifice or surgical manipulation—especially important for this study, as the glass frog’s transparency can be affected by activity, stress, anesthesia and death. And interrupt.
The researchers focused on a specific species of glass frog, transparent frogThey found that resting glass frogs increased transparency two to three times by removing nearly 90 percent of their red blood cells from circulation and packing them in livers containing light-reflecting guanine crystals. Whenever the frog needs to be active again, they bring red blood cells back into the bloodstream, allowing the frog to move around — at which point the cells’ light absorption disrupts the transparency.
In most vertebrates, aggregated red blood cells lead to potentially dangerous blood clots in veins and arteries. But glass frogs don’t clot, raising a host of important questions for biological and medical researchers.
“This is the first in a series of studies documenting the physiology of transparency in vertebrates, and it promises to stimulate biomedical work that translates the extreme physiology of these frogs into new targets for human health and medicine,” Delia said.
Other authors of the study include Maomao Chen, Chenshuo Ma, Xiaorui Peng, Xiaoyi Zhu, Tri Vu, Junjie Yao and So?nke Johnsen from Duke University; Laiming Jiang and Qifa Zhou from USC Los Angeles; and Stanford University University’s Lauren O’Connell.
This research was supported in part by the National Geographic Society, grant #NGS-65348R-19; Human Frontier Science Program Postdoctoral Fellowship #LT 000660/2018-L; the Gerstner Family Foundation and Richard Gere of the American Museum of Natural History Gerstner Scholars Fellowship from the German Graduate School; Startup Funding at Stanford University; Startup Funding at Duke University; National Institutes of Health, Award #s R01 EB028143, R01 NS111039, RF1 NS115581 BRAIN Initiative; Duke Brain Institute Incubator Award; American Heart Association Collaborative Science Award 18CSA34080277; and Chan Zuckerberg Initiative Grant 2020-226178.