New research shows that glass frogs – known for their highly transparent undersides and muscles – perform their “disappearing acts” by stowing away almost all of their red blood cells in their uniquely reflective livers. The study, led by researchers at the American Museum of Natural History and Duke University, is published Friday in the journal Science. The work could lead to new avenues of research linked to blood clots, which the frogs somehow avoid as they pack and unpack about 90 percent of their red blood cells in their livers daily.
“There are more than 150 known species of glass frogs in the world, and yet we’ve only just begun to learn about some of the truly incredible ways they interact with their environment,” said co-author Jesse Delia, a Gerstner postdoctoral fellow. in the museum’s department of herpetology.
Living in the American tropics, glass frogs are nocturnal amphibians that spend their days sleeping upside down on translucent leaves that match the color of their backs – a common camouflage tactic. However, their bellies show something surprising: transparent skin and muscles that make their bones and organs visible, giving the glass frog its common name. Recent research has suggested that this adaptation masks the frogs’ outlines on their leafy perches, making them harder for predators to spot.
Transparency is a common form of camouflage among animals that live in water, but it is rare on land. In vertebrates, transparency is difficult to achieve because their circulatory system is full of red blood cells that interact with light. Studies have shown that kingfish and larval eels achieve transparency by not producing hemoglobin and red blood cells. But glassfrogs use an alternative strategy, according to the results of the new study.
“Ice frogs overcome this challenge by essentially hiding red blood cells from view,” said Carlos Taboada, the study’s co-author from Duke University. “They almost pause their respiratory system during the day, even at high temperatures.”
At Duke, the researchers used a technique called photoacoustic imaging, which uses light to induce sound wave propagation from red blood cells. This allows researchers to map the location of the cells in sleeping frogs without restraints, contrast agents, sacrifices, or surgical manipulation—especially important for this study because glass frog transparency is disrupted by activity, stress, stunning, and death.
The researchers focused on a particular species of glass frog, Hyalinobatrachium fleischmanni. They found that resting glass frogs increase transparency two to three times by removing nearly 90 percent of their red blood cells from circulation and packing them into the liver, which contains reflective guanine crystals. Whenever the frogs need to become active again, they bring the red blood cells back into the blood, which gives the frogs the ability to move – then the light absorption from these cells breaks the transparency.
In most vertebrates, the accumulation of red blood cells can lead to potentially dangerous blood clots in veins and arteries. But glass frogs do not experience coagulation, which raises a number of important questions for biological and medical researchers.
“This is the first of a series of studies documenting the physiology of vertebrate transparency, and it will hopefully stimulate biomedical work to translate the extreme physiology of these frogs into new targets for human health and medicine,” Delia said.
Other authors on the study include Maomao Chen, Chenshuo Ma, Xiaorui Peng, Xiaoyi Zhu, Tri Vu, Junjie Yao and So?nke Johnsen of Duke University; Laiming Jiang and Qifa Zhou, from the University of Southern California, Los Angeles; and Lauren O’Connell, of Stanford University.
This study was supported in part by the National Geographic Society, grant # NGS-65348R-19; Human Frontier Science Program Postdoctoral Fellowship # LT 000660/2018-L; Gerstner Scholars Fellowship provided by the Gerstner Family Foundation and the Richard Gilder Graduate School at the American Museum of Natural History; start-up funds from Stanford University; start-up funds from Duke University; National Institutes of Health, grant #s R01 EB028143, R01 NS111039, RF1 NS115581 BRAIN Initiative; a Duke Institute of Brain Science Incubator Award; American Heart Association Collaborative Sciences award 18CSA34080277; and a grant from the Chan Zuckerberg Initiative 2020- 226178.