New research shows that glass frogs – known for their highly transparent undersides and muscles – perform their “disappearing acts” by storing almost all of their red blood cells in their single-reflecting livers. The study, conducted by scientists from the American Museum of Natural History and Duke University, is published Friday in the journal Science. The work might open up new avenues of research related to blood clots, which frogs somehow avoid by packing and unpacking around 90% of their red blood cells in their livers daily.
“There are more than 150 species of known glass frogs in the world, and yet we’re only just beginning to learn regarding some of the truly amazing ways they interact with their environment,” said co-lead author Jesse Delia, Gerstner Postdoctoral Fellow. in the museum’s herpetology department.
Glassfrogs, 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 tactic. Their bellies, however, show something startling: translucent skin and muscles that allow their bones and organs to be seen, giving the glass frog its common name. Recent research has proposed that this adaptation obscures the outlines of frogs 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, achieving transparency is difficult because their circulatory system is full of 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 glass frogs use an alternative strategy, according to the results of the new study.
“Glass frogs overcome this challenge by essentially hiding red blood cells from view,” said Carlos Taboada, co-lead author of the Duke University study. “They almost pause their respiratory system during the day, even in high temperatures. »
At Duke, the researchers used a technique called photoacoustic imaging, which uses light to induce sound waves to propagate from red blood cells. This allows researchers to map the location of cells in sleeping frogs without coercion, contrast agents, sacrifice or surgical manipulation – especially important for this study because glass frog transparency is disrupted by activity, stress, anesthesia and death.
The researchers focused on a particular species of glass frog, Hyalinobatrachium fleischmanni. They found that resting glass frogs increased transparency two to three times by removing nearly 90% of their red blood cells from circulation and packing them into their liver, which contains reflective guanine crystals. Whenever the frogs need to become active once more, they bring the red blood cells back into the blood, which gives the frogs the ability to move – at which point the absorption of light from these cells breaks the transparency.
In most vertebrates, clumping of red blood cells can lead to the formation of potentially dangerous blood clots in veins and arteries. But glass frogs do not undergo coagulation, which raises a set of important questions for researchers in biology and medicine.
“This is the first in a series of studies documenting the physiology of vertebrate transparency, and we hope it will stimulate biomedical work to translate the extreme physiology of these frogs into new targets for human health and medicine,” Delia said.
Other study authors 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 of 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; the Human Frontier Science Program Postdoctoral Fellowship # LT 000660/2018-L; the Gerstner Scholars Fellowship offered by the Gerstner Family Foundation and the Richard Gilder Graduate School of the American Museum of Natural History; start-up funds from Stanford University; Duke University start-up funds; the National Institutes of Health, grant number R01 EB028143, R01 NS111039, RF1 NS115581 BRAIN Initiative; a Duke Institute of Brain Science Incubator Award; the American Heart Association Collaborative Science Award 18CSA34080277; and a Chan Zuckerberg Initiative grant 2020-226178.