One of the few terrestrial animals capable of becoming transparent, glass frogs transfer red blood cells from their blood to their liver while sleeping.
Glass frogs are named for their remarkable ability to turn transparent while asleep, a strategy to evade predators. In a study published in the journal Science on December 22, researchers revealed their secret: they move red blood cells into their liver. This new discovery could have implications for medicine.
Glass frogs sleeping (left) and awake (right), showing how red blood cells are concealed. (Photo: Jesse Delia)
Found throughout the tropical regions of the Americas, glass frogs only display their green color when active at night. During the day, they sleep on the undersides of leaves. At this time, their skin and muscle tissues become transparent, revealing only their bones, eyes, and internal organs. If not observed closely, predators can hardly detect them.
Many aquatic creatures also employ transparency as a strategy to avoid predation, but glass frogs are among the few terrestrial animals that can achieve this. A significant challenge is that red blood cells are typically very visible. Some fish species avoid this by not producing red blood cells, but frogs do not take this route.
To investigate further, scientists from the American Museum of Natural History and Duke University studied the glass frog Hyalinobatrachium fleischmanni in a controlled environment using photoacoustic imaging techniques.
They discovered that while sleeping, the frogs transfer nearly 90% of their red blood cells from their blood to their liver, making their skin and muscle tissues more than 2 to 3 times transparent. Additionally, their livers have a reflective outer membrane, which further reduces visibility.
In most vertebrates, such a concentration of red blood cells could lead to dangerous blood clots forming in the bloodstream. However, this does not occur in glass frogs. The research team is working to understand why and hopes their findings will aid in developing anticoagulant medications for humans.
“This is the first study to document the physiological process of transparency in vertebrates. We hope it will spur biomedical research aimed at applying the unique physiological process of these frogs to medicine and human health,” said Jesse Delia, a postdoctoral researcher at the American Museum of Natural History and co-author of the study with Carlos Taboada from Duke University.
