Glass frogs become see-through by hiding their blood | Science

They’re called glass frogs for a reason. Flip the paperclip-size amphibians over, and you’ll see their bones, innards, and beating heart through a translucent belly.

Now, scientists have figured out how some of these tiny frogs, which reside in tropical forests throughout Central and South America, keep their skin so clear—they divert their blood into their livers to help them disappear.

In nature, transparency is largely reserved for fully aquatic creatures such as eel larvae and gelatinous jellyfish. Terrestrial animals and those that straddle land and water have a tougher time going clear because light reflects differently through air than water.

Another issue is blood. Red blood cells employ rust-tinted hemoglobin proteins that bind to oxygen. These proteins absorb light and give blood its crimson color, keeping skin opaque. Only Antarctic icefish, which inhabit the Southern Ocean’s frigid depths, have done away with hemoglobin entirely, giving their blood a cloudy white color.

To figure out how glass frogs overcome this hurdle, researchers used highly calibrated cameras to capture the transparency of Fleischmann’s glass frogs (Hyalinobatrachium fleischmanni), which congregate near streams throughout Central America. At night, when the amphibians breed and feed, they’re opaque. But during the day, when they snooze on leaves, most of their bodies, save for the lime green hue of their backs, turn transparent. This helps the frogs blend in like drops of dew, keeping them safe from spiders and snakes while they rest.

The scientists brought a few of the frogs back to the lab and monitored how their transparency shifted as they slept, exercised, chirped, or were under anesthesia. Sleeping glass frogs were between 34% and 61% more transparent than when they were active, the team reports today in Science.

A group of glassfrogs sleeping together upside down on a leaf, showing their leaf camouflage in transmitted (downwelling) light.
As they snooze, several glass frogs blend into the underside of a leaf save for the shadowy smudge of their internal organs.Jesse Delia

This increased transparency appeared to be linked with a lack of red blood cells coursing through their veins. “We could see that there was no blood there as the animals went to sleep,” says Carlos Taboada, a biologist at Duke University and an author of the new study. When they woke up, their blood started pumping again, reducing their transparency.

To determine where the blood cells went, Taboada and his colleagues utilized a technique called photoacoustic imaging, which maps the ultrasonic waves produced when red blood cells absorb light. During the day, blood vessels in the frogs’ livers were brimming with red blood cells, swelling the size of the organ by about 40%. Compared with other tree frogs, which can only store about 12% of their red blood cells in their livers, glass frogs can store a whopping 89%—nearly all of the red blood cells in their body.

How the animals survive this extreme adaptation is unclear, says study co-author Jesse Delia, a biologist at the American Museum of Natural History. “They’re basically not transporting very much oxygen for 12 hours a day.”

Another mystery is how glass frogs are able to move so many blood cells into one place without creating a potentially fatal clot. Solving that could lead to better blood clot treatments for humans, says Richard White, an oncologist at the University of Oxford who has studied the spread of cancer and other diseases in translucent zebrafish but was not involved with the new study. “This seemingly basic observation about glass frogs leads to very clear implications for human health.”

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