What makes glass frogs transparent? The secret is in their blood.

Native to forests of Central and South America, glass frogs in the family Centrolenidae get their name from their translucent skin and muscles that blend them seamlessly into their jungle environment. Flip the amphibians over, where the effect is most impressive, and you’ll see their hearts, livers, and squiggly coils of intestines—no dissection needed.

And now, in a study published in the journal Science, researchers have discovered yet another amazing mechanism the animals use to become so transparent.

When fleischmanni's glass frogs (Hyalinobatrachium fleishmanni) go to sleep, they siphon off 89 percent of their brightly coloured red blood cells into crystal-lined sacs in their liver, which reflect incoming light and make the frogs appear nearly invisible. With their red blood cells out of view, the frogs become two to three times more transparent—a trick scientists believe helps the animals avoid predators. (Related: “These see-through frogs are full of surprises.”)

“Transparency is both rare and really hard to do, because our tissues are full of things that absorb and scatter light,” says study co-author Jesse Delia, a postdoctoral researcher and National Geographic Explorer at the American Museum of Natural History in New York City.

“Red blood cells also absorb a lot of light, and we found that the frog can actually hide them by packing them into the liver.”

Sleep secrets

Many aquatic creatures, like krill and salps, are see-through, but it’s exceedingly rare on land—which is why scientists have been long been intrigued by the glass frog’s ability to blend into its surroundings.

But before this study, no one had noticed the red blood cell phenomenon, perhaps because it only occurs while the frog sleeps throughout the day. (See more beautiful photos of frogs.)

What’s more, because glass frogs are nocturnal, those who study them tend to become nocturnal, too. “I worked at night, and so all the [wild] frogs I would see were awake,” says Delia, who studied parental care in glass frogs as part of his Ph.D. dissertation.

“It wasn’t until I brought one into captivity, and we saw it sleeping on glass, that we sort of realised, Holy cow, there’s something going on here,” he says.

The next question was how to study it.

The sound and the froggy

While the glass frog’s trick is so impressive that it can be seen by the naked eye, understanding how it works required an imaging technique known as photoacoustic microscopy.

“When pigments absorb light, part of the light that is absorbed then produces heat,” says study leader Carlos Taboada, a biologist at Duke University. “And that heat creates a local change in pressure, which creates sound waves.”

“It’s happening all around you, constantly,” adds Delia. “Everything that’s absorbing light, in theory, is also generating sound waves.”

In the lab, scientists took captive fleischmanni's glass frogs and put them through a series of imaging techniques. For instance, optical spectroscopy allowed the team to quantify how transparency increases during sleep, while the photoacoustic microscopy technique identified the red blood cells after they disappeared inside the liver.

This type of microscopy can also detect haemoglobin, a protein that transports oxygen in the blood. The team found that sleeping fleischmanni's glass frogs had an average of 96.6 percent less oxygenated haemoglobin in circulation than when they were active.

“The conclusions are, as far as I know, 100 percent novel!” Juan Manuel Guayasamín, an evolutionary biologist at Ecuador’s Universidad San Francisco de Quito and a National Geographic Explorer, says by email.

“Understanding the evolution of transparency has always had two main questions,” says Guayasamín, who wasn’t involved in the study. “Why is it produced? And how do animals become transparent? This paper beautifully answers the second question.”

A boon for people?

Not only are the findings fascinating and bizarre, but the researchers say they could lead to advances in human medicine.

That’s because many red blood cells in one place usually form a clot, which can block a blood vessel and lead to a potentially life-threatening condition, such as thrombosis. But the frogs can seemingly condense and expand their red blood cells at will—without any negative effects.

“And indeed, the animals can still clot normally when they’re wounded,” Taboada says.

This may mean the animals already possess what medical researchers have been seeking for decades: A biological mechanism that prevents excessive bleeding while also preventing excessive clotting.

“It’s been called the Holy Grail of haematology,” he says—and it may be hidden in plain sight, through the see-through skin of a tiny rainforest frog.