How transparent glass frogs hide their red blood from predators

At first glance, one might overlook the Costa Rican rainforest glass frog. As the name suggests, it is almost transparent. Except for a light green patch on its back, its skin, muscles, and other tissues are transparent. Then there are its tiny organs, which seem to float in that clear flesh like a pale cocktail of fruit in the strangest Jell-O lettuce to ever grace a branch.

As handy as translucency is to evade predators, it is rare in animals that live on land. Their bodies are full of substances that the light cannot penetrate, many of them essential to life. Glass frogs appear to have evolved see-through versions of some of these anatomical features, but they also have some tricks for hiding lingering colors when they are most vulnerable.

In a study published Thursday in the journal Science, researchers report that almost all red blood cells retreat to the liver when a glass frog falls asleep. They hide in the organ and allow the frog to become nearly invisible while it is dormant. The discovery not only reveals another remarkable adaptation in nature, but could also provide clues as to how deadly blood clots can be prevented.

Like humans, glass frogs rely on hemoglobin, a colored protein found in red blood cells, to help carry oxygen around the body. Jesse Delia and Carlos Taboada, biologists and authors of the new publication, spent a lot of time observing the frogs when they noticed that this red color sometimes seemed to disappear.

“When they’re awake, the circulatory system is red,” said Dr. Delia, who works at the American Museum of Natural History in New York. “When they sleep, it’s not.”

Where did the red blood cells go?

To solve the mystery of the disappearing blood cells, the researchers and their colleagues wanted to take pictures of the frogs under anesthesia – when the blood cells were clearly visible circulating through their bodies – and asleep when the cells were nowhere to be seen. To do this, they had to find a way to look inside the frog’s organs, which have a mirror-like exterior that helps the frog blend in. dr Taboada, a researcher at Duke University, said they suspected the blood would retreat to different organs when they weren’t circulating.

The researchers ended up relying on sound rather than light to show them what was inside. They provoked the molecules in the sacks to release ultrasonic waves that could be used to identify the contents.

As soon as they compared the images of sleeping and anesthetized frogs, a big difference was noticed.

“All the signals came from the liver,” said Dr. Taboada. About 89 percent of the frogs’ red blood cells were packed into this organ.

That made sense: The liver, which filters blood, is a logical destination for red blood cells, he said.

What was stranger, and what researchers still don’t understand, was how the frogs were able to stuff all of these cells together without dying from blood clots. When blood cells bump into each other, it causes clotting in most vertebrates. The resulting clot can form a scab to seal a wound—or, if the clot is in a blood vessel, clog the circulatory system and kill the creature. Up to 100,000 people die from blood clots in the United States each year, according to the Centers for Disease Control and Prevention.

Glass frogs can control when their blood clots, new research suggests. When injured, they form a scab in the usual way. But when they sleep, with red blood cells packed like sardines in the liver, no clot forms.

The finding implies that glass frogs could teach us about how to prevent clots from forming in our own bodies. If future research can shed light on what keeps the frogs safe, it could lead to treatments to reduce deaths from clots in humans.

More immediately, the researchers said, the results raise other questions. If 89 percent of the oxygen-carrying cells hide in the liver while the frog sleeps, how does it breathe? They wonder if the frogs can switch their metabolism to a mode that requires little oxygen, similar to other hibernating frogs.

The new paper is just the beginning of this line of research. The team has already improved their imaging techniques to scan the frogs more quickly and reveal substances other than blood as they move inside the creatures.

“We’re in the lab now,” said Dr. Delia during a phone interview. “There’s literally a frog scan in the system right now. I’ll have to check in a minute.”