How a transparent frog hides its red blood from predators

At first glance, you might not see the glass tree frog, a species that inhabits tropical regions such as Costa Rica. As the name suggests, it is almost transparent. Aside from a lime-green patch on her back, her skin, muscles, and other tissues are transparent.

Then there are your tiny organs, which seem to float within this translucent flesh, like a cocktail of pale fruit in the strangest jelly you’ve ever seen on a tree branch.

As useful as transparency can be for escaping predators, it is rare in animals that live on land. Their bodies are full of substances that light cannot penetrate, many of them essential for life.

Glass tree frogs seem to have evolved transparent versions of some of these anatomical features, but they also have some tricks for hiding persistent colors when they are most vulnerable.

In a study published in the journal Science on Thursday, researchers report that when a glass tree frog falls asleep, nearly all of its red blood cells retreat to the liver. They hide in the organ and allow the tree frog to achieve near invisibility while resting. As well as revealing another remarkable adaptation in nature, the discovery could lead to clues about how to prevent deadly blood clots.

Like humans, glass tree frogs depend on haemoglobin, a colored protein found in red blood cells that supplies the body with oxygen. Biologists Jesse Delia and Carlos Taboada, authors of the new paper, spent a long time observing the amphibians when they noticed that, at times, that red color seemed to disappear.

“When they’re awake, their circulatory system is red,” said Delia, who works at the American Museum of Natural History in New York. “When they’re sleeping, no.”

Where did the red blood cells go?

To solve the mystery of the blood cells’ disappearance, the researchers and their colleagues wanted to take pictures of tree frogs under anesthesia — when blood cells were clearly visible circulating through their bodies — and sleeping, when the cells were nowhere to be seen.

To do this, they needed to find a way to peer inside the frog’s organs, which have a mirror-like exterior that helps the frog disguise itself. Taboada, a researcher at Duke University, said he suspected that blood backed up to various organs when it was not circulating.

The researchers ended up relying not on light but on sound to show what was inside. They provoked the molecules inside the vacuoles (sort of bags for carrying molecules inside cells) to release ultrasonic waves, which could be used to identify the contents.

As soon as they compared the images of sleeping and anesthetized frogs, a big difference jumped out at them.

“All the signal came from the liver”, said Taboada. About 89% of the frogs’ red blood cells accumulated in that organ.

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

The strangest thing, and what the researchers still don’t understand, is how the frogs managed to gather all these cells together without dying from blood clots. In most vertebrates, when blood cells collide, they cause clotting.

The resulting clot can form a scab to seal a wound—or, if the clot is in a blood vessel, it can clog the circulatory system and kill the creature. In the United States, according to the Centers for Disease Control and Prevention (CDC), about 100,000 people die from blood clots each year.

Glass frogs can control when blood clots, new research suggests. If they are injured, they will form a scab in the usual way. But when they’re asleep, with their red blood cells packed like sardines in their livers, no clots form.

The discovery suggests that glass tree frogs may have something to teach us about how to prevent clots from forming in our bodies. If future research can shed light on what keeps frogs alive, it could lead to treatments to reduce deaths from clots in humans.

More immediately, the researchers said, the results raised other questions. If 89% of the cells that carry oxygen are hidden in the liver while the tree frog sleeps, how does it breathe? They wonder if frogs can switch their metabolisms to a mode that requires almost no oxygen, perhaps similar to what other frogs do when hibernating.

The new article is just the beginning of this line of research. The team has already honed its imaging techniques to scan the amphibians more quickly and reveal substances other than blood as they move through the creatures.

“We’re in the lab now,” Delia said during a phone interview. “There is literally a frog being scanned in the system right now. I need to check that out in a bit.”

^{Translated by Luiz Roberto M. Gonçalves}