How transparent glass frogs hide their red blood from predators

At first glance, you might miss the Costa Rican rainforest glass frog. It is, as the name suggests, almost transparent. Apart from a lime green spot on its back, its skin, muscles and other tissues are transparent. Then there are his tiny organs, which seem to float within that pale flesh, like a pale fruit cocktail in the weirdest jello salad ever made on a tree branch.

As useful as translucency is for evading predators, it is rare in land-dwelling animals. Their bodies are full of substances that light cannot penetrate, many of them essential for life. Glass frogs seem to have evolved transparent versions of some of these anatomical features, but they also have some tricks to hide the persistent colors when they are most vulnerable.

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in the journal Science, researchers report that when a glass frog falls asleep, nearly all of its red blood cells are withdrawn to the liver.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

Like people, glass frogs depend on hemoglobin, a colored protein in red blood cells that provides oxygen to the body. Jesse Delia and Carlos Taboada, biologists and authors of the new paper, had been observing the frogs for a long time when they noticed that sometimes that red color 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, said. “When they’re asleep, it’s not.”

Where did the red blood cells go?

To solve the mystery of the disappearance of the 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 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 integrate. Dr. Taboada, a researcher at Duke University, said they suspected that the blood would be withdrawn to various organs when it was not circulating.

The researchers ended up relying not on light but on sound to show them what was inside. They caused the molecules inside the sacs 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.

“All the signal was coming from the liver,” said Dr. Taboada said. 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 strange, and what the researchers still don’t understand, was how the frogs could pack all those cells together without dying of blood clots. In most vertebrates, when blood cells collide with each other, it leads to 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, up to 100,000 people die from blood clots each year.

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

The finding implies that glass frogs might have something to 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 clot deaths in humans.

More immediately, the researchers said, the results raised other questions. If 89 percent of the oxygen-carrying cells are in the liver while the frog sleeps, how does it breathe? They wonder if frogs can switch their metabolism to a mode that requires almost no oxygen, perhaps similar to what other frogs do when they hibernate during the winter.

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

“We’re in the lab now,” said Dr. Delia said during a phone interview. “There’s literally a frog scanning the system right now. I have to go check it out in a little while.”

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