The same glassfrog photographed during sleep and while active, using a flash, showing the difference...

Frogger

These frogs can become “invisible” and scientists just figured out their secret

No red blood cells mean frog stealth mode.

Jesse Delia

What if you had the power of invisibility, but it only works when you’re asleep? That might be useful for a surreptitious nap in public, but it still probably wouldn’t suit you as well as it does the Northern glass frog.

The Hyalinobatrachium fleischmanni, native to humid forests throughout Central and South America, has this dubious superpower. It’s just one of more than 100 species of glass frogs that are so-called because of their translucent skin and organs. Back in 1980, one study from the Journal of Morphology found that their bellies lack pigment, so their internal organs are visible. Then, a 2020 study in the journal Proceedings of the National Academies of Sciences proposed that this translucency is actually a camouflage technique.

Now, we know how they accomplish this vanishing act. A paper published today in the journal Science offers the results of experiments on 13 of these little guys. Northern glass frogs, and likely other species of glass frogs, can achieve quasi-invisibility through a physiological act that’s even more impressive than disappearing itself.

Frogs photographed in transmitted light on leaves, showing how blood perfusion increases the glass frogs’ silhouette. The species from the opaque sister taxa is Allophryne ruthveni, which is the closet related species to glass frogs that is not transparent.Jesse Delia

What’s new — Thanks to this study, we learned that these tiny glass frogs — measuring between three-quarters of an inch and one-and-a-half inches — go into stealth mode when they’re asleep. We also now know how they do it. When they snooze during the day, these little guys corral most of their red blood cells into their liver, rendering them nearly transparent.

“They basically remove over 83 percent of their red blood cells from circulation, and they hide them in their liver, and they somehow do this without forming a massive clot,” says co-author Jesse Delia, a herpetologist and postdoctoral scholar at the American Museum of Natural History. In fact, these frogs can sift out up to about 89 percent of red blood cells.

In 2018, Delia and co-author Carlos Taboada, a biology researcher at Duke University, were working on tissue transparency in these frogs’ muscles, but then noticed one day while the frogs snoozed that there were no red blood cells visible.

“That was just crazy,” Delia tells Inverse. “So we just dropped everything and started working on that.”

Red blood cells reflect green light, so removing the pigmented cells from circulation lets the frogs’ translucent skin transmit more light. What’s more, since light passes through them, they cast less of a silhouette that predators might spot. The authors observed that, on average, these frogs become 34 to 61 percent more transparent in sleep. These critters are able to achieve and sustain this state of transparency in dreamland. Once the frogs awaken, the red blood cells recirculate and make the skin opaque again.

While he’s not certain, Delia believes this is a camouflage technique that allows the glass frogs to sleep safely during the day without fear of predators.

A glass frog sleeps as a photoacoustic microscope scans it, measuring acoustic waves created by molecules in red blood cells absorbing light and producing ultrasonic waves.Jesse Delia

Why it matters — Transparency in vertebrates is extremely rare. This discovery makes glass frogs a model species for investigating transparency in vertebrates, especially while the underlying biological mechanisms are still in question. For most vertebrates, packing red blood cells together causes coagulation and a clot.

“These frogs have some unique ability to survive with an extremely depressed cardiorespiratory system for 12 hours a day,” Delia tells Inverse. Not only do the packed red blood cells not clot, but the frog can sleep soundly with just a fraction of its circulatory system functioning, leading Delia to postulate the animal can live on less oxygen temporarily.

On the other hand, it’s more common for sea creatures like jellyfish, octopuses, and squids to enter ghost mode. This tactic of transmitting light rather than reflecting it comes in handy in the deep sea, where there’s lots of shimmering blue light and little to hide behind.

The jury’s still out on whether other species of glass frog can pack their red blood cells into their liver, but Delia says it seems likely.

Humans can leverage this amphibian’s ability, too. Richard White, a biologist who just recently left Memorial Sloan Kettering Cancer Center for the University of Oxford, wrote an article accompanying this paper in Science on this insight’s high points. He was not involved in the research.

“The implication, which I think would be really neat, would be if you can figure out how the frog doesn't clot its blood,” White tells Inverse. “Maybe it gives us this insight into novel blood clotting mechanisms and ways to prevent blood clotting because clearly, the frogs have figured it out.” Knowledge of this mechanism could help humans develop better drugs for common clotting disorders like pulmonary emboli and deep venous thrombosis.

Changes in the density of circulating red blood cells as a glass frog awakes. Video covers a small region of abdominal tissues (skin and muscle) in transmitted light (backlit).Carlos Taboada

Digging into the details — The real challenge to this study was looking at these frogs’ innards as they slept. Any invasive procedure could wake them up from their crucial slumber.

A lab at Duke University took care of this issue. Specializing in photoacoustic microscopes, the lab helped the team conduct optical spectroscopy and photoacoustic microscopy on 13 of these frogs. Optical spectroscopy detects light all throughout the electromagnetic spectrum — extending beyond the range of visible light to infrared and ultraviolet light as well.

The photoacoustic effect, Delia says, uses light to do sound wave propagation. When molecules absorb light, some of that light is converted into ultrasonic waves, which are not on the electromagnetic spectrum. Molecules in red blood cells are reflecting light as well as sound waves, which are at a frequency that humans can't hear.

The photoacoustic microscope acoustically measures this ultrasonic wave production, creating an image of these frogs’ innards based on both optics and acoustics — all without waking the sleepy frogs.

This dual technique allows for deeper imaging of the frogs’ blood vessels, targeting hemoglobin pigments in red blood cells for maximum precision.

Glass frogs are at their most translucent as they sleep, packing red blood cells into their liver.Jesse Delia

What’s next — The big questions for Delia and his team are: Why did glass frogs evolve this ability, and what mechanisms underly the process?

“We’re definitely interested in some of the predator-prey dynamics that may have led to transparency,” Delia says.

White believes this is evolution at its finest. “It's like a beautiful example of evolution,” he tells Inverse. “Evolution never finds the solution — it finds a solution. This is just such a rare and unusual of a usual evolutionary solution to camouflage.”

Finding biological mechanisms that allow frogs to round up red blood cells will take much more digging. These systems are all intricate universes unto themselves. “Even red blood cells require a whole suite of mechanisms, so the ability to pack red blood cells together without forming a clot is really interesting,” Delia says.

If this sleepy vanishing act has let glass frogs survive and thrive, then it’s a worthwhile superpower for them.

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