Scientists just revealed the mystery behind how glass frogs maintain transparency

The finding has huge implications for biomedical research.
Deena Theresa
A transparent glass frog.
A transparent glass frog.

Thorsten Spoerlein/iStock 

Scientists have revealed the secret powers of the humble glass frog and how they protect themselves in the thick forests of Central and South America.

Allow us to explain. Glass frogs get their name from their translucent, glass-like skin. Now, at night when they're out, they turn green, seamlessly blending with the thick foliage. But, during the day, the northern glass frog, Hyalinobatrachium fleischmanni, becomes the true master of camouflage when they arm themselves with their invisible coat.

"When glass frogs are resting, their muscles and skin become transparent, and their bones, eyes, and internal organs are all that's visible," Carlos Taboada, a post-doctoral fellow at Duke and a co-first author of the paper, said in a statement.

This special effect makes them almost invisible to their predators.

Scientists just revealed the mystery behind how glass frogs maintain transparency
Glassfrog photographed during sleep and while active, using a flash, showing the difference in red blood cell perfusion within the circulatory system.

How do glass frogs become invisible?

The glass frogs temporarily store nearly all of their red blood cells in their reflective livers.

Fun fact: Despite packing and unpacking red blood cells daily into a small space, the nocturnal amphibians do not experience dangerous clotting. This finding has huge implications for biomedical research.

How do these glass frogs achieve transparency? 

Ironically, for a see-through animal, it wasn't the least bit easy to decipher the creature's biology.

Using photoacoustic imaging to investigate the transparency

"If these frogs are awake, stressed or under anesthesia, their circulatory system is full of red blood cells, and they are opaque," Jesse Delia, a post-doctoral fellow at the American Museum of Natural History, said in a statement. "The only way to study transparency is if these animals are happily asleep, which is difficult to achieve in a research lab. We were really banging our heads against the wall for a solution."

To investigate further into the matter, a multi-disciplinary team of biologists and biomedical engineers used a technique called photoacoustic imaging at Duke University. As per a release, the method uses light to induce sound-wave propagation from red blood cells, allowing the team to map the cells' location within sleeping frogs without using restraint, contrast agents, sacrifice, or surgery. 

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The team found that resting glass frogs increase transparency two- to threefold by removing almost 90 percent of their red blood cells from circulation and hiding them within their liver, which contains "reflective guanine crystals that shield the cells from light."

And at night, when the frogs become active, they bring back the red blood cells into the blood.

Scientists just revealed the mystery behind how glass frogs maintain transparency
A group of glassfrogs sleeping together upside down on a leaf, showing their leaf camouflage in transmitted (downwelling) light.

Glass frogs could be a very useful model for research

"The primary result is that whenever glass frogs want to be transparent, which is typically when they're at rest and vulnerable to predation, they filter nearly all the red blood cells out of their blood and hide them in a mirror-coated liver -- somehow avoiding creating a huge blood clot in the process," said Sönke Johnsen, a professor of biology at Duke who specializes in studying transparency. "Whenever the frogs need to become active again, they bring the cells back into the bloodstream, which gives them the metabolic capacity to move around."

The team's work isn't over. The process raises questions about how the glass frogs store their red blood cells in their liver without clotting or damaging their tissues. This mechanism could be studied to find out how it could apply to vascular issues in humans.

This study could also introduce glass frogs as a useful model for research, especially when paired with state-of-the-art photoacoustic imaging.

"We can learn more about the glass frog's physiology and behavior, or we can use these models to optimize imaging tools for biomedical engineering," Delia said. 

The study was published in the journal Science on December 22.

Study Abstract:

Transparency in animals is a complex form of camouflage involving mechanisms that reduce light scattering and absorption throughout the organism. In vertebrates, attaining transparency is difficult because their circulatory system is full of red blood cells (RBCs) that strongly attenuate light. Here, we document how glass frogs overcome this challenge by concealing these cells from view. Using photoacoustic imaging to track RBCs in vivo, we show that resting glass frogs increase transparency two- to threefold by removing ~89% of their RBCs from circulation and packing them within their liver. Vertebrate transparency thus requires both see-through tissues and active mechanisms that "clear" respiratory pigments from these tissues. Furthermore, glass frogs' ability to regulate the location, density, and packing of RBCs without clotting offers insight into metabolic, hemodynamic, and blood-clot research.