A male glassfrog photographed in ventral view

A male glassfrog photographed in ventral view using a flash, showing its transparency. The team measured muscle and ventral skin and found that these tissues transmit over 90% of visible light and are still perfectly functional. These tissues are so transparent that their organs and bones can be viewed through them. (Credit: Jesse Delia)

DURHAM, N.C. — Scientists have discovered the mechanism which allows glassfrogs to become transparent. The breakthrough could lead to new treatments for blood clots.

The amphibians blend into the background when they become see-through as they sleep. Now, researchers have discovered they do it by hiding 90 percent of their red blood cells in their liver — which is mirror-coated! Their beady eyes, bones, and internal organs are all that remain visible.

Surprisingly, the glassfrogs manage to avoid getting a massive blood clot during this process. When they wake up, the blood flows back out into their whole body again.

“The primary result is that whenever glassfrogs 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,” says Sönke Johnsen, a professor of biology at Duke, in a media release. “Whenever the frogs need to become active again, they bring the cells back into the blood stream, which gives them the metabolic capacity to move around.”

A male glassfrog photographed in ventral view
A male glassfrog photographed in ventral view using a flash, showing its transparency. The team measured muscle and ventral skin and found that these tissues transmit over 90% of visible light and are still perfectly functional. These tissues are so transparent that their organs and bones can be viewed through them. (Credit: Jesse Delia)

It’s extremely hard to spot a glassfrog in nature

The creatures – which live in the southern United States, Central America, the Caribbean, and northern South America – are nocturnal animals which sleep upside down on translucent leaves that match the color of their backs. They can be almost impossible to spot when they are awake because they are just a few centimeters long and are most active at night when their green skin helps them blend in with surrounding grass and leaves. However, they become true masters of camouflage when they are asleep.

While many sea creatures including ice fish and larval eels can become transparent or change the color of their skin, that skill is far less common on land. Glassfrogs do it to make it harder for predators to spot them. Red blood cells in the circulatory system make it difficult for most animals to become transparent.

These cells are good at absorbing green light, which is the color of light normally reflected by plants and other vegetation. In return, oxygen-rich red blood cells reflect red light which makes blood and the circulatory system highly visible, especially against a bright green leaf.

Glassfrogs are one of just a few land-based vertebrates that can become transparent, which has made researchers particularly interested in them, but few have understood the process until now.

A group of glassfrogs sleeping together upside down on a leaf
A group of glassfrogs sleeping together upside down on a leaf, showing their leaf camouflage in transmitted (downwelling) light. (Credit: Jesse Delia)

These frogs have a ‘shockingly’ difficult biology

For the study, the team focused on one particular type of glassfrog called hyalinobatrachium fleischmanni. The researchers travelled the world collecting glassfrogs to examine.

They conducted imaging tests on the animals and used them to produce optical models which proved that they were able to become transparent by pushing red blood cells out of their vessels. The team suspected that the cells were being stored in one of the frog’s inner organs which are packaged in a reflective membrane.

They explain that for a see-through animal, its biology was “shockingly” difficult to decipher. Experts from across the U.S. had to help the team get a grip on it.

“If these frogs are awake, stressed or under anesthesia their circulatory system is full of red blood cells and they are opaque,” explains study author Dr. Jesse Delia, who now works at the American Museum of Natural History.

“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.”

PAM image of RBC distribution and oxygenation
Photoacoustic Microscopy images showing circulating red blood cells within a glassfrog while asleep and under anesthesia (which breaks blood storage). This system uses light to induce absorption in a hemoglobin and an ultrasound transducer to map the soundwaves back to the point of optical absorption. The color scheme shows the percent of hemoglobin bound with oxygen. These images were acquired on an OR-PAM system developed by Dr. Junjie Yao’s lab. (Credit: Junjie Yao, Duke University)

New technology literally sheds light on glassfrogs

An imaging technology called photoacoustic microscopy (PAM) helped researchers make this discovery. This process involves shooting a safe laser beam of light into a tissue, which is absorbed by molecules and converted into ultrasonic waves.

These sound waves are then used to make detailed biomedical images of the molecules. The imaging tool is non-invasive, quiet, and sensitive.

“The red blood cells themselves provide the contrast, because different types of cells absorb and reflect different wavelengths of light. We could optimize our imaging systems to specifically look for red blood cells and track how much oxygen was circulating in the frog’s bodies,” explains Junjie Yao, an assistant professor of Biomedical Engineering at Duke.

During the study, the frogs slept upside down in a petri-dish which is similar to how they would sleep on a leaf. The researchers then shined a green laser at them. Red blood cells in the frog’s body absorbed the green light and emitted ultrasonic waves, which were then picked up by an acoustic sensor to trace their whereabouts.

A pair of mating glassfrogs sleep together on a leaf.
A pair of mating glassfrogs sleep together on a leaf. (Credit: Jesse Delia)

The team now want to look at the process of how the frogs manage to store so many cells in their livers to understand more about human vascular health.

“This is the first of a series of studies documenting the physiology of vertebrate transparency, and it will hopefully stimulate biomedical work to translate these frogs’ extreme physiology into novel targets for human health and medicine,” Delia concludes.

The findings are published in the journal Science.

South West News Service writer Gwyn Wright contributed to this report.

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