When a tiny glass frog dozes off to sleep, its body becomes so transparent it’s almost invisible. The amphibian’s glass-clear skin casts no shadows. Even the red blood disappears from its veins. It’s an unusual trick—most see-through animals are aquatic, such as icefish or jellyfish, which don’t produce hemoglobin or red blood cells.
“Transparency is both rare and really hard to do, because our tissues are full of things that absorb and scatter light,” Jesse Delia, a researcher at New York’s American Museum of Natural History, tells National Geographic’s Jason Bittel. “Red blood cells also absorb a lot of light.”
But when the nocturnal frogs are active, blood begins to snake again through their circulatory systems, forming a visible maze of bright red. So, scientists set out to discover what happens to all that blood during the day.
In a new study published Thursday in Science, Delia and other researchers detail how, while sleeping, glass frogs divert most of their red blood cells to their liver, which grows in size by about 40 percent to accommodate the extra cells. Like its heart and digestive organs, the amphibian’s liver is encased in a mirrored sac that helps the frogs camouflage.
To crack the case of the vanishing blood, researchers used imaging techniques on both asleep and awake frogs—an undertaking that initially proved difficult.
“If these frogs are awake, stressed or under anesthesia, their circulatory system is full of red blood cells, and they are opaque,” explains Delia 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.”
But Carlos Taboada, a biologist at Duke University and first author of the paper, had heard about a non-invasive imaging technology called photoacoustic microscopy, which can detect red blood cells using a laser, per the statement. When molecules absorb the light from the lasers, they release sound waves that researchers can detect and use to map the frogs’ bodies.
“Everything that’s absorbing light, in theory, is also generating sound waves,” Delia tells National Geographic. “It’s happening all around you, constantly.”
The team used this technique on frogs while they were sleeping and under anesthesia. They found the animals moved a shocking 89 percent of their red blood cells to their liver while snoozing.
“It’s not like they put some blood in their liver—they put almost all their blood in their liver,” Karen Warkentin, a biologist at Boston University who was not involved in the work, tells New Scientist’s Corryn Wetzel. “I just find that pretty amazing.”
The team also used calibrated color photography to image 11 glass frogs while they were asleep, awake, under anesthesia, calling mates and recovering after exercise. They measured the animals’ transparency during these activities and found frogs became an average of 34 to 61 percent more see-through when sleeping.
How exactly these frogs can pack their red blood cells together without getting blood clots remains a mystery. Most other animals’ blood coagulates if the cells bump into each other, which can help heal a wound or—in a worse scenario—clog up a vein or artery, stopping blood from flowing to crucial areas, per the New York Times’ Veronique Greenwood. Understanding more about how the frogs stay healthy while jam-packing their livers full of red blood cells could help advance blood clot research in humans, write the authors.
“These researchers have nailed down how the camouflage works, which is really novel and exciting,” Kate Thomas, a visual ecologist at the University of Texas at Arlington and the Natural History Museum in London, who was not involved in the research, tells Defector’s Sabrina Imbler. “Hiding away red blood cells in the liver during rest is a fascinating strategy to maximize the amount of light that can pass through a complex body.”