Glass frogs become transparent by masking their blood

They are called glass frogs for a reason. Turn the paperclip-sized amphibian over, and you’ll see its bones, internal parts, and beating heart through its translucent abdomen.

Now, scientists have discovered how some of these tiny frogs, which live in tropical forests across Central and South America, keep their skin so pristine — they divert their blood into their livers to help them hide.

In nature, transparency is largely reserved for aquatic organisms such as eel larvae and jellyfish jellyfish. Terrestrial animals and those that span both land and water have a tougher time because light reflects through air differently than from water.

Another issue is blood. Red blood cells use the rust-colored hemoglobin proteins that bind to oxygen. These proteins absorb light and give blood its crimson color, keeping the skin opaque. Only the thickness of the ice in Antarcticawhich live in the frigid depths of the Southern Ocean, completely shed hemoglobin, which gave their blood a cloudy white color.

To see how glass frogs overcome this obstacle, the researchers used high-calibration cameras to capture the transparency of Fleischmann’s glass frogs (Hyalinobatrachium fleischmannii), which congregate near streams throughout Mesoamerica. At night, when amphibians breed and feed, it is opaque. But during the day, when they doze off on the leaves, most of their bodies, save for the lime-green color of their backs, turn translucent. This helps the frogs blend together like dew drops, protecting them from spiders and snakes while they rest.

The scientists brought a few of the frogs back into the lab and watched how their transparency changed while they slept, exercised, chirped, or were under anesthesia. Sleeping glass frogs were between 34% and 61% more transparent than when they were active, the team reports today. science.

As they snooze, many glass frogs blend into the underside of the leaf except for the fuzzy blotch of their internal organs.Jessie Delia

This increased transparency appears to be related to the lack of red blood cells running through her veins. “We could see that there was no blood where the animals were sleeping,” says Carlos Tapwada, a Duke University biologist and author of the new study. When they woke up, they started pumping blood again, which reduced their transparency.

To determine where the blood cells were going, Tabuda and his colleagues used a technique called photoacoustic imaging, which identifies ultrasounds produced when light is absorbed by red blood cells. During the day, the blood vessels in the frog’s liver filled with red blood cells, causing the organ to swell in size by about 40%. Compared to other tree frogs, which can only store about 12% of their red blood cells in their livers, glass frogs can store a whopping 89% – almost all of the red blood cells in their bodies.

How the animals survive such extreme adaptation is unclear, says study co-author Jesse D’Elia, a biologist at the American Museum of Natural History. “They basically don’t move around a lot of oxygen for 12 hours a day.”

Another mystery is how glass frogs are able to transport so many blood cells to one location without forming a fatal clot. Solving this could lead to better blood-clotting therapies for humans, says Richard White, an oncologist at the University of Oxford who has studied the spread of cancer and other diseases in transparent zebrafish, but he was not involved in the new study. “This seemingly basic observation about glass frogs leads to very clear implications for human health.”

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