Scientists have figured out how tiny frogs living in the rainforests of Central and South America can become almost transparent - they send all their blood to the liver and almost disappear, writes Science.

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In nature, transparency is only found in completely aquatic creatures, such as eel larvae and gelatinous jellyfish.. Land animals and those between land and water have a harder time achieving transparency because light reflects differently from air than it does from water..

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Another problem is blood.. Red blood cells use rust-colored hemoglobin proteins that bind to oxygen. These proteins absorb light and give the blood its crimson color, keeping the skin opaque.. Only Antarctic icefish, living in the cool depths of the Southern Ocean, have completely abandoned hemoglobin, giving their blood a cloudy white color..

To find out how glass frogs overcome this obstacle, the researchers used highly calibrated cameras to capture the transparency of Fleischmann's glass frogs (Hyalinobatrachium fleischmanni), which congregate along streams throughout Central America.. At night, when amphibians breed and feed, they are opaque.. But during the day, when they nap on the leaves, most of their body, with the exception of the lime-green hue of the back, becomes transparent.. This helps the frogs blend into the foliage like dewdrops and keep out spiders and snakes..

Scientists returned several frogs to the lab and observed them for several days. Sleeping glass frogs were 34% to 61% more transparent than active ones..

It turned out that this increased transparency is due to a lack of red blood cells flowing through their veins.. " When they woke up, the blood began to pump again, reducing its transparency."

To determine where the blood cells had gone, Taboada and his colleagues used a photoacoustic imaging technique that maps ultrasonic waves produced when red blood cells absorb light.. During the day, the blood vessels in the frogs' livers were overflowing with red blood cells, which led to an increase in the size of the organ by about 40%.. Compared to other tree frogs, which can only store 12% of red blood cells in their livers, glass frogs can store 89%—almost all of their body's red blood cells..

Study co-author Jesse Delia, a biologist at the American Museum of Natural History, says it's unclear how animals survive such extreme adaptations.. "

Another mystery is how glass frogs are able to move so many blood cells to one place without forming a potentially fatal clot.. Solving this problem could lead to better treatments for blood clots in humans, says Richard White, an oncologist at the University of Oxford who has studied the spread of cancer and other diseases in translucent zebrafish but was not involved in the new study.. "