Glass frog

We’ve all wanted the ability to disappear at one point or another. But for glass frogs, several species of tree frogs that call the rainforests of Central and South America home, this dream is a reality… sort of. The bottle cap-sized glass frog’s body boasts a translucent belly and chest.

This is a particularly useful form of camouflage because it prevents predators from spotting the frog’s otherwise recognizable silhouette. Now new research, published in Science in December revealed how amphibians pull off this trick: They divert almost all of their red blood cells straight to the liver.

Clear as day

Many aquatic species, including some jellyfish and crustaceans, also have transparency on their evolutionary bingo cards—though it’s a little easier for these sea-dwelling invertebrates. For one thing, light reflects differently when it’s in air than it does in water.

Read more: Some aquatic species have evolved transparent bodies for camouflage

But for glass frogs and other invertebrates, the biggest challenge to transparency is their red blood cells, which naturally absorb light. In fact, until recently, exactly how glass frogs weave their “invisibility cloaks” remained a mystery to scientists.

In the latest study, the authors focused exclusively on Fleischmann’s glass frog, or Hyalinobatrachium fleischmanni. These nocturnal amphibians are opaque at night while they are actively feeding and breeding. During the day, however, when they cling to the undersides of tree leaves to catch some z, their bodies become almost transparent.

Finding refuge in the liver

The researchers used photoacoustic imaging — a technique that delivers laser pulses to body tissues and maps the ultrasound waves that are subsequently produced — to find out where the rust-stained frogs are red blood cells were disappearing to.

In laboratory conditions, the researchers report, amphibians increase transparency two or threefold by packing 89 percent of their red blood cells into the liver.

Just like in humans, this organ is responsible for clearing toxins from the blood. But while other tree frogs can only store about 12 percent of their red blood cells in the liver at a time, glass frogs can store nearly 9 out of every 10 red blood cells in the liver when needed.

Read more: These little frogs traded their sense of balance for their size

The researchers found that the organ, which is covered in reflective mirror crystals to hide what’s inside, grew by about 40 percent during the day. Then, when the frog wakes up, its body redistributes this supply of red blood cells in just seconds.

Blood Clot Conundrum

Of course, researchers are still puzzled as to how the glass frog survives in such a state for up to 12 hours every day. However plasma, the colorless part of the blood, is still circulating during this time, the red blood cells are solely responsible for delivering oxygen to the body’s tissues. Essentially, the frog just goes without.

Some speculate whether amphibians go into a a state of hibernation that reduces the need for oxygensimilar to what some other frogs do in winter, although only further research can confirm this.

Another mystery is how H. fleischmanni prevents the formation of blood clots. For most vertebrates, including humans, blood cells stick together when they bump into each other, a process called coagulation. This is helpful in some cases, such as when scabies seals a scratch, but it can also lead to serious health problems.

In the US, for example, approx 100,000 people die from blood clots every year. Determining how the glass frog manages to pack roughly 90 percent of its red blood cells into a space as small as a liver — without a clot — could revolutionize blood clotting research in humans as well.

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