A circulatory disappearing act
In the cloud forests of Costa Rica, a small amphibian performs an incredible physiological feat every day. The Fleischmann's glass frog (Hyalinobatrachium fleischmanni) achieves near-invisibility while it sleeps by actively removing its red blood cells from circulation. This nocturnal frog, measuring only 19 to 32 millimeters, spends its days sleeping upside down on the underside of broad leaves. To a predator looking up, a frog full of red blood would be conspicuous against the green leaf. To solve this, the frog temporarily packs away almost all of its most visible cells.
Using a technique called photoacoustic imaging, which uses light to generate ultrasonic waves from red blood cells, researchers tracked the frog's circulatory system. They found that when a glass frog goes to sleep, it removes approximately 89% of its red blood cells from its bloodstream. These cells are sequestered inside its liver, which is covered in reflective guanine crystals that help shield the red cells from view. This process increases the frog's transparency by two to three times, making it between 34 and 61 percent more see-through than when it is awake and active. Once the frog awakens, the red blood cells are released back into the bloodstream, and the animal becomes opaque again.