The mechanics of invisibility
Glass frogs, members of the amphibian family Centrolenidae, possess one of the most effective forms of camouflage on land. While their backs are typically a shade of lime green that matches the foliage they rest on, their abdominal skin is transparent. This see-through underside reveals their internal organs, including the heart, liver, and digestive tract. For vertebrates, transparency is a biological challenge because their circulatory systems are filled with light-absorbing red blood cells.
A recent discovery revealed the glass frog's primary method for achieving this feat. During the day, when the nocturnal frogs are asleep and most vulnerable, they withdraw nearly 90% of their red blood cells from circulation. These cells are then packed densely into the frog's liver. This process makes the frog two to three times more transparent than when it is active at night. The liver itself is concealed by reflective guanine crystals, which act like a mirror to shield the concentrated red blood cells from view. When the frog awakens, it releases the red blood cells back into its bloodstream, without developing blood clots—a phenomenon of great interest to medical researchers.