How glass frogs -- er, I guess it's one word, glassfrogs -- become transparent has been figured out. "Glassfrogs make themselves transparent while they rest by taking red blood cells from circulation and concealing them in their livers."
"In their imaging set-up, the frogs slept upside down in a petri dish, similar to how they would sleep on a leaf, and the team shined a green laser at the animal. The red blood cells in the frog's body absorbed the green light and emitted ultrasonic waves, which were then picked up by an acoustic sensor to trace their whereabouts, with high spatial resolution and high sensitivity."
"The results were startlingly clear: When the frogs were asleep, they removed nearly 90 percent of their circulating red blood cells and stored them in their liver." The liver has a mirror coating.
Glassfrogs hide red blood cells in their liver to become transparent
