Glass frogs already have translucent ventral skin, but circulating blood is opaque and red, and a visible network of vessels would break the illusion. Work published in 2022 on Hyalinobatrachium fleischmanni showed that a sleeping frog removes close to 90 per cent of its red blood cells from circulation and concentrates them in the liver, raising transparency two- to threefold.
The measurement was made with photoacoustic imaging, which detects haemoglobin without disturbing the animal, because handling wakes the frog and reverses the effect within minutes. When active, the frog returns the cells to circulation and becomes noticeably less transparent.
Packing blood cells at that density would normally trigger clotting and tissue damage in a vertebrate. Glass frogs do this daily without apparent harm, and the mechanism protecting them is not understood. The transparency itself is also partial: the frogs are green from above, and the effect chiefly softens their outline against a backlit leaf rather than making them invisible.