Glass frogs have transparent bellies that reveal their beating hearts and internal organs.

Glass frogs belong to the family Centrolenidae, which includes over 150 species primarily found in the Neotropics. A 2022 study published in the journal Science by researchers from Duke University and the American Museum of Natural History revealed the secret to their transparency. While many aquatic animals are transparent, it is extremely rare in land animals because red blood cells absorb light and create a visible silhouette.To overcome this, glass frogs perform a remarkable physiological feat by sequestering approximately 89% of their red blood cells into their liver during sleep. This process increases their transparency by two to three times, making them nearly invisible to predators like birds and snakes. The liver itself is coated in reflective guanine crystals, which helps mask the concentrated blood stored inside.This biological mechanism is particularly fascinating to medical researchers because the frogs pack their red blood cells together so tightly without forming life-threatening blood clots. In humans, such a high concentration of blood cells would typically trigger massive clotting. Understanding how these frogs manage their circulatory system could lead to breakthroughs in treating human vascular diseases and preventing thrombosis.When the frog becomes active at night to hunt or mate, the red blood cells circulate back through the body, and the frog loses its high level of transparency. This daily cycle of 'cloaking' is a unique evolutionary adaptation that allows a terrestrial vertebrate to achieve near-total camouflage. The study utilized photoacoustic imaging to track the movement of blood cells without disturbing the frogs' natural behavior.