GLASS FROG (family Centrolenidae), tree-dwelling amphibian notable for its translucent underbelly. There are 160 known species of glass frog, all of which are native to South and Central America. The glass frog is a nocturnal carnivore that lives in warm, wet environments. Virtually all species are green in color; some have white or yellow spots that resemble their eggs. The glass frong varies in size, ranging from that of a paperclip to a marshmallow.
Though a glass frog will appear opaque when seen from above, its chest and stomach are largely transparent and its skeleton, intestines, and beating heart are often visible. This translucency is adapted for camouflage: when the frog is asleep upside-down on the underside of a leaf, its transparency minimizes the shadow it casts through the leaf.
Transparency is common in aquatic creatures like jellyfish and eels but rare in terrestrial animals. Vertebrates’ circulatory systems make true transparency difficult to achieve; red blood cells will absorb light and create opacity even if the skin and muscles are translucent. (Antarctic icefish and larval eels overcome this problem by not producing red blood cells at all, giving their blood a milky-white color.)
The transparency of a glass frog increases dramatically when it sleeps. When it is awake and moving, its blood circulates visibly through its veins, but when the frog is asleep, its blood seemingly disappears, rendering it all but invisible. Where the blood went remained a mystery until 2022, when researchers using advanced imaging technology known as photoacoutistic microscopy, which uses sound waves to capture images, determined that around eightynine percent of the sleeping frog’s blood retreats to its liver.
How glass frogs are able to store so much blood in one place without dangerous clotting remains an open question. Since the frogs' blood clots normally after an injury, it is theorized that they must possess a hitherto unknown mechanism for controlling clotting. This information could prove useful to researchers investigating how to prevent blood clots in humans and other species.
According to the International Union for Conservation of Nature’s Red List, approximately half of glass frog species are threatened with extinction, with ten listed as critically endangered. The biggest threats facing them are deforestation, climate change, and the illegal pet trade.