Glass Frogs Are So Transparent You Can See Their Hearts Beating

The small, lime-green glass frogs of Central and South America have one very peculiar characteristic: Their undersides are so completely transparent that their internal organs are fully visible through the skin. You can see the liver, intestines, and most remarkably of all, the heart — a small, red, rhythmically pulsing structure visible to the naked eye through living tissue. 

In 2022, a study by researchers at Duke University revealed the mechanism behind this transparency: When glass frogs sleep, they pump almost all of their red blood cells into the liver. This reduces the blood’s visibility, the primary obstacle to the frog’s transparency, by around 89%. In effect, the creatures hide their own blood while they sleep, which lends them an incredible degree of transparency, allowing them to blend in with the surrounding environment.