Glass frogs turn translucent by ‘hiding’ blood in their liver

Glass frogs can boost their transparency by up to 61 per cent by storing most of their blood in their liver while they sleep. Researchers hope that understanding how the frogs manage to pool their blood this way without experiencing blood clots could provide new insights into preventing dangerous clots in other animals, including humans.

The tropical, marshmallow-sized amphibians spend their days sleeping on bright green leaves and foraging for food under the cover of dark. Being semi-see-through helps glass frogs avoid being spotted by predators, but it’s a challenging biological task, as most animals need to continuously pump red blood cells throughout their body to deliver oxygen to their tissues.

“Transparency is rare in general for vertebrates,” says Jesse Delia at the American Museum of Natural History in New York. Only a few species of fish and amphibians have managed the feat, including Fleischmann’s glass frog (Hyalinobatrachium fleischmanni). “If it wasn’t for that green skin on their back, you would probably be able to read a newspaper through them,” says Delia.

Delia and his colleagues began investigating the frogs’ transparency more closely after noticing that the animals appeared much more translucent while sleeping compared with when they were awake. So the researchers measured the frogs’ opacity by shining different wavelengths of light through the animals while they were active and while they were resting. They found the frogs become up to 61 per cent more transparent when asleep.

When they traced the movement of blood in the living animals in real time using photoacoustic imaging, the researchers discovered that glass frogs can “hide” around 89 per cent of their red blood cells in their liver while they slumber. To accommodate this, the frogs’ livers enlarged by an average of 40 per cent. Once the frogs awoke and their circulation increased, they became more opaque and their livers decreased in size.

“It’s not like they put some blood in their liver – they put almost all their blood in their liver,” says Karen Warkentin at Boston University who was not involved in the work. “I just find that pretty amazing.”

In most vertebrates, packing red blood cells together leads to clotting, but these amphibians don’t appear to have negative health consequences from storing blood in their liver. Delia says they aren’t sure yet how glass frogs manage to evade coagulation but hope the work can advance researchers’ treatment of blood clots in humans.