During the day, glassfrogs hide under leaves and become almost transparent, leaving only their eyes, bones and internal organs visible. This camouflage is so effective that they can sleep peacefully during the day, in preparation for the night, during which they hunt for prey. 

Scientists have long wondered how glassfrogs are able to bring about their "cloak of transparency."

A recent study has shed some light on this topic, revealing that the amphibians were able to conceal as much as 90 percent of their circulating red blood cells by storing them in the liver during the day. 

The research team, which comprised biologists and biomedical engineers, studied northern glassfrogs (Hyalinobatrachium fleischmanni)—also known as Fleischmann's glassfrogs—as the amphibians slept upside down in a petri dish. The findings of their study were published on 23 December in the Science journal. 

The findings

According to co-author Sönke Johnsen, a professor of biology at Duke University, "whenever glassfrogs want to be transparent, which is typically when they're at rest and vulnerable to predation, they filter nearly all the red blood cells out of their blood and hide them in a mirror-coated liver—somehow avoiding creating a huge blood clot in the process."

At night, when it was time for the frogs to be active, the red blood cells would flow back into the bloodstream, thus giving the frogs the metabolic capacity to move around. They subsequently become less transparent.

In many vertebrates, high concentrations of red blood cells can lead to clotting or damage to their peripheral tissues, thus this discovery holds implications for the development of anticoagulants and other cardiovascular drugs.