Journal club, simply put #1

Glass frogs camouflage by hiding their blood while sleeping

This is the very first post in a new blog series, “Journal club, simply put”, where I aim to take exciting scientific studies and papers I hear about, and write about them in a way that’s easy to digest!

Camouflage is a necessary part of how animals evade predators, and one of the most fascinating methods some animals use to hide is to make their bodies transparent or translucent. Take glass frogs for example. These tree frogs are native to Central and South America, and are about as small as your fingers. When viewed snoozing on a leaf, the frogs’ transparency allows them to blend in with the leaf and evade vision almost entirely!

Sleeping glass frogs resting on a leaf. Image by Jesse Delia

But how exactly do these frogs turn themselves transparent? In a study published in Science magazine in December 2022, scientists revealed some of the secrets of frog transparency — they divert their blood to their livers while sleeping.

Linking blood and transparency

Glass frogs are very well known for their highly transparent muscles and skin — their organs and blood vessels can be easily seen as a result. However, circulating blood can potentially give the frogs away to predators. Blood gets its distinctive red color from the hemoglobin found in red blood cells binding to oxygen. If blood circulates through the frogs’ transparent bodies normally, then we should expect to see the frogs because of their blood! How then do they stay transparent while resting?

To figure out exactly how glass frogs handle their blood to maintain transparency, researchers used advanced imaging techniques to monitor specimens of Fleischmann’s glass frog (Hyalinobatrachium fleischmanni) while sleeping, active, and under anesthesia. First, they observed that sleeping frogs circulate plasma with extremely low red blood cell concentration, and blood cell concentration spiked when awake. Coupled with additional imaging studies of the frogs, researchers determined a link between red blood cell concentration during sleep and the transparency of the frogs.

Taken from Figure 1 in the paper. Note the increase of blood circulation (and as a result, no transparency/translucency) when the frogs are active or under anesthesia!

Where’s the blood even going?

In the next part of the study, researchers asked where red blood cells in the frogs were going when they go to sleep. To measure the presence of red blood cells, they used photoacoustic microsopy (PAM), a hybrid imaging technique that relies on the photoacoustic effect — the formation of ultrasonic waves after molecules absorb light. When measuring red blood cell concentration using PAM in sleeping glass frogs, they found that red blood circulation decreased by almost a whopping 90%, and that red blood cells localize in the frogs’ livers.

Selected figures from Figure 3 in the paper. Note in A) and C) the high degree of red blood cell signal detected near the liver region pre-exercise and post-exercise, in contrast to the spread of red blood cell signal throughout the rest of the body (like the abdomen) while exercising. B) is a plot of red blood cell signal near the liver, which shows a decrease during exercise. D) and E) are plots of red blood signal and oxygen saturation in the abdomen, which increase during exercise.

Further experiments done on sleeping versus active frogs only confirmed this observation. The researchers found that the liver volume of glass frogs, associated with this red blood cell packing, increased by 40% on average when sleeping versus when the frogs were active. Meanwhile the liver volume of another species of tree frogs Allophryne ruthveni — the closest non-transparent relative of glass frogs — did not change significantly!

Why does this matter?

Researchers determined glass frogs pack their red blood cells in their liver while sleeping, leaving their transparent skin and muscles to camouflage and blend in to the surrounding environment, chiefly the leaves that they sleep on. This process however leads to an extremely reduced circulation of red blood cells — just 3.4% of hemoglobin found in their blood is bound with oxygen. That raises one question: how can these frogs survive with hardly any blood circulation? They’re basically not transporting any oxygen for half of the day!

A second question has a very interesting implication for human blood clots. Normally in animals, packing so much blood together results in thrombosis, or blood clots. Surprisingly, glass frogs can pack so many blood cells in their livers without causing potentially fatal blood clots. Further study on glass frogs can prove extremely useful for blood-clot research. The mechanisms for which blood clots don’t form in these frogs could perhaps be translatable to blood clots in humans, and reveal interesting therapeutic solutions for humans!

I hope that with this blog series, “Journal club, simply put”, I can help make science more approachable and easy to digest. Behind all the jargon and graphs lies extremely profound and interesting research, and everyone deserves to be excited by it! See here for why I decided to start this blog!