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Glass frogs divert blood during sleep to become transparent, study finds

New research says that glass frogs can regulate their blood flow and increase transparency during sleep for protection against predators.

(CN) — If you’re ever in the rain forests of Colombia, don’t expect to see any glass frogs hanging out; the tiny frogs are masters of camouflage, thanks to their transparent skin.

The glass frog, found throughout southern Mexico, Central America and South America, is distinguished by the translucency of its underbelly, through which it is possible to see the frog’s internal organs. The glass frog’s transparency is believed to help them blur their outline against their leafy surroundings to better disguise themselves against predators.  

The frogs’ capability for transparency is unique, especially among creatures that live primarily on land. According to a study published Thursday in Science, transparency is usually a challenge for terrestrial vertebrates to achieve, due to the presence of red blood cells flowing throughout the body.

Carlos Taboada of Duke University and colleagues studied the Hyalinobatrachium fleischmanni, or Fleishmann’s glass frog, and determined that the frogs are able to maintain transparency in their sleep by redirecting blood cells to ‘hide’ in the liver while sleeping.

A previous study had determined that the Fleishmann’s glass frog reflected near-infrared light in experiments using calibrated color photography, an adaptation would render the frog nearly invisible in both visible light and the infrared vision of some of its predators.

Taboada and his fellow researchers used this method of calibrated color photography in combination with photoacoustic imaging, which traced the movement of red blood cells in the frogs. They discovered that the glass frogs would become 34 to 61% more transparent while they were sleeping.

Further investigation showed that the frogs manipulated transparency levels by taking nearly 90% of their red blood cells out of circulation while sleeping. The cells would be ‘hidden’ in the liver during this time. Researchers did not document any negative side effects to vascular or metabolic health as a result of the disruption in blood flow and subsequent storing of red blood cells.

The additional transparency would protect the glass frog during its vulnerable sleeping hours; upon waking, the researchers noted, blood flow in the frogs would resume and the frogs would become more opaque and visible.

This ability to reallocate red blood cells in the body once again sets the glass frog apart from other vertebrates, in which the ‘hiding’ of the blood cells in the liver would usually cause dangerous clotting.

Authors of the study hope that an understanding glass frog red blood cell activity may be useful for understanding blood flow in general and will aid in the development of treatments for cardiovascular conditions.

However, Nelly Cruz and Richard White of Memorial Sloan Kettering Cancer Center push back on some of Taboada’s conclusions in a Perspective released with the paper.

 “The mechanism that drives RBC re-distribution in glassfrogs is not understood. It is unclear if the glassfrog can actively manipulate the changes in RBC circulation – for example, in the presence of a predator,” they write. “Another intriguing question is how sequestration of RBCs in the liver affects cellular respiration and whether the glassfrog has a special metabolism that adapts to the drastic changes in RBC circulation.”

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