Secret of the glass frogs revealed

A glass frog presents its transparent underside. © Jesse Delia

Amazing camouflage strategy: Glass frogs can make their bodies transparent during sleep by storing the opaque red blood cells in their livers, a study shows. When the nocturnal amphibians wake up, the oxygen carriers are then released back into the bloodstream. The researchers emphasize that this discovery is not only exciting from a biological point of view: How the frogs manage to pack and unpack their red blood cells every day without blood clots forming could be of medical interest.

In order to avoid the hungry eyes of predators, many animals try to appear as inconspicuous as possible. A tried and tested strategy is to adapt the color to the environment as well as possible. This concept has taken a group of amphibians to extremes: the glass frogs of the American tropics appear greenish when viewed from above, like many other frogs. But when viewed from below, something amazing becomes apparent: They have transparent skin and muscles that make their bones and organs visible. "These frogs sleep clinging to the underside of large leaves, and their translucency enables them to blend in perfectly with the colors of the vegetation," explains lead author Carlos Taboada of Duke University in Durham. When they are asleep, they appear significantly more transparent than during their nocturnal activity. What this phenomenon is all about was previously unclear.

Blood cells normally block vision

As Taboada and his colleagues explain, terrestrial vertebrates cannot normally develop transparent bodies because so many red blood cells are carried through their vascular system. In contrast to tissue structures, these oxygen-transporting structures cannot be made transparent because their structure means they interact strongly with light. In the case of glass frogs, this led to the suggestion that their increased transparency during sleep might be due to a reduction in circulating blood cells. Taboada and his colleagues followed this lead by studying the glass frog species Hyalinobatrachium fleischmanni.

The researchers used a sophisticated technique called photoacoustic imaging. Harmless laser light beams are sent into the tissue, which are then absorbed by certain molecules and converted into ultrasound waves. These acoustic signals can then provide information. "It is the ideal tool for non-invasive imaging of red blood cells," explains co-author Junjie Yao of Duke University. “The red blood cells themselves provide the contrast because different cell types absorb different wavelengths of light. We managed to optimize our imaging system so that it could look specifically for and track red blood cells. During the investigations using the laser technology, the frogs were able to sleep undisturbed upside down in a Petri dish, similar to how they would sit on a leaf in nature.

Biologically and medically interesting

Tracking the red blood cells in frog bodies revealed: The resting glass frogs increase their transparency by two to three times by removing nearly 90 percent of their red blood cells from the bloodstream. The researchers were also able to determine exactly where they disappear to: they are stored in the liver. Interestingly, this organ is encased along with the others in a reflective coating that contributes to the camouflage effect, the team reports. The investigations showed that when the frogs become active – and an increased supply of oxygen in the body is therefore required – the blood cells are fed back into the circulatory system. This makes the frogs much less translucent when they are nocturnal. The scientists conclude that the secret of their increased transparency during sleep is that they can temporarily hide their red blood cells.

As they emphasize, the study results now raise further exciting questions that go beyond the biology of these unusual amphibians: it is now necessary to clarify which physiological mechanisms enable the glass frogs to achieve their amazing ability. Because in other vertebrates, such an accumulation of red blood cells would lead to dangerous blood clots in veins and arteries. However, this effect does not appear to occur in glass frogs, which could be an interesting starting point for medical research. “This is perhaps the first in a series of studies documenting the physiology of vertebrate transparency. The topic could stimulate biomedical research to translate the extreme physiology of these frogs into new avenues for human health and medicine," says co-author Jesse Delia of Duke University.

Source: Duke University, American Museum of Natural History, professional article: Science, doi: 10.1126/science.abl6620

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