At first glance, you might miss the
glass frog of the Costa Rican rainforest. It is, as the name suggests, nearly
transparent. Apart from a lime green smear across its back, its skin, muscle,
and other tissues are see-through. Then there are its tiny organs, which seem
to float within this clear flesh, like a pale fruit cocktail in the weirdest
Jell-O salad ever to grace a tree branch.
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As handy as translucence might be for
evading predators, it is rare in animals that live on land. Their bodies are full
of substances that light cannot penetrate, many of them essential for life.
Glass frogs seem to have evolved see-through versions of some of these
anatomical features, but they also have some tricks to hide lingering colors
when they are at their most vulnerable.
“When they are awake, the circulatory system is red. When they are asleep, it’s not.”
In a study published in the journal
Science, researchers report that when a glass frog falls asleep, almost all of
its red blood cells retreat into its liver. They hide in the organ and allow
the frog to achieve near invisibility while it rests. In addition to revealing
another remarkable adaptation in nature, the discovery could lead to clues for
how to prevent deadly blood clots.
Disappearing bloodLike people, glass frogs rely on
hemoglobin, a colored protein in red blood cells that delivers oxygen around
the body. Jesse Delia and Carlos Taboada, biologists and authors of the paper,
had been spending a lot of time observing the frogs when they realized that
sometimes, that red color seemed to disappear.
“When they are awake, the circulatory
system is red,” said Delia, who works at the American Museum of Natural History
in New York. “When they are asleep, it’s not.”
Where were the red cells going?To solve the mystery of the disappearing
blood cells, the researchers and their colleagues wanted to take images of the
frogs under anesthesia — when the blood cells were clearly visible circulating
through their bodies — and asleep, when the cells were nowhere to be seen. To
do that, they needed to find a way to peer inside the frog’s organs, which have
a mirror-like exterior that helps the frog blend in. Taboada, a researcher at
Duke University, said they suspected the blood would retreat to various organs
when not in circulation.
The researchers wound up relying not on
light but on sound to show them what was inside. They provoked the molecules
within the sacs to release ultrasonic waves, which could be used to identify
the contents.
Glass frogs, the new research suggests, can control when their blood clots. If they are wounded, they will form a scab in the usual way. But when they are asleep, with red blood cells packed like sardines in the liver, no clot forms.
As soon as they compared the images of
sleeping and anesthetized frogs, one big difference jumped out.
“All the signal was coming from the liver,”
Taboada said. About 89 percent of the frogs’ red blood cells had packed
themselves into that organ.
No clotting?That made sense: The liver, which filters
blood, is a logical destination for red blood cells, he said.
What was stranger, and what the researchers
still do not understand, was how the frogs could cram all these cells together
without dying from blood clots. In most vertebrates, when blood cells bump into
each other, it leads to coagulation. The resulting clot can make a scab to seal
a wound — or, if the clot is in a blood vessel, it can plug up the circulatory
system and kill the creature. In the US, according to the Centers for Disease
Control and Prevention, as many as 100,000 people die from blood clots each
year.
Glass frogs, the new research suggests, can
control when their blood clots. If they are wounded, they will form a scab in
the usual way. But when they are asleep, with red blood cells packed like
sardines in the liver, no clot forms.
Human research and further questionsThe finding implies that glass frogs could
have something to teach us about how to prevent clot formation in our own
bodies. If future research can illuminate what keeps the frogs safe, it could
lead to treatments to reduce deaths from clots in humans.
More immediately, the researchers said, the
results raised other questions. If 89 percent of the cells that carry oxygen are
holed up in the liver while the frog sleeps, how is it breathing? They wonder
whether the frogs can shift their metabolisms to a mode that requires barely
any oxygen, perhaps akin to what other frogs do when they hibernate for the
winter.
If 89 percent of the cells that carry oxygen are holed up in the liver while the frog sleeps, how is it breathing?
The paper is just the beginning of this
line of research. The team has already improved their imaging techniques to
scan the frogs more quickly and to reveal substances other than blood as they
move around in the creatures.
“We’re in the lab now,” Delia said during a
phone interview. “There’s literally a frog scanning right now in the system. I
have to go check on it in a bit.”
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