Every animal with bones has blood with hemoglobin, which binds with oxygen and makes the blood appear red.
Every animal, that is, except one.
The ocellated icefish (Chionodraco rastrospinosus) has gin-clear blood. And it has no scales. And it lives nowhere but the inky depths down to 3,200 feet (1 kilometer) in the icy waters off Antarctica. Other than that, it's just an ordinary fish.
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The Tokyo Sea Life Park is the only place with ocellated icefish in captivity, Agence France-Presse reports. "Luckily, we have a male and a female, and they spawned in January," Satoshi Tada, an education specialist at the center, told AFP.
The ocean's depths are rich with odd sea life, from giant squid to translucent sea anemones. Researchers now believe life around deep-sea vents may have arisen following the last mass extinction on Earth 65 million years ago, after a giant meteor impact killed off dinosaurs and other animals.
Scientists hope the mated pair of icefish and their offspring in Tokyo will help researchers unlock the secrets of how the fish manages to survive without hemoglobin to carry oxygen to its cells.
It's possible, some scientists speculate, that the icefish's unusually large heart might help move oxygen through its body using blood plasma instead of hemoglobin.
Also, with no scales to get in the way, the icefish may absorb some oxygen directly through its skin: Cold, polar water is richer in oxygen than warmer waters.
But the mystery surrounding the icefish's lack of hemoglobin may take years to solve. "More studies are needed on the question," Tada said.
Follow Marc Lallanilla on Twitter and Google+. Follow us @livescience, Facebook & Google+. Original article on LiveScience.com.
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