The power of natural selection

The blackfin icefish, Chaenocephalus aceratus, lacks hemoglobin and lives in Antarctic waters, where the temperature is often close to the freezing point of seawater.

New York Times - How the Icefish Got Its Transparent Blood and See-Through Skull

Research shows how the Antarctic blackfin icefish differs from its close relatives on the genetic level.

By JoAnna Klein

Feb. 28, 2019

The Southern Ocean around Antarctica was once warmer. Then about 30 million years ago, the temperature dropped. Few fish could survive temperatures that were just above seawater’s freezing point, and they either migrated to warmer waters or went extinct.

One bottom-dweller held on. Through the power of natural selection, its descendants developed traits that let them survive these unlikely conditions. Today, the Antarctic blackfin icefish, or Chaenocephalus aceratus, thrives in these frigid waters with no scales, blood as clear as water and bones so thin, you can see its brain through its skull.

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How this creature — no longer a bottom-dweller — can live in such a hostile environment has long fascinated scientists, who have mapped its genome and continued exploring its unusual traits. In a paper published Monday in Nature Ecology and Evolution, a team of scientists compared the genome of the Antarctic blackfin icefish to those of its close relatives. They found that, across these genomic maps, and tens of millions of years of evolution, gene families had shrunken or expanded, giving rise to some of the icefish’s most unusual features. In addition to revealing how the icefish managed to adapt to extreme Antarctic conditions, the team’s findings provide a new way to look at the genetics behind human diseases such as anemia and osteoporosis.

“A trait that’s maladaptive in one environment can be adaptive in another,” said H. William Detrich, a marine scientist at Northeastern University who has been studying icefish for decades and helped lead the study. He added that, “we can learn a lot about human physiology and medicine by studying these evolutionary outliers.”

The icefish first surprised science with its clear blood after a Norwegian zoologist caught one in the early 20th century. The species no longer makes red blood cells and hemoglobin to carry oxygen through its body. Those traits are essential to the survival of other vertebrate species, all 60-some-thousand of us.

Now, why would the icefish go and do a thing like that?

Becoming the modern icefish required millions of years of natural gene hacking. Parts of their genomes that, in adults, were dedicated to making antifreeze for blood were greatly expanded. More genome space became dedicated to making ice-preventing proteins in the shell-like casings that surround icefish embryos.

Frigid water holds more gasses, including oxygen, than warmer water does. But in water so cold, red blood becomes gunky, hard to pump and more likely to freeze. So the fish basically “evolved a therapy for anemia,” said John Postlethwait, a developmental biologist at the University of Oregon who also worked on the paper. It developed supersize gills and lost its scales, which enabled it to absorb the water’s plentiful oxygen through its skin. It also expanded its circulatory system with extra vasculature and a heart four times the size of closely related, red-blooded species.

Over evolutionary time, the icefish accumulated lipids, or fats, which, like oil, float in water. It also developed floppy bones that were less mineralized than those of their ancestors. This allowed the icefish to rise in the water column, like spaghetti in boiling water, and eat krill and other creatures that couldn’t be found near the sea floor.

The research team is still working to understand how these adaptations in bone density are reflected in the icefish genome. That may lead to insights into how humans lose bone density and develop osteoporosis with age, the researchers said.

Similarly, by looking at how the icefish gets by with severe anemia, scientists may learn something about why humans don’t.

Creatures such as the icefish are what Charles Darwin called “wrecks of ancient life,” because they lost important traits of their ancestors, like red blood cells and dense bones. Dr. Postlethwait calls them “evolutionary mutant models,” because their genomes could help explain why some physiological traits can be adaptive for one animal but disease states in another.

The icefish may be a wreck of ancient life, but it’s a wonderful wreck.