How did the icefish get its clear blood and transparent skull?



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The Southern Ocean around Antarctica was once warmer. Then, about 30 million years ago, the temperature dropped. Few fish could survive temperatures just above the freezing point of seawater. They then migrated to warmer waters or died out.

An inhabitant of the bottom held it. Thanks to the power of natural selection, her descendants have developed traits that allow them to survive in these unlikely conditions. Today, the Antarctic black-finned icefish, or Chaenocephalus aceratus, thrives in these icy waters without scales, blood as clear as water and so thin bones., you can see his brain through his skull.

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How this creature – who is no longer a grassroots citizen – can live in such a hostile environment has long fascinated scientists, who have mapped its genome and have continued to explore its unusual traits. In an article published Monday in Nature Ecology and Evolution, a team of scientists compared the genome of the Antarctic black-finned icefish to that of its close relatives. They found that with these genomic maps and tens of millions of years of evolution, gene families have narrowed or expanded, giving rise to some of the most unusual features of icefish. In addition to revealing how the ice fish has managed to adapt to the extreme conditions of Antarctica, the team's findings provide a new way to examine the genetics that cause human diseases such as anemia. and osteoporosis.

"A mis-matched trait in one environment may fit into another," said H. William Detrich, a marine science scientist at Northeastern University, who has been studying icefish for decades and helped guide the study. He added that "we can learn a lot about human physiology and medicine by studying these aberrant values ​​of evolution".

The ice fish first surprised science with its pureblood after a Norwegian zoologist caught one at the beginning of the 20th century. The species no longer makes red blood cells and hemoglobin carry oxygen through its body. These traits are essential for the survival of other vertebrate species, all of which have about 60,000 people.

Now, why would the icefish go do such a thing?

Picture
The blood of an ice fish, right, is clear, while the blood of its Antarctic parent, the marbled baguette, on the left, is purple and contains red blood cells.CreditNortheast University

Becoming the modern ice fish has required millions of years of natural piracy of genes. Parts of their genomes that, in adults, were dedicated to manufacturing antifreezes for blood have been considerably expanded. More genome space has become dedicated to making protein-preventing ice in the shell-shaped envelopes that surround icefish embryos.

The icy water holds more gas, including oxygen, than the warmer water. But in such cold water, the blood becomes bloody, difficult to pump and more likely to freeze. Thus, the fish "has basically developed a therapy against anemia," said John Postlethwait, a developmental biologist at the University of Oregon, who also worked on the paper. He developed very large gills and lost his scales, which allowed him to absorb the abundant oxygen from the water through his skin. He also expanded his circulatory system with an extra vasculature and a heart four times larger than closely related, red-blooded species.

During the course of evolution, the ice fish has accumulated lipids, or fats, which, like oil, float in the water. He also developed less mineralized floppy bones than those of their ancestors. This allowed the ice fish to climb into the water column, like spaghetti in boiling water, and eat krill and other creatures that could not be found near the bottom of the sea.

The research team is still working to understand how these adaptations of bone density are reflected in the icefish genome. This could help understand how humans lose bone density and develop osteoporosis with age, the researchers said.

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