The Antarctic blackfin icefish (Chaenocephalus aceratus) is an extraordinary animal that inhabits the Earth’s coldest marine environment and can survive in temperatures below the freezing point of freshwater.
Now, an international team of researchers has mapped more than 30,000 of the fish’s genes, completing its genome—the complete set of genetic material in an organism—according to a study published in the journal Nature Ecology & Evolution.
The latest project could help researchers to better understand how the animal adapted to such extreme environments over the course of their evolution.
Scientists have long been puzzled by C. aceratus and other animals in the icefish family, due to the fact that they are the only vertebrates that lack functional hemoglobin genes. This means their bodies do not produce the red blood cells that carry oxygen in the blood of all other vertebrates. As a result, their blood is a ghostly white color.
Essentially, the fish exist in a constant state of anemia—a common disorder that occurs when blood lacks sufficient healthy red blood cells, or hemoglobin (the specific protein that binds oxygen.) Furthermore, icefish have oversized hearts and their bones have a very low mineral density.
If a human was to have all of these traits, a doctor would likely conclude that they were suffering from a disease or disorder. However, these characteristics are essential to the survival of icefish, according to the researchers.
The icefish’s suborder—Nototheniodei—diverged from the lineage that produced sticklebacks around 77 million years ago, according to the study. Since then, they have gained several important features.
“Icefishes evolved mechanisms that appear to compensate for loss of oxygen-binding proteins, including enormous hearts with increased stroke volume relative to body size, enhanced vascular systems, and changes in and morphology [body structure,]” the authors wrote in the study.
Perhaps most beneficially, they evolved the ability to manufacture proteins that act as a kind of "antifreeze"—enabling them to survive as the Southern Ocean cooled. Today, average water temperatures in the region hover around 29 degrees Fahrenheit (it is important to note that seawater has a lower freezing point than freshwater.)
Icefish have also lost many features over the course of their evolution, including sets of genes associated with circadian rhythms. These disappeared as the fish became accustomed to an environment where summers and winters involved either continuous light or darkness for months on end.
According to a co-author of the study, John Postlethwait from the University of Oregon, investigating the Icefish genome could even have implications for our knowledge of the human body.
"Understanding how these traits arose over evolutionary time in icefish may help us to appreciate how similar traits—bone loss, decrease in the ability to make blood cells, problems of the circulatory system, obesity—arise in aging humans over developmental time,” he said in a statement.