Few animals have risen to meet the challenge of enduring life in one of the coldest and most severe environments on the planet: the Southern Ocean, which surrounds Antarctica. The highest temperatures in the waters near Antarctica’s northernmost peninsula reach 1.5° Celsius (34.7° Fahrenheit) and that only during the brief summer months.
These temperatures have allowed a group of particularly gritty animals to stake their claim on this singularly severe sea: about 90 percent of the fishes in the Southern Ocean belong to a single suborder, Notothenioidei. To survive in an environment where water temperatures drop below the freezing point of blood, notothenioids have evolved incredible characteristics. Many species produce antifreeze proteins that thwart ice crystals from forming in their bodies during a big chill.
The species that belong to the family Channichthyidae, which branched off from the lineage about 5.5 to 2 million years ago, opted for a different tactic. Somewhere along the way, channichthyids shed their red blood cells and hemoglobin, the iron-rich protein found in red blood cells that transports oxygen throughout the body. They became the only vertebrates without red blood cells and hemoglobin—an oddity that manifests itself in their strange, colorless blood.
When biologists took a closer look, blood free of hemoglobin started to look more like an evolutionary mishap than a clever adaptation. Read more.