With a ghostly pale, translucent appearance, a large, unshapely jaw and milky white blood, the icefishes are certainly the most mysterious and strange group of fishes found in the region of your Antarctica cruise. There are currently 16 recognized species of icefish in Antarctica, all of which surprisingly lack two elements that are key to life, haemoglobin and red blood cells. On top of that, they live in one of the coldest places on planet Earth, the bottom of the Southern Ocean. How was it that evolution allowed this fish to survive in these extreme conditions and lack of oxygenated blood?
Discovering the Icefish
Icefishes were first discovered the same way all scientifically dumbfounding fish are; when someone went fishing in Antarctica. The zoologist, Ditlif Rustad, pulled the extraordinary fish, which he described as a “white-blooded crocodile fish”, from his net in 1927 while participating in a Norwegian expedition to claim Bouvet Island as a whaling station. His descriptions were like no other species of fish that had been described before. Its skin was snow-white and translucent in some areas. When he cut it open, there was no red blood, but a milky white substance instead, which Rustad described as “blod farvelöst” or “colorless blood”. Clearly, it lacked hemoglobin and red blood cells, which are found coursing through the veins of every other vertebrate, and are essential to survival. Where did this creature come from, and how could it exist on Earth without these very important parts?
No Red Blood Cells? No Haemoglobin? No Problem!
Since the time of its discovery, scientists have been grappling with the idea of a vertebrate that doesn’t posses the same oxygen delivering cells and oxygen binding proteins as every other vertebrate. As more studies emerged, scientists began realizing that the icefish had exceptionally large veins and vessels in its body, and an enlarged heart. This turned out to be an adaptation to move blood throughout their bodies at a faster rate than most animals. Why would an icefish need to quickly move blood throughout its body? Since it lacks haemoglobin, which binds oxygen to red blood cells and transports it throughout the body, the icefish only carries about 10% of oxygen that normal blood can carry. The large vessels speed up circulation, and ensure all of its tissues are well supplied by the oxygen it can carry. The absence of red blood cells, which also gives blood its crimson pigment, causes icefish blood to have a more diluted, or runny, appearance. When it comes down to it, red blood cells can be heavy to push around the circulatory system, so evolution decided to do away with the extra weight.
Blame Continental Drift
As Antarctica began to split away from the rest of the world and move south, the temperature of the water surrounding the continent began to drop. Decreasing ocean temperatures became less habitable for marine life, causing major die-offs for those who couldn’t adapt accordingly. While temperature dropped, oxygen levels increased dramatically. This abundance of oxygen is thought to have led the icefish in the direction of a haemoglobin-less existence. Why should the body continue to produce this protein if it’s so readily available within its environment? You can’t have haemoglobin without red blood cells, and since the fishes’ red blood cells were being scrapped, the haemoglobin had to go too.
Another very important adaptation for cold-water species, including the icefish, is the development of anti-freeze proteins that prevent the formation of ice crystals within cells that can lead to rupturing. Even as the oceans approach freezing temperatures of -2°C (28°F), icefish are able to survive and swim about freely without freezing solid. As it turns out, Antarctic icefish and your car have a lot more in common than you’d think!
Threats to the Antarctic Icefishes
It’s no mystery that the Southern Ocean, which surrounds Antarctica, is currently experiencing an increase in temperature due to global climate change. The many species that depend on these waters for their survival, including the icefishes, face a very serious threat if temperatures continue to increase over time. For a fish that has been evolutionarily shaped and molded by the ocean’s extremely cold temperatures, this could be bad news. They may be experts at surviving in freezing cold temperatures, but have no adaptations for living in warmer temperatures. With rising ocean temperatures comes an increase in ocean acidity. Like warmer temperatures, increasing acidity causes disruptions in ecosystems and food webs. The Antarctic icefishes feed primarily on fish, krill, and other crustaceans, which will experience a dramatic decline if acidity continues to increase.