What do you think of this fact?
Around Antarctica, seawater can cool to about -2 degrees Celsius (28 degrees Fahrenheit). Salt water stays liquid at that temperature, but the blood of most fish would freeze. Fish live there anyway, and some of them are built so strangely they seem impossible at first glance.
Homemade antifreeze
Many Antarctic fish produce antifreeze glycoproteins. These circulate in the blood and stop tiny ice crystals from growing, so the blood stays liquid. It doesn’t work by keeping ice from ever appearing, but more like a bouncer that keeps every crystal small.
Where this ability came from is the cool part. In the Antarctic toothfish, the antifreeze gene evolved from pieces of a gene that originally made a pancreatic digestive protein. Digestion turned into frost protection.
Fish without red blood
Crocodile icefish take it even further. At least 16 species of this family are known, and they are the only known vertebrates that have no hemoglobin as adults. Red blood cells are usually missing entirely, and when there are any, they’re rare and don’t work.
So how does oxygen get around? It’s simply dissolved in the blood plasma. That works because cold water can hold more oxygen than warm water. On top of that, icefish have bigger hearts, bigger blood vessels and about 4 times as much blood as other fish. Their hearts pump around 5 times as much blood per minute.
They also save energy: icefish mostly sit still on the sea floor and wait for prey. Less movement means less oxygen needed.
Arctic cod also have antifreeze proteins in their blood, but they use completely different genes. Evolution invented the same solution twice, independently.
No hemoglobin, built-in antifreeze and an XXL heart: icefish prove there’s more than one blueprint for life in the ice.