Science Puzzle
Why Don’t Antarctic Fish Freeze?
The sea around Antarctica is about -1.9 °C. Salt water stays liquid at that temperature, but the blood of most fish would freeze at about -0.7 °C. Yet whole families of fish live there all year round, swimming among floating crystals of ice.
Why don't Antarctic fish freeze?
The Answer
Their blood contains antifreeze proteins. These cling to the surface of any tiny ice crystal that gets into the fish and stop it growing, so the blood stays liquid below the temperature of the sea around it.
A fish's body is the same temperature as the water, so warmth isn't the answer. The danger is that a tiny crystal of ice, swallowed or touching the gills, would act as a seed: water in the blood would freeze onto it and the crystal would grow until it killed the fish.
The antifreeze proteins bind to the faces of an ice crystal as soon as it appears, like sticky patches on its surface. New water molecules can't join the crystal where the proteins sit, so it can't grow. That lowers the temperature at which the fish's blood freezes to below -1.9 °C, just enough to be safe.
This is a different trick from the salt we spread on roads, which works by dissolving into the water. To do the same job with dissolved salt, a fish would need blood far saltier than the sea. The proteins were discovered in the 1960s by the American biologist Arthur DeVries. Arctic cod, at the other end of the world, have evolved very similar antifreeze proteins quite independently.
The principle: Antifreeze proteins. Proteins that stick to the surface of tiny ice crystals stop them growing, so a fish's blood can stay liquid below the temperature at which it would normally freeze.