If you live in Canada, winter eventually gets in your veins. But the animal kingdom has one-upped people in this department: there are actually fish that have antifreeze running through their veins!
Antifreeze is a substance that keeps liquids from freezing. It lowers their freezing temperature, so it prevents damage that could be caused by ice formation. For example, if you’ve ever driven or ridden in a car in winter, antifreeze is what’s keeping liquids in that car’s radiator from freezing.
Fish living in cold climates have evolved their own solution to freezing: antifreeze proteins. Arctic and Antarctic fish families have these proteins in their blood. They’re part of why these fish can live in waters that other fish can’t.
The Arctic Cod (Boreogadus saida) is an example of one fish species that has evolved to use anitfreeze proteins as protection from frigid temperatures. (Public Domain, Wikimedia Commons)
Did you know? Arctic water reaches freezing temperatures without actually freezing. That’s because, thanks to its salt content, salt water has a freezing temperature below 0°C.
When the water is below freezing, fish need a way to keep themselves from freezing. Antifreeze proteins have long strands of repeating amino acid units that can bind ice crystals. Ice crystal formation in blood leads to cell death. Small ice crystals form, then snowball into larger crystals by adding on surrounding water. Antifreeze proteins can prevent this by binding to the small crystals. This prevents the crystals from growing, and prevents the blood from freezing.
Did you know? Humans have found ways to use antifreeze proteins, too. For example, ice cream manufacturers use them to prevent ice crystals from forming. Scientists and lab workers also use them when they store tissues or organs.
This is an example of an antifreeze protein molecule. The long chain of repeating amino acids is folded up into a species-specific shape. The light blue portion of this protein binds ice crystals. This prevents them from binding to more ice and growing into large crystals (Public Domain, Wikimedia Commons)
Some of these antifreeze proteins came to be millions of years ago, and have evolved multiple times. They’ve evolved whenever a type of fish has needed to change in order to survive.
Antifreeze proteins are an example of convergent evolution. This is when two lineages of the same species develop the same trait over time, even though they don’t have a common ancestor in their recent history. This usually happens because they’re trying to survive in similar environmental conditions.
For example, the Antarctic toothfish and the Arctic cod have no close relatives. They also live very far away from each other, almost on opposite ends of the Earth! But they have both evolved antifreeze proteins. That’s because they both live in environments where, without these proteins, they’d freeze to death.
What does all this mean? Thanks to antifreeze proteins, fish can live in areas where they otherwise couldn’t. What an amazing evolutionary adaptation!