Why don't polar fish freeze?

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If saltwater at the poles is below freezing, how do the fish avoid becoming frozen?

Although saltwater doesn’t turn to ice immediately when below zero, ice crystals can still form and if these develop inside fish cells, then they can cause significant damage which may result in death. To prevent ice crystals harming them, polar fishes have evolved antifreezes that prevent them from freezing in saltwater with a temperature that is below freezing.

At both the North Pole and South Pole different groups of fish have evolved similar antifreeze proteins, called antifreeze glycoproteins or AFGPs. The proteins act as biological antifreezes and bind to tiny ice crystals and prevent them from growing larger and damaging cells.

At the Arctic, members of the cod family Gadidae are famous for these, while at the South Pole Notothenioids have evolved similar proteins. Although the two groups of fish are unrelated, evolution has seen them separately evolve proteins that perform identical roles and are structurally very similar.

Antifreeze proteins are being used by some ice cream manufacturers as an additive to prevent ice crystals forming in ice creams and frozen yoghurts and keep them soft and creamy.

This item was first published in the November 2009 issue of Practical Fishkeeping. It may not be reproduced without written permission.