Sunday, January 18, 2015

Why (some) fish don't freeze

Early in a bike ride this cold January afternoon, I hit some hidden loose gravel under a big icy puddle on a bike trail and fell, soaking my feet, legs, and hands. Nice job, I know.

No cold feet here - curious Weddell seals under the ice.
Photo: changehali, Wikimedia Commons
I decided to keep going, partly to accompany my companion and partly it seemed silly to just turn back, especially when the next few km were mainly up hill, so I stayed warm for most of the ride (except those cold feet!).

We humans can't survive more than about an hour submerged in icy water (and normally much less, due to exhaustion or blacking out). But what about all those crazy animals that live in the world's various frigid oceans?

You've got your polar bears and sea otters, with their thick, heat-trapping fur, and your pinnipeds (seals, sea lions, walruses) and cetaceans (whales and dolphins), protected by layers of fat, and of course penguins, who use both strategies.

And then there are the antifreeze artists, the animals and plants that survive freezing environments without the benefit of fur or feathers, using just the chemical composition of their body fluids.

No, the antifreeze artists do not decorate your car with antifreeze in cold weather. They are animals and plants that live in frozen environments but escape cell damage from ice formation by producing antifreeze proteins (AFPs) that help them either avoid freezing altogether or tolerate it somehow.

Antarctic toothfish
photo: Paul Cziko
One example is the Antarctic toothfish, the southern cousin of the famous and now overfished Patagonian toothfish (a.k.a. Chilean seabass, a name invented by a fish wholesaler named Lee Lantz in 1977 to improve its marketability - today's factoid).

Antarctic toothfish, like a number of other cold-water fish and other non-furry, non-feathered animals, produce ATPs in their livers and surface skin/scales/gills that stop the growth of ice crystals inside their bodies.

Scientists have known since the 1960s that the antifreeze proteins interact at their surface with "nano" ice crystals. This interaction prevents larger ice crystals from forming and harming the animal.

A 2014 study found that the AFPs that prevent fish in the Southern Ocean from freezing also allow the internal ice crystals in their bodies to persist at temperatures that normally would melt them. They never enter warm waters naturally, but might blockage or damage be a problem if they did?

And yes, clever entrepreneurs have already figured out how to use fish AFPs to maintain textures in frozen human foods, particularly in low-fat ice cream to prevent ice recrystallization and maintain that soft, creamy texture we love even when we forget to put it back in the fridge right away.

FYI: In case you were wondering about your own AFP-producing capacity:

Lesson:  stay dry to avoid hypothermia
chart: Seagrant, University of Minnesota


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