Origins ANTARCTICA, Scientific Journeys from McMurdo to the Pole
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Fish: Fresh, Not Frozen
by Michael Matz

Ecologists drool over Antarctica. It’s an ideal place to study evolution, they say. This is partly because the continent’s harsh environment has contributed to the evolution of an abundance of elegant biological adaptations. Life forms running the gamut—from bacteria and fish to plants and birds—have figured out ingenious ways to survive the continent’s frigid, dry climate and relentless winds.

Meet the Notothenioids, a group of more than 120 marine fish species, most of which are native to Antarctic waters. Antarctic Notothenioids have remarkable proteins in their bloodstream that prevent them from freezing. These "antifreeze proteins", as they are commonly known, bind to tiny ice crystals in their bodies, inhibiting further growth.

Arctic Cod (Boreogadus saida)
Arctic cod (Boreogadus saida)
(click for a larger image)

© Dr. Christopher Zimmerman, ISH
To survive the frigid waters of the Arctic Ocean, the Arctic cod—a genetically unrelated fish to the Notothenioids—developed an antifreeze protein nearly identical to the Notothenioid one. This independent development of the same adaptation in response to similar environmental conditions is known as convergent evolution.

Most marine fish from temperate and tropical regions don’t have antifreeze proteins, and would freeze to death in the frigid Antarctic waters. That’s because the water temperature (28°F or –1.9°C) is below the freezing point of their blood. (While pure water freezes at 0°C, the salt in sea water lowers the freezing point to about –2°C.)

The Notothenioids dominate all of Antarctica’s marine habitats. Not only do they comprise about half of Antarctic marine fish species, they account for 95 percent of all fish biomass in the region. As a result, there’s not much of a variety of marine fish species outside the Notothenioids. In contrast, temperate and tropical regions, such as coral reefs, have much higher fish diversity, with ample distribution of species among many species groups.

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