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Cold-blooded and Clear-blooded

by AMNH on

On Exhibit posts

Ice Fish
Antarctic icefish like this one eat smaller fish and krill in the cold Southern Ocean.

Few animals have risen to meet the challenge of enduring life in one of the coldest and most severe environments on the planet: the Southern Ocean, which surrounds Antarctica. The highest temperatures in the waters near Antarctica’s northernmost peninsula reach 1.5° Celsius (34.7° Fahrenheit) and that only during the brief summer months.

These temperatures have allowed a group of particularly gritty animals to stake their claim on this singularly severe sea: about 90 percent of the fishes in the Southern Ocean belong to a single suborder, Notothenioidei. To survive in an environment where water temperatures drop below the freezing point of blood, notothenioids have evolved incredible characteristics. Many species produce antifreeze proteins that thwart ice crystals from forming in their bodies during a big chill.

The species that belong to the family Channichthyidae, which branched off from the lineage about 5.5 to 2 million years ago, opted for a different tactic. Somewhere along the way, channichthyids shed their red blood cells and hemoglobin, the iron-rich protein found in red blood cells that transports oxygen throughout the body. They became the only vertebrates without red blood cells and hemoglobin—an oddity that manifests itself in their strange, colorless blood.

Young Ice Fish
A young icefish, shown here in its larval stage.
Wikimedia/Uwe kils

When Antarctic icefishes, as channichthyids are commonly known, were discovered in the 1920s, biologists were puzzled. Many assumed that the animals’ curious lack of red blood cells had some adaptive value. Because they’re so cold, Antarctica’s waters are also incredibly rich in oxygen, and scientists thought that icefishes might soak up enough dissolved oxygen through their gills and skin to be able to afford to thin out their blood, saving energy by circulating a watered-down version instead.

After all, some fishes in warmer waters show a decrease in red blood cells during colder seasons for this very reason.

As it turns out, that’s not actually the case. Without hemoglobin, icefish blood is thinner but it carries less than 10 percent of the oxygen found in the veins of their redblooded notothenioid cousins, putting significant pressure on the cardiovascular system to distribute enough oxygen.

To compensate, icefishes have considerably larger hearts that pump higher volumes of blood—and spend about twice as much energy in the process. They also move that blood through wider blood vessels and devote a much denser network of blood vessels to sensitive organs like the retina of the eye than do red-blooded fishes.

Being an ice-veined icefish turns out to be quite expensive, metabolically speaking. Instead of a clever adaptation, blood free of hemoglobin starts to look more like an evolutionary mishap. Still, these clear-blooded icefishes have thrived for millions of years—and it’s hard to argue with success.

To learn more about icefish and other animals that can brave the chill of life in Antarctica, come see the new exhibition Life at the Limits

A version of this article originally appeared in our member magazine, Rotunda.