On a winter day in interior Alaska, a hike along a frozen slough reveals a patch of open water. Just below the surface, a school of small fish mills about. They're blackfish, a hardy, 7-inch-long fish found only in Alaska and Siberia. They have gills, but they also have a modified esophagus and can absorb oxygen by gulping mouthfuls of air. That's what these blackfish in this oxygen depleted slough water are doing - in fact, their activity is keeping this patch of water ice free.
In summer, blackfish can be found in tundra pools and seasonal ponds in interior and western Alaska, ponds that may go dry. When seasonal tundra pools dry up, the blackfish survive by wiggling down into the smelly bottom sludge and mud, keeping moist and breathing air until the next rain.
Blackfish are a Beringian holdover from the ice age. Beringia was a great expanse of open land that included Interior Alaska, Siberia, and land that is now under the Bering Sea. Sea levels were about 300 feet lower, and the shallow Bering Sea was dry land. Giant short-faced bears, woolly mammoths and mastodons roamed this landscape, and Alaska blackfish shared the freshwater with northern pike and Arctic grayling. The range of blackfish is entirely Beringian, in fact, these little freshwater fish are found on three islands in the Bering Sea, Saint Lawrence, Saint Matthew and Nunivak, islands that were once low hills rising above the dry, windswept plain of the Bering land bridge.