The Arctic has fewer endemic fishes due to the combined effects of less geographic isolation and younger age. The oceanic environment between subarctic or boreal and Arctic areas is fairly continuous. On the western, Pacific side, the Bering Sea flows into the Arctic Ocean and has done so since the Bering Strait opened up 3.5 million years ago. Similarly, on the eastern, Atlantic side, the Arctic Ocean is directly connected to the Greenland Sea. Hence,Arctic fishes are either species that evolved there since the current climate developed or are cold-tolerant Pacific or Atlantic species that experience gene flow from source areas rather than being endemic to the Arctic itself. The Arctic has undergone repeated warming and cooling until about 3 million years ago when the present cold conditions stabilized, leaving less time for organisms to adapt to current conditions (Briggs 1995). Consequently, fishes in the northern polar region have had less time to speciate.
Adaptations to cold are evident in Arctic fishes, where species have converged with Antarctic fishes in the production of antifreeze compounds (Farrell & Steffensen 2005). Glycoprotein antifreeze occurs in Arctic and Greenland Cod, whereas Warty Sculpin, Canadian Eel-pout, and Alaska Plaice possess peptide antifreezes (Clarke 1983). Arctic Cod are frequently observed resting in contact with ice and taking refuge inside holes in ice, so their potential for encountering seed crystals is very high. In some of these fishes, kidney glomeruli are convergently reduced to help retain antifreeze compounds in the body (Eastman 1993). Several boreal cods, sculpins, eel-pouts, and flatfishes whose ranges extend into Arctic water also have antifreeze compounds in their blood.
Water temperatures show greater annual and latitudinal variation in the Arctic than in the Antarctic, which means that fishes are likely to encounter extreme winter cold but also relatively high summer temperatures. Winter temperatures do commonly drop to −1.8°C as in the Antarctic, but water can reach 7 or 8°C during the summer. The greater seasonal range is reflected in the tolerance of different species to warm temperatures, as well as differences in seasonal production of antifreeze. Few Antarctic fishes can tolerate water temperatures above 7 or 8°C regardless of acclimation temperature, whereas Arctic species have upper lethal temperatures of 10–20°C depending on species and acclimation temperature (DeVries 1977). Several north polar species produce less antifreeze during the summer, particularly among boreal fishes that may encounter temperatures well above freezing. Winter Flounder, Pleuronectes americanus, have a blood volume of 3% antifreeze in winter and 0% in summer. Reduced antifreeze production during warmer months probably saves energy and may also increase the blood’s capacity to carry oxygen or nutrients.