Polar Seas: Life Under the Ice
Situated on opposite ends of the globe, polar seas share many similarities but are imperfect mirrors of each other. Both the Arctic and the Southern Ocean are cold and nutrient-rich, but the latter is colder and boasts a richer marine fauna. In both, the Killer Whale (Orcinus orca) is the top marine predator and both are host to many of the same species of migratory filter-feeding whales during summer months, but their respective summers are six months’ out-of-phase — so that when it is summer in one hemisphere, it is winter in the other. Both have a variety of elephant seal, fur seal, and right whale, but — while superficially similar — each hemisphere’s species is distinct from the other’s. The closer one examines the Arctic and Southern Oceans, the more differences become apparent.
The Arctic Ocean is almost completely surrounded by the landmasses of North America, Eurasia, and Greenland. Roughly 80% of water entering and leaving this ocean basin passes through the narrow channel between Greenland and Spitzbergen. Arctic weather is influenced by warming and cooling of the mainlands and larger islands that rim it. Thus the climate is warmer and more seasonal than that of the Antarctic and terrestrial wildlife is more speciose. Arctic Ocean surface temperatures range from 28 to 41 degrees Fahrenheit (-2 to 5 degrees Celsius). Its cold waters are home to the world’s largest jellyfish (the Lion’s Mane, Cyanea capillata, the swimming bell of which can measure 6 feet or 1.8 metres across), auks, auklets, puffins, murres, guillemots, the hawk-like parasitic jaeger, the Walrus (Odobenus rosmarus), Belugas (Delphinapterus leucas), Narwhals (Monodon monoceros), and the world’s only seagoing bear, the Polar Bear (Ursus maritimus). Although it appears solid, the far Arctic has no land, consisting entirely of dense sea ice (frozen seawater), distinct from the thousands of icebergs (composed of frozen freshwater, broken off from glaciers) that dot the surrounding sea.
In contrast, the Southern Ocean surrounds the Antarctic continent and comprises the southernmost portion of the Atlantic, Pacific, and Indian Oceans. Although it has no rigid northern boundaries, the Southern Ocean is defined by the Antarctic Convergence — which is the limit of cold Antarctic water at the surface. The Southern Ocean is thus a distinct body of water with its own physical characteristics and a common fauna. Antarctic weather features strong, cold winds blowing down from the high polar plateau, cooling the surrounding ocean. Southern Ocean surface temperatures range from 28 to 36 degrees Fahrenheit (-2 to 2 degrees Celsius). The Southern Ocean’s marine ecosystem is thus more isolated, and colder than that of the Arctic, but it is also more prolific. Its cold waters swarm with krill, squids, notothenoid cods (which produce a biological anti-freeze), the gull-like South Polar skuas, at least 11 species of penguin, and the only seal to prey on warm-blooded animals, the Leopard Seal (Hydrurga leptonyx). These waters are also home to the southernmost-living mammal and champion diver among seals (as long as an hour to depth as great as 1,900 feet or 580 metres), the Weddell Seal (Leptonychotes weddelli). Unlike the high Arctic, Antarctica does feature a continental landmass, but it is almost entirely covered by thick ice and remains bitterly cold year-round.
Unlike the vast majority of the deep-sea environment, polar seas are very nutrient rich and highly productive. But, like the deep-sea, they are also fiercely cold, imposing many of the same thermal challenges to the sharks and other creatures that live there. As a consequence, polar shark diversity is rather low but some species grow to enormous size.