Young polar cod are barely longer than a finger, making them easy prey for larger fish. The larvae and juvenile fish can only survive by hiding; and the best hiding place in their Arctic home waters is the sea ice. Millions of them seek shelter in the cracks and crevices under the ice floes, because the underside of the sea ice is hardly smooth as polished glass. On the contrary! It’s a virtual labyrinth of caves, cracks and sharp ridges. Sometimes these floes overlap, while at others they collide, as can be seen by the metre-wide sheets of ice jutting up vertically from the water. In some cases, terraces also form on the underside.
This craggy ice world not only offers the polar cod a safe haven. They also seek their own prey on the underside of the sea ice, where algae and countless amphipods and copepods can be found. In fact, the sea ice is practically an ocean floor densely populated by small life forms – the only difference being that everything is upside down.
Marine biologists still only partly understand how the food web under the ice works: given how difficult it is to access the underside of the pack ice, observations of polar cod or other fish there have been few and far between. It’s only been in the last few years that scientists have slowly begun to unlock the secrets of the food web, thanks to the advent of underwater robots and nets specially designed for use under the ice. Just when the polar cod eats, and which organisms are on the menu, are questions that no one can answer for sure. Our knowledge of another key polar species, the Antarctic krill, is also fragmentary. The small crustacean is similar to the North Sea brown shrimp and is considered a staple for the penguins, seals and whales of the Southern Ocean. As such, its role in the waters of the Antarctic is just as central as that of the polar cod in those of the Arctic.