It’s like something out of a science-fiction horror movie: creatures that feed on slime coating the underside of sea ice, and swim in swarms of as many as 35,000 per square meter, advancing in a narrow band or sheet. They eat by gathering food between their legs and stuffing it in a sort of sausage casing while they digest it. If they run low on food, adults can “develop in reverse”, losing their sex organs and regressing through previous life stages to a more immature life form that can absorb nutrients directly from the water.

These creatures may seem like science fiction, but they exist; they are krill, crustaceans that somewhat resemble shrimp. Krill are the primary food for Antarctic fish, squids, penguins, albatrosses, petrels, whales, and some seals. Penguins eat so much krill that their droppings are stained bright red by the krill’s red pigments. Blue whales can eat up to 5 tons (4,500 kg) of krill in one day. One of the largest species of krill in the world weighs just over 1 gram as an adult, so that means one blue whale can eat about 4.5 million individual krill a day—that is more than the human populations of Philadelphia, Phoenix and San Antonio combined! Baleen whales eat so much krill they can increase their body weight by 50% during the summer months they spend feeding in Antarctic waters.

Because virtually all of the animals in Antarctica rely either directly or indirectly on krill, krill are called a keystone species. In construction, a keystone is the stone at the summit of an arch that locks the whole structure together. Without the keystone, the arch would fall. The same idea holds true for the krill as a keystone species; without krill the entire Antarctic ecosystem would collapse.

Krill depend on the slime coating on the underside of sea ice to survive through the Antarctic winters. Over the last twenty years the extent of winter sea ice has decreased, greatly reducing the krill populations.