The Southern Ocean is extremely rich in food. Millions of penguins and seals and thousands of whales feed here. Most of them rely on krill for their food. Krill are small shrimp-like creatures about six centimetres long. In winter they are dispersed widely, mostly under the ice surface. But in summer they assemble in vast swarms, some of which may contain a billion individuals. They are the most numerous animals on earth and their combined weight far exceeds that of the total human population.