Antarctic Krill (Euphausia superba)

node · earth · Southern Ocean - circumpolar, south of the Antarctic Polar Front

A shrimp-like euphausiid crustacean, a few centimetres long as an adult, that forms the single load-bearing link of the Southern Ocean food web: whales, seals, penguins, flying seabirds and fish either eat it directly or eat something that does. Its dependence on sea ice is a matter of life history rather than preference - the larvae overwinter under the pack ice, grazing the algal layer on its underside through the months when there is nothing in the open water to eat, so the extent and duration of winter ice sets how many larvae survive to the following year. A 2004 analysis of net-haul records from the Scotia Sea sector reported a long-term decline in krill density correlated with reduced winter sea-ice extent, alongside an increase in salps; the magnitude of the decline has been argued about since, but the mechanism linking recruitment to winter ice is not seriously disputed. Krill is also a commercial fishery, which puts an ecosystem bottleneck and a managed catch limit in the same accounts.