Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Crustacea
Class: Malacostraca
Superorder: Eucarida
Order Eupausiacea
Over 90 different species.

Krill look like a small shrimp. Ironically, this is their distinguishing characteristic. If you are a serious biologist, you might note that shrimp have ten legs, while krill have 12 to 16.

Krill live in every ocean on Earth, and have a cumulative biomass in the billions of billions of tons. Just about everything bigger than a krill eat krill; many species of whales, seals, penguins, squid, and fish depend on krill. Krill, in turn, are filter feeders that eat phytoplankton and zooplankton. They range from 10 to 150 millimeters, and thus some of them are indeed big enough for humans to catch with nets. If humans do go after krill, they must freeze them immediately, as krill turn poisonous at room temperature.

Krill live in in the photic zone, although many travel deeper during the day to escape predators. Many species take part in the grand diurnal vertical migration, traveling to the surface at night to hunt plankton. Other species will swim towards the surface whenever they are hungry, and may make the trip to the surface multiple times a day. Krill travel in large swarms to protect themselves from smaller predators. Of course, a large swarm of krill can also attract larger predators, and swarms can disband at the drop of a hat. Some fleeing krill have been seen molting to leave behind exoskeleton fragments as decoys.

Most species of krill have light-making organs, an array of ten of them in lines along each side. These photophores are comparatively complex, with lenses allowing them to be 'focused', and with the necessary musculature to allow the krill to rotate them. We do not know what these are used for, although it is generally assumed that they either have a social/sexual function, or that they are some form of camouflage.

The word comes from the Norwegian word krill, meaning 'young fry'. A group of krill is known as a swarm.

Last night I dreamed of krill again. The swarm whirled around like a flock of small birds. But the flock was bigger than a thundercloud. And zeppelins soared through it.

A krill is small and has a lot of legs. It doesn't like being mistaken for prawns or shrimps, which have fewer. How would you like to be taken for a seagull? Krill are euphausiids, not decapods.

Krill eat anything smaller than krill. Krill live in all of the oceans. But the northern krill of the lukewarm waters are not the krill of my dreams. They are not the billions of billions that swarm beneath the eternal ice. They are not Antarctic krill, Euphausia superba.

Algae grows on the underside of the ice in the frozen wastes of the Antarctic sea. But the sea isn't frozen, and it isn't a wasteland (nor any other land at all). The sunlight steals through the ice and the algae use it to make more algae. Then the krill scrape off the the algae and make more krill. Then the great baleen whales come by, and eat them.

Seals and penguins, squid and fish eat krill, as well. And penguins eat fish, and seals eat penguins, and they all eat anything that's smaller than them. Except for crabeater seals: they hardly eat anything but krill. But the whale swims past them all, and scoops up the krill to make more whales. And everybody likes whales.

Krill live in swarms. There can be millions of krill in a swarm, tens of thousands in a cubic meter. Imagine that a flock of birds married a big white cloud and their children went to live in the water. Five hundred megatonnes of children. The mass of krill in the Antarctic sea outweighs all the humans on Earth. Half of it is eaten every year. Two hundred and fifty billion kilogrammes of it: around the weight of the human race. The krill replace it all, with more krill.

People catch krill, in relatively small quantities. A couple of hundred thousand tonnes a year. They feed it to fish. Some Japanese eat it. Of course.

Krill are born of sinking eggs, two or three kilometers below the surface. They swim upwards while living off their egg yolk. They moult and metamorphose several times until finally they can eat. They have to moult to grow. They can also moult to shrink, if there isn't enough food. They can grow to up to six centimeters and can weigh two whole grammes

Adult krill swim so well they are no longer considered plankton.

When they aren't scraping algae off the bottom of the ice, they filter-feed on phytoplankton. And on non-alliterating zooplankton. They spit out balls of left over gunk, which sink along with long strings of krill poo to the bottom of the ocean, kilometers below. The sunken carbon is safely sequestered and out of the atmosphere for the next millennium. This may be the single most significant part of the carbon cycle in terms of the mass involved.

Krill stop swimming when they are full up, and start sinking. When they get hungry, they swim upwards again. Two or three times a day. Usually they swim with a selection of their legs. If you scare them they swim backwards quickly using their tail.

But why would you want to frighten a krill?

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