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?