Life in the Antarctic depends on the lowest members of the global food chain - krill. This would be reason enough for scientists to investigate the comings and goings of the small crustaceans. But researchers also wanted to know why trillions of krill formed swarms so vast the superswarms can be seen from the air.

A research team from the British Antarctic Survey studied the composition of 4,525 swarms in the Scotia Sea. The team used echo-sounding equipment in an area equivalent to the eastern half of the Atlantic Ocean.

The researchers found that superswarms, which often stretch over one kilometer (0.6 mile) in length and 30 meters (98 feet) deep, are densely packed, with trillions of krill in one swarm. “It was astonishing how much biomass could be concentrated into such a small area,” said Geraint Tarling, the lead researcher quoted in a BBC article. It also means the majority of krill living in the Antarctic Ocean at any one time will exist within a few superswarms.

The investigation also revealed that superswarms are formed by juvenile krill, but small, diffuse swarms were formed by adult krill. Although the researchers don’t know why this should be, they suspect that the juveniles may seek safety from marine predators in the dense packs. Adult krill may seek small swarms where there is less competition for their food supply — phytoplankton.

Ten or twelve swarms could account for 60 to 70 percent of the krill in an area the size of the eastern Atlantic, according to Tarling.

This finding has implications for the ecology of the Antarctic Ocean. Commercial fishing fleets that scoop up swarms of krill could be removing the majority of the densely packed krill. Several nations have commercial fishing operations in the sea. Krill is processed for food used in aquariums and aquaculture (farming of freshwater and saltwater organisms) and for bait for sport and commercial fishing.