When krill come together, they form some of the largest gatherings of life on the plant.
Now scientists have discovered just how these small marine crustaceans do it.
Huge ‘superswarms’ containing trillions of krill are formed by juveniles not adults, and these swarms are even denser than experts supposed.
That suggests that all krill in the Southern Ocean are more vulnerable to overfishing then previously thought, the scientists warn.
Krill are small shrimp-like crustaceans that gather in huge numbers.
Previous research has found that some gatherings of Antarctic krill (Euphausia superba) can stretch for tens of kilometres.
But while huge swarms are known to exist, scientists did not really understand why some swarms are bigger than others, and what drives krill to gather in this way.
So researchers working for the British Antarctic Survey (BAS) decided to investigate the phenomenon.
Led by Dr Geraint Tarling, a BAS researcher based in Cambridge, UK, the research team studied the composition and structure of 4525 separate krill swarms in the Scotia Sea, a vast expanse of water in the Southern Ocean.
The team used echo-sounding equipment, which works much like underwater radar, to find the krill across an area of water equivalent to the eastern half of the Atlantic Ocean.
What they found surprised them.
Krill tend to gather into two distinct types of swarm.
Some krill gather into smaller swarms, no longer than 50m long and up to 4m deep.
These swarms are not very tightly packed, with just ten individual krill per cubic metre, on average.
However, other much bigger swarms also occur.
Dubbed “superswarms”, these are an order of magnitude larger in area, often stretching over one kilometre in length, and averaging almost 30m deep.
What is more, these superswarms are much more densely packed, containing up to ten times greater density of animals.
“I was coming at it thinking there might be small swarms tightly packed, and then large swarms that were a bit more diffuse,” says Dr Tarling.
“But what we actually found was the opposite.
There were small swarms that were quite diffuse and large swarms that were tightly packed.”
That means that the majority of krill living in the Antarctic Ocean at any one time will exist within a few, huge superswarms.
“We talking trillions of krill in one aggregation,” explains Dr Tarling.
“Ten or 12 swarms could explain 60 or 70% of the biomass in an area the size of the eastern Atlantic.”
“It was astonishing how much biomass could be concentrated into such a small area.”
The scientists then searched for reasons why such superswarms form.
Certain factors made superswarms more likely.
“The factors we identified included whether there was more likely to be a lot of food around or not, and when there wasn’t that much food around, they tended to form larger swarms,” says Dr Tarling.
The small, diffuse swarms are usually formed by mature, adult krill, the researchers discovered.
However, the huge superswarms are formed by juvenile krill.
“Where the animals were less mature, they were more likely to form the larger swarms,” says Dr Tarling.
“Why they do that I don’t know.”
One possible explanation could be that swarming together offers individual krill protection against marine predators such as whales or seals.
“All types of swarms are probably to a greater or lesser extent an antipredator response. There is safety in numbers, the predator confusion affect,” Dr Tarling says.
But swarming comes at a cost, as each individual shrimp has to compete with millions of others for food.
Adult krill are quite negatively buoyant, and have to keep swimming to stay afloat. That takes a lot of energy, which must be supplied by food, so adult krill likely want to avoid competing with millions of others for their next meal.