Planet Earth

The Krill Drill

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Antarctic krill, small shrimplike creatures, swim in dense schools of 20,000 or so animals per cubic yard. To avoid turbulence they adopt a staggered formation so that no krill swims directly in another’s wake. That obviously requires some kind of coordination. Konrad Wiese, a zoologist at the University of Hamburg, thinks he’s figured out how krill keep in touch: their tiny antennae pick up pressure waves from their neighbors. Wiese glued a tiny Lucite block to a krill to immobilize it, as shown here, and placed a small pressure sensor near the swimmerets--the bristle-covered legs that propel the animal. He detected four distinct signals: a low-frequency wave coming from the paddling swimmerets themselves and three higher-frequency signals from the bristles, which increase the surface area of the swimmerets by folding and unfolding as the krill swims. The low-frequency wave, Wiese thinks, tells each krill how fast the krill ahead is paddling. If they detect that the leader is changing the pace, he says, the signal propagates through the formation very quickly. Meanwhile, the high-frequency waves, whose strength falls off rapidly with distance, help a krill keep the proper distance from its neighbors--close, but not so close as to drift into their wake.

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