Eighteen krill species, including the most common varieties, have been observed to congregate in large groups called swarms. Depending on the species and location, krill swarms may pack as many as one million individuals, or hundreds of pounds (kilograms) of biomass, into each cubic foot (0.03 cubic m), and can extend over several hundred square miles (square kilometers). The density of organisms is not constant; in some areas, there may only be a few individuals per ft2 (0.09 m2). There may also be aggregations called shoals, less dense than swarms, with 10-100 krill per 35 cubic ft (1 cubic m). Swarming behavior is generally less apparent during the winter than at other times of year.

In most krill species, the swarms tend to be found at lower depths during the day to escape predation. The cooler temperatures also allow them to conserve energy by slowing their metabolism. At night, they rise to the surface in order to feed, accomplishing these vertical migrations through control of their buoyancy.

The pleopods of euphausiids allow them to maneuver, and in fact, being heavier than water, they must swim in order to stay afloat. They do this sporadically, alternating with peri ods of rest. Despite their swimming ability, krill do not propel themselves over large horizontal distances, and although not strictly planktonic (drifting) during most of their lifecy-cle, they are to some degree at the mercy of the current. However, by adjusting their buoyancy, they may be able to take advantage of current variations at different depths.

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