Krill. (McGraw-Hill Inc. Reproduced by permission.)

Marine crustaceans in the order Euphausiacea. Krill are zooplankton, and most feed on microalgae by filtering them from the water. In high latitudes, krill may account for a large proportion of the total zooplankton. Krill often occur in large swarms and in a few species these swarms may reach several hundred square meters in size with densities over 60,000 individuals per square meter. This swarming behavior makes them valuable food sources for many species of whales and seabirds. Humans have also begun to harvest krill for use as a dietary protein supplement.