Krill,or euphausiids,are plank-tonic, shrimp-like crustaceans of moder-ate size, up to 6 cm (2.5 in). The head isfused with some of the body segments toform a distinctive
carapacethat coversthe anterior half of the body like armor(see Fig. 17.12). Most krill are filterfeeders that feed on diatoms and otherplankton. They are extremely common inpolar waters, aggregating in giganticschools of billions of individuals. They are an almost exclusive food source for whales, penguins, and many types of fish(see Fig. 10.12). Other species of krilllive in the deep sea (see Fig. 16.4 a )