Krill: Euphausiacea

Krill are shrimplike in appearance and measure 1.57 to 5.9 inches (40 to 150 millimeters) in length. Their red-spotted bodies are transparent, with their internal organs visible from the outside. They have two pairs of antennae, both of them branched, or biramous (BY-ray-mus). Their large compound eyes are set on stalks. Each compound eye has many individual lenses. The head and segmented thorax are closely joined together, forming a region known as the cephalothorax (SEH-feh-lo-THOR-acks). The cephalothorax is covered by a shieldlike carapace that does not cover the feathery gills located on the bases of some legs.

The thorax has eight pairs of biramous limbs. Thoracic (thuh-RAE-sik) limb pairs six through eight form a netlike structure that is used to strain food out of the water. Krill do not have any thoracic limbs associated with the mouth. The abdomen has six segments, plus a flaplike tail segment, or telson. The first five segments each have a pair of paddlelike limbs, or pleopods (PLEE-oh-pawds). These special pleopods are used for swimming and are also called swimmerets (SWI-meh-rehts).

Many species are called "light-shrimp" because they have light-producing tissues in their eyestalks, legs, and abdomen. These displays of yellowish green or blue light are probably used to locate mates or confuse predators.