There are about 85 species of krill, ranging in size from under 0.5 inch (1 cm) up to 5.5 inches (14 cm) long. The dominant krill in the southern polar oceans is the Antarctic krill (Euphausia superba), which is up to 2.3 inches (6 cm) long and weighs about 0.035 ounces (1 g). Antarctic krill have a life span of about 5 to 10 years. Antarctic Krill is considered to be a keystone species, an organism upon which very many Antarctic predators depend.

Diet: Krill eat phytoplankton, single-celled plants that float in the seas near the surface. Some tropical krill also eat zooplankton. Krill spend their days in the dark depths of the ocean (about 320 feet = 100 m deep), safe from their major predators (like baleen whales and sea birds). They swim to the surface each night to eat phytoplankton. They can fast (go without eating) for up to 200 days, shrinking during this time.

Anatomy: Krill have a hard exoskeleton, many legs (used for swimming and gathering food), and a segmented body. Females produce almost 1,000 eggs each summer; the eggs are laid at the surface, but fall to great depths. The hatchlings swim back to the surface to feed. Like all crustaceans, krill molt their exoskeleton as they grow.

Food Web: Phytoplankton is eaten by krill (and many other organisms). Krill are eaten by many organisms, including fish, squid, sea birds, and mammals (like baleen whales and some seals).

Classification: Kingdom Animalia (animals), Phylum Arthropoda, Subphylum Crustacea, Class Malacostraca, Order Euphausiacea.