krill,any member of the crustacean order Euphausiacea or of the genus Euphausia within that suborder. Euphausiids are shrimplike marine animals that are pelagic in habit (i.e., they live in the open sea). They differ from true shrimp (order Decapoda) in that their gills are located on the swimming legs, and fewer legs are modified for feeding. They range in size from 8 to 60 mm (about 14 to 2 inches). Eighty-two species have been described. Most have bioluminescent organs (photophores) on the lower side, making them visible at night. They are of great importance in certain regions of the sea as food for various fishes, birds, and whales, particularly blue whales (Balaenoptera musculus) and finback whales (B. physalus). Krill occur in vast swarms that may gather near the ocean surface or at depths greater than 2,000 m metres (about 6,600 feet).

The body of E. superba is about five centimetres 5 cm (2 inches) long and translucent, with reddish brown blotches. The swimming larvae pass through nine stages of development. Males mature in about 22 months, females in about 25 months. During a spawning period of about five and a half months, the eggs are shed at a depth of about 225 m metres (740 feet). The krill larvae gradually move toward the surface as they develop, feeding on microscopic organisms. From January to April swarms of E. superba in the Antarctic Ocean may reach concentrations of 20 kg per cubic metre (about 35 pounds per cubic yard).

Krill serve as integral parts of marine food chains in Antarctic waters; they are the main prey for several penguin, whale, and fish species in the region. Krill populations in the waters adjacent to the Antarctic Peninsula have declined significantly since the 1970s as a result of reduced sea-ice coverage caused by climate change; sea ice protects krill and the blooms of phytoplankton they feed on from storms and predators. Some ecologists attribute population declines of Adélie penguins (Pygoscelis adeliae) and chinstrap penguins (P. antarcticus) to low krill abundance caused by climate change.

Because of their vast numbers and nutritive qualities, krill have been regarded by ecologists increasingly harvested as a potential food source for humans, raising concerns about the potential impact on penguin, whale, and fish populations. They are an especially rich source of vitamin A. In addition, krill oil, which is rich in omega-3 fatty acids, is used to produce dietary supplements. Many ecologists are concerned that the continued development of the Antarctic krill fishery by humans will reduce the amount of krill available for wildlife and further disrupt the region’s penguin, whale, and fish populations.