Thysanoessa raschii, sometimes known as Arctic krill, is one of the most common euphausiid

Krill

Krill is the common name given to the order Euphausiacea of shrimp-like marine crustaceans. Also known as euphausiids, these small invertebrates are found in all oceans of the world...

 species of the subarctic

Subarctic

The Subarctic is a region in the Northern Hemisphere immediately south of the true Arctic and covering much of Alaska, Canada, the north of Scandinavia, Siberia, and northern Mongolia...

 and Arctic

Arctic Ocean

The Arctic Ocean, located in the Northern Hemisphere and mostly in the Arctic north polar region, is the smallest and shallowest of the world's five major oceanic divisions...

 seas. They may reach 20–25 mm (0.78740157480315–0.984251968503937 ) long, and are sexually mature

Sexual maturity

Sexual maturity is the age or stage when an organism can reproduce. It is sometimes considered synonymous with adulthood, though the two are distinct...

 above 14 mm (0.551181102362205 in).

T. raschii is a major prey item of several taxa, planktivorous fishes and marine mammal

Marine mammal

Marine mammals, which include seals, whales, dolphins, and walruses, form a diverse group of 128 species that rely on the ocean for their existence. They do not represent a distinct biological grouping, but rather are unified by their reliance on the marine environment for feeding. The level of...

s. It is also a common prey item of seabird

Seabird

Seabirds are birds that have adapted to life within the marine environment. While seabirds vary greatly in lifestyle, behaviour and physiology, they often exhibit striking convergent evolution, as the same environmental problems and feeding niches have resulted in similar adaptations...

s, including shearwater

Shearwater

Shearwaters are medium-sized long-winged seabirds. There are more than 30 species of shearwaters, a few larger ones in the genus Calonectris and many smaller species in the genus Puffinus...

s.

T. raschii goes through a number of stages in its development. Macdonald defined the characteristics of fourteen stages, or 'furcilia'.