euphausiids

Euphausiids, commonly known as krill, are small to medium-sized crustaceans in the order Euphausiacea, within the class Malacostraca. They inhabit oceans worldwide, from polar to temperate waters, typically measuring about 1 to 6 cm in length, though some species are larger. Their bodies are laterally compressed with a hard carapace covering the head and thorax and a flexible abdomen. They possess a set of thoracopods used for swimming and feeding, and the abdomen bears pleopods that propel them through the water.

Many euphausiids are bioluminescent, bearing photophores along the body, which may aid schooling, counter-illumination, and predator avoidance.

Ecology: They are primary consumers or detritivores that feed on phytoplankton, microzooplankton, and organic detritus, using specialized mouthparts to filter minute particles. They occur in dense swarms and perform diel vertical migration, living close to surface at night to feed and descending to deeper waters by day to avoid predators.

Life cycle: Females brood eggs attached to pleopods and release free-swimming larvae after fertilization. Larval development passes through several stages before reaching adulthood.

Role in ecosystem and fisheries: They are a major trophic link between phytoplankton and higher predators such as fish, seabirds, penguins, seals, and whales, and they support large commercial krill fisheries, notably Euphausia superba (Antarctic krill) and Euphausia pacifica (Pacific krill). Harvests are managed by international bodies such as CCAMLR, with concern about climate change, especially loss of sea ice in the Antarctic affecting reproduction.