Euphausia superba

Lore & Background

In the pelagic zone south of the Antarctic Convergence, krill assembles into swarms so dense that satellite imagery resolves them as dark smudges against the green-blue of open ocean. Field researchers describing these aggregations note that a single cubic metre can contain several million individuals, their combined bioluminescence producing a faint blue-green shimmer at twilight when the photophores fire in unison.

Reproduction in Euphausia superba involves a distinctive courtship: the male produces a spermatophore, a gelatinous packet of sperm, which he deposits on a substrate or presents to the female. The female then picks up the packet and stores it in a spermatheca, fertilising eggs over a period of days. This decoupling of mating and fertilisation allows the species to maintain high reproductive output even in the short austral-summer window when phytoplankton blooms peak.

Krill's ecological role is so central that a decline in its biomass—whether from warming waters, reduced sea-ice extent, or commercial trawling—cascades through every higher trophic level in the Southern Ocean. Marine biologists frequently describe it as the single most important prey species in any marine ecosystem, a claim supported by the sheer proportion of predator diets it constitutes.

Reader's Guide

Trigger: dawn phytoplankton bloom in the upper 50 m. Krill ascends in a slow, coordinated vertical migration, orienting its maxilliped filter-feeding apparatus toward the diatom-rich layer. Observed action: individuals extend their maxillipeds in a rhythmic, basket-like motion, trapping diatoms and detritus in a mucous net that is periodically swept to the mouth. Ecological consequence: the krill swarm converts primary production into animal biomass, making it available to higher trophic levels within hours.

Trigger: predation pressure from a passing seal or penguin. Krill in the outer swarm layers flash their photophores in a rapid, synchronous burst, then execute a coordinated lateral jet using their pleopods. Field observations (e.g., submersible footage in the Bransfield Strait) show the swarm splitting into two lobes that re-merge within 30–60 seconds. Ecological consequence: the escape response reduces per-individual predation risk but concentrates the remaining biomass in a tighter volume, making the core swarm a more efficient foraging target for the next predator.

Trigger: male courtship in mid-water. The male produces a spermatophore and presents it to a nearby female; she retrieves it with her antennae and stores it in a ventral spermatheca. This behaviour is documented in field collections and laboratory observations of captured individuals; it is not an aquarium-tank observation, as Euphausia superba is rarely maintained outside short-term research holding. Ecological consequence: decoupled fertilisation allows the female to release fertilised eggs over several days, extending the reproductive window beyond the brief austral-summer bloom.

Did You Know?

• Antarctic krill swarms are large enough to be detected by satellite ocean-colour sensors, appearing as anomalous dark patches in the Southern Ocean.
• The species' reddish colouration comes from the carotenoid astaxanthin, which it sequesters from its diatom diet and stores in the cuticle.
• A single blue whale can consume up to four tonnes of krill per day, making Euphausia superba the largest single food item in the diet of the largest animal ever to live.
• Krill possess a blue-coloured circulatory fluid based on hemocyanin, the copper-containing respiratory protein, rather than the iron-based haemoglobin found in vertebrates.