Antarctic Krill

The Antarctic krill is the most abundant wild animal on Earth. A single adult is a translucent, shrimp-like crustacean about six centimetres long and weighing only two grams, yet the species collectively masses somewhere between 300 and 500 million tonnes -- a biomass greater than every human being on the planet and larger than any other single wild vertebrate or invertebrate species yet measured. Euphausia superba is the organism on which the entire Southern Ocean ecosystem rests, and the organism whose slow retreat from the warming Antarctic Peninsula threatens to unravel that ecosystem over the coming century.

Everything about this species is extreme. It forms swarms so vast they can be seen from space. It glows in the dark with ten chemical lanterns built into its body. It can reverse its own growth and physically shrink when food is scarce. Its larvae spend eleven months developing under the shelter of pack ice. It migrates 500 metres vertically through the water column every day. And it sits at the centre of an industrial fishery managed under an international treaty because getting krill management wrong would cascade into whale, penguin, and seal populations across an entire ocean.

This guide covers the biology, ecology, and conservation status of the Antarctic krill: taxonomy, anatomy, bioluminescence, swarm dynamics, life cycle, the 500-million-tonne biomass question, climate impacts, and the CCAMLR fishery. It is a reference entry, not a summary, so expect specifics -- tonnages, percentages, decades, and the names of the regions where this animal is thriving or vanishing.

Etymology and Classification

The scientific name Euphausia superba combines the genus Euphausia, coined in 1852 by the Norwegian zoologist Michael Sars, with the Latin species epithet superba, meaning 'magnificent' or 'proud'. The genus name is constructed from Greek roots meaning 'well-shining', a direct reference to the bright photophores that characterise the family. The common English name 'krill' comes from the Norwegian krill, meaning 'small fry of fish' or 'young fish', a term borrowed by nineteenth-century whalers who saw these animals as the staple food of the baleen whales they were hunting.

The species belongs to the order Euphausiacea, a group of roughly 85 described krill species found in every ocean on Earth. The family Euphausiidae contains most of these, and within that family Euphausia superba is by far the most abundant. Other Southern Ocean krill species exist -- Euphausia crystallorophias (ice krill), Thysanoessa macrura, and a handful of others -- but none approach superba in total biomass or ecological weight.

Krill are morphologically similar to shrimp and prawn, and they share the broader crustacean class Malacostraca with crabs, lobsters, and true shrimp. They differ from true shrimp (order Decapoda) in several structural details: krill have externally visible gills, they carry their eggs free in the water rather than glued to the underside of the female, and they have a different arrangement of thoracic appendages. Krill are an ancient lineage; fossil evidence and molecular clocks place their divergence from other malacostracans at least 130 million years ago.

Size and Physical Description

Antarctic krill are small animals. A mature adult is typically between 5 and 6 centimetres long and weighs up to about 2 grams. Males and females are similar in size, with females slightly larger on average. The body is laterally compressed and built on the standard krill plan: a fused head-thorax region (the cephalothorax) protected by a carapace, a segmented abdomen bearing swimming legs, and a paddle-shaped tail fan at the rear.

Adult dimensions:

• Body length: up to 6 cm
• Weight: up to 2 g
• Antennae: approximately half the body length
• Eye diameter: roughly 2-3 mm (unusually large for the body size)

Colour and transparency:

Living krill are largely translucent, with the internal organs visible through the shell. The carapace carries patches of orange-red pigment, concentrated in the thorax and along the edges of the tail fan. In dense swarms the collective pigment is strong enough to stain square kilometres of ocean a visible pink or red. When krill are eaten and passed through the gut of a predator, the pigment carries through -- penguin droppings on Antarctic snow are characteristically pink because of the krill that fed the bird.

The body is divided into three functional regions. The cephalothorax carries the eyes, antennae, mouthparts, and a set of thoracic legs that together form a feeding basket for filtering phytoplankton from the water. The pleon or abdomen carries five pairs of swimming legs called pleopods that drive the animal through the water with a steady rhythmic beat. The telson at the rear, flanked by uropods, makes up the tail fan that the krill flicks for escape responses.

Anatomy and Sensory Systems

Antarctic krill are fully marine and cannot survive in freshwater or out of water. The anatomy reflects a life spent in cold, dark, pelagic ocean water.

Gills and respiration: Unlike many crustaceans whose gills are hidden under the carapace, krill carry their gills externally along the bases of the thoracic legs. This gives them a large gas-exchange surface relative to body mass, useful in the cold, highly oxygenated waters of the Southern Ocean. It also makes them more sensitive to water quality and chemistry than animals with protected gills.

Eyes: The compound eyes are unusually large for the body size -- a pair of black, hemispherical globes that take in light from roughly 180 degrees. This is an adaptation to life in low-light conditions: krill migrate daily between well-lit surface waters and the deep scattering layer several hundred metres down, and during the polar winter they must function in near-total darkness under ice.

Antennae and chemoreception: Two pairs of antennae extend forward from the head. They carry dense fields of chemoreceptor sensilla that detect trace chemicals in the water -- phytoplankton metabolites, predator scents, and conspecific pheromones associated with swarming and spawning.

Thoracic feeding legs: Six pairs of thoracic legs bear fine bristles that together form a basket-like filter. The krill pumps water through this basket, straining out phytoplankton cells, ice algae, and small zooplankton. The filtration apparatus is efficient enough to strip particles down to a few micrometres in diameter -- small enough to capture individual diatom cells.

Photophores: Ten bioluminescent organs distributed along the body emit blue-green light. The structural complexity of these photophores is striking, with lenses, reflectors, and pigment screens that allow directional control of the emitted light.

Bioluminescence

Antarctic krill are one of the most brightly bioluminescent animals in the Southern Ocean. Each adult carries ten photophores arranged in a precise pattern: one pair near each eye, one pair on the thorax, and four pairs along the underside of the abdomen. The light is produced by a chemical reaction between a luciferin substrate and a luciferase enzyme, similar in general principle to the reaction used by fireflies but evolutionarily independent.

The emitted light is blue-green, centred around 480 nanometres, a wavelength that matches the peak transmission of open ocean water. This is the same wavelength used by most deep-sea bioluminescent animals and by many open-ocean fish.

Likely functions of krill bioluminescence:

• Counter-illumination camouflage. A krill swimming in the upper water column is visible in silhouette against downwelling surface light to any predator looking up from below. By emitting light of the right intensity and colour on its underside, the krill matches the background brightness and disappears from view.
• Swarm coordination. Flashing signals may help individuals maintain position, spacing, and orientation within dense swarms.
• Mate signalling. Bioluminescent flashes during spawning season may identify receptive individuals in otherwise dark water.
• Startle response. A sudden bright flash can momentarily dazzle a predator and create a window for escape.

The collective output of a large swarm is substantial. Divers working at night in Antarctic waters have reported that an active krill swarm produces enough light to read instruments by. Satellite sensors have occasionally detected bioluminescent surface patches that may be linked to krill aggregations, although such observations are difficult to separate from other bioluminescent plankton.

Swarms and Swarming Behaviour

The swarm is the defining feature of krill ecology. A typical Antarctic krill swarm contains thousands to millions of individuals packed into a coherent cloud that moves, feeds, and reproduces as a unit. Densities within swarms routinely exceed 10,000 individuals per cubic metre and can reach millions per cubic metre in the densest aggregations. Swarms range from compact patches a few metres across to continuous super-swarms tens of kilometres long.

The 1981 super-swarm: The most famous documented krill swarm was surveyed acoustically in March 1981 in the southwest Atlantic sector. Echo sounders measured a single coherent swarm covering approximately 450 square kilometres of ocean, with an estimated total biomass of 2 million tonnes of krill. That single swarm contained more biomass than the annual global production of all wild-caught fish combined. No swarm of remotely comparable size has been documented since, partly because such events are rare, partly because survey effort has never matched the coverage needed to catch another one, and partly because regional krill populations have declined substantially since the 1980s.

Why swarm?

• Predator satiation. Predators can only eat so many krill in a given encounter; beyond that point additional krill in the swarm are safe.
• Hydrodynamic efficiency. Individuals in the middle of a coordinated swarm expend less energy swimming than solitary animals in open water.
• Reproduction. Dense swarms allow reliable fertilisation because males and females are close together when females release eggs.
• Feeding. Swarms track phytoplankton blooms, and coordinated group movement may improve detection of productive water.

Swarms can be seen from space under the right conditions. Surface swarms colour the ocean red-pink over areas large enough to register on satellite ocean-colour imagery, and subsurface swarms are routinely detected by airborne lidar used in oceanographic research.

Diet and Feeding

Antarctic krill are primarily filter-feeders, straining phytoplankton, ice algae, and small zooplankton from seawater. Their diet varies strongly with season and life stage.

Summer diet (phytoplankton bloom): In the austral summer, when the Southern Ocean experiences an intense phytoplankton bloom driven by melting sea ice and long daylight hours, krill feed almost entirely on diatoms and other single-celled algae. A single krill can filter up to three litres of water per hour during peak feeding.

Winter diet (ice and detritus): During the polar winter, when open-water phytoplankton is scarce, krill shift to a diverse subsistence strategy: grazing algae growing on the underside of pack ice, scraping bacterial films, eating detritus sinking from the surface, and consuming small copepods and other zooplankton. Adult krill can also fast for long periods, reabsorbing tissue and shrinking rather than dying of starvation.

Larval diet: Larvae depend almost entirely on ice algae during their first winter. This dependency is why loss of winter sea ice has such a dramatic effect on population recruitment -- without the ice-algae food source, larval survival drops sharply.

The Vertical Migration

One of the most striking behaviours of Antarctic krill is the diel vertical migration. Every day, massive portions of the krill population move hundreds of metres up and down through the water column in a coordinated rhythm driven by light.

At dusk, krill rise from deep water -- sometimes from 500 metres or more -- toward the surface, where they feed on phytoplankton through the night. At dawn, they sink back into deeper, darker water to shelter from visual predators during daylight. The round trip can cover a kilometre of vertical distance every twenty-four hours.

The energetic cost of this migration is substantial, but the benefits are clear: feed where the food is (the surface), hide where the predators cannot see (the depths). Multiplied across a species of several hundred trillion individuals, the daily krill migration is arguably the largest daily movement of animal biomass on Earth.

Life Cycle and Reproduction

Antarctic krill have a multi-year life history with long development times and a heavy dependence on sea ice during early stages.

Mating: Adults mate during the austral summer. Males transfer a sperm packet to the female's gonopore during a brief encounter.

Spawning: Females release fertilised eggs directly into surface waters, usually in batches of several thousand per spawning event. A single female may spawn multiple times per summer, producing up to 10,000 eggs in total across several episodes.

Sinking eggs: The eggs sink through the water column to depths of 700 to 1,000 metres before hatching. This vertical dispersal keeps early larvae away from surface predators.

Nauplius and metanauplius stages: Hatchlings develop through several larval stages -- nauplius, metanauplius, calyptopis, and furcilia -- over about eleven months. During this time the larvae ascend back toward the surface, feeding on ever-larger food items as they grow.

First winter under ice: Larvae must survive their first Antarctic winter. Most shelter on the underside of pack ice, where they feed on ice algae and hide from open-water predators. This is the bottleneck that ties recruitment success to winter sea-ice extent: no ice, no surviving larvae.

Juvenile and adult stages: Juveniles join adult swarms after their first year. Sexual maturity arrives at roughly two to three years. Adults may then spawn over several successive summers.

Lifespan: Five to six years in the wild, unusually long for the Euphausiacea order where one to two years is typical.

The 500-Million-Tonne Question

How much Antarctic krill actually exists? The current consensus figure is 300 to 500 million tonnes, but arriving at that number involves significant uncertainty and methodology.

Acoustic surveys. Research vessels tow echo sounders tuned to the acoustic signature of krill swarms. By mapping swarm density across survey transects and extrapolating across ocean sectors, researchers estimate regional biomass. The CCAMLR 2000 survey of the southwest Atlantic produced one of the best single-basin estimates: approximately 60 million tonnes in that sector alone.

Net sampling. Fine-mesh nets towed through swarms provide ground-truth samples for species identification, size distribution, and density calibration.

Satellite and remote sensing. Chlorophyll measurements, sea-ice extent, and ocean colour data help constrain where krill are likely to be abundant.

Predator studies. Backward inference from the energetic needs of krill-eating predators -- whales, penguins, seals -- provides independent minimum estimates for how much krill must exist to sustain those populations.

The low end of the estimate (about 300 million tonnes) reflects conservative assumptions; the high end (500 million tonnes or more) reflects optimistic interpretations of acoustic and satellite data. Either way, Antarctic krill are the most abundant wild animal species on Earth by biomass.

Comparative biomass figures:

The Foundation of the Southern Ocean Food Web

Antarctic krill are a keystone species -- the single organism that links primary producers to almost every vertebrate predator in the Southern Ocean. Remove krill and the entire ecosystem collapses.

Principal krill predators:

• Blue whale, fin whale, humpback whale, minke whale, sei whale
• Crabeater seal, leopard seal (partially), Antarctic fur seal
• Adelie penguin, chinstrap penguin, gentoo penguin
• Emperor penguin (indirectly, via fish that eat krill)
• Antarctic silverfish, mackerel icefish, Patagonian toothfish
• Wandering albatross, black-browed albatross, snow petrel, cape petrel, Antarctic petrel
• Colossal squid and various smaller Southern Ocean squid

A single blue whale engulfs an estimated 2,000 kilograms of krill in one feeding lunge and consumes roughly 16 tonnes of krill per day during the summer feeding season. Adelie penguins rely on krill for more than 90 per cent of their diet during breeding. Crabeater seals, despite the name, eat almost exclusively krill, which they strain from seawater using teeth that have evolved into specialised filter combs.

Conservation Status and Threats

Antarctic krill have never been formally assessed by the IUCN Red List. The species' sheer abundance has made conventional threat categorisation awkward, but that does not mean the population is secure.

Regional declines. Long-term survey data show that krill populations in the southwest Atlantic sector -- the species' historic heartland and the region where both commercial fishing and vertebrate predator populations are concentrated -- have declined by approximately 80 per cent since 1976. The population centre of mass has also shifted southward, tracking the retreat of winter sea ice.

Climate change. The Antarctic Peninsula has warmed by nearly three degrees Celsius over the past seventy years, faster than almost anywhere else on Earth. Winter sea ice has retreated and thinned, undercutting the ice-algae habitat that larval krill depend on. Ocean acidification is an additional stressor because krill larvae struggle to develop their exoskeletons in more acidic water. If these trends continue, the foundation species of the Southern Ocean could be pushed into serious decline within decades.

The CCAMLR fishery. The Commission for the Conservation of Antarctic Marine Living Resources, established in 1982 under the Antarctic Treaty, sets binding catch limits for krill. The current fishery takes approximately 400,000 tonnes per year, almost entirely from the southwest Atlantic. CCAMLR limits are precautionary -- a small single-digit percentage of estimated regional biomass -- and the fishery is considered well managed by fisheries standards. The concern is spatial: catches are concentrated in the same small areas where penguins, seals, and whales forage, and the overlap may matter even if total removals look modest at the basin scale.

Industrial demand. Krill oil is marketed as an omega-3 supplement, and krill meal is used as an aquaculture feed ingredient. Demand has grown steadily since the 2000s, and fishing nations have added larger and more efficient vessels to the krill fleet. Without strict quota enforcement and expanded marine protected areas, the fishery could grow beyond sustainable limits.

Antarctic Krill and Humans

Human contact with Antarctic krill has shifted across the twentieth century from scientific curiosity to commercial resource. Early Antarctic expeditions documented krill as whale food; the collapse of commercial whaling in the second half of the twentieth century freed enormous krill biomass that had previously fed the large whale populations. Soviet and Japanese fleets began experimenting with krill harvest in the 1960s and 1970s, and by the 1980s the fishery was regulated under CCAMLR.

Today the main uses of Antarctic krill are:

• Krill oil supplements. Sold at premium prices as a source of omega-3 fatty acids (EPA and DHA) plus astaxanthin, a red carotenoid pigment.
• Aquaculture feed. Krill meal is used in salmon farming, shrimp farming, and other aquaculture operations because it supplies protein and natural colouring.
• Sport fishing bait. A minor market but well established.
• Direct human food. Limited, mostly in Japan and South Korea; krill flesh is highly perishable and hard to process at scale.

The future of the fishery is contested. Conservation groups have pushed for expanded Marine Protected Areas in the southwest Atlantic and around the Antarctic Peninsula, partly to shield krill swarms from overlap with predator foraging and partly as a hedge against climate-driven declines. CCAMLR has adopted some precautionary spatial measures but has faced political resistance to larger protected areas from fishing nations. The balance between extractive use and ecosystem protection will define the next decades of krill management.

Related Reading

• Crustaceans: The Armoured Wonders of the Ocean
• Coconut Crab
• Mantis Shrimp: The Fastest Punch in the Sea
• Pistol Shrimp: The Loudest Animal on Earth

References

Information in this entry draws on peer-reviewed research on Antarctic krill ecology published in journals including Nature, Marine Ecology Progress Series, Deep-Sea Research, and CCAMLR Science. Biomass estimates reflect the CCAMLR 2000 acoustic survey and subsequent updates. Population decline figures in the southwest Atlantic follow Atkinson et al. 2019 in Nature Climate Change. Fishery catch statistics are drawn from the CCAMLR Statistical Bulletin. Bioluminescence descriptions follow work on euphausiid photophores in Journal of Experimental Biology and related sources. Life history data reflect long-term field and laboratory studies at British Antarctic Survey, Alfred Wegener Institute, and the Australian Antarctic Division.