Euphausiaceans are small, shrimp-like crustaceans found in every ocean on Earth, better known by their common name: krill. Despite rarely growing longer than a finger, they rank among the most abundant and ecologically consequential animals alive. Antarctic krill alone have an estimated post-larval biomass of roughly 379 million tonnes spread across a summer range of about 19 million square kilometers, making them one of the largest aggregations of any single animal species on the planet.1Deep Sea Research Part I: Oceanographic Research Papers. A re-appraisal of the total biomass and annual production of Antarctic krill Their role in marine food webs, carbon cycling, and even human nutrition gives these tiny animals an outsized influence on ocean systems.
What Makes a Euphausiacean a Euphausiacean
The order Euphausiacea contains about 86 known species across two families, found from the tropics to polar seas and from surface waters to depths beyond a thousand meters. They look superficially like small shrimp, and for a long time scientists assumed they were most closely related to decapods, the order that includes shrimp, crabs, and lobsters. Molecular work using ribosomal DNA sequences overturned that assumption, finding instead that euphausiaceans are more closely related to the Mysida, sometimes called opossum shrimp.2PubMed. 28S rDNA evolution in the Eumalacostraca and the phylogenetic position of krill The resemblance to true shrimp is a case of convergent body plans rather than shared ancestry.
A few features set euphausiaceans apart from lookalikes. Their gills are exposed and visible along the sides of the body, unlike in true shrimp where a shell flap covers them. Many species have light-producing organs called photophores on the underside of the body and near the eyes. And all euphausiaceans are strictly marine; you will never find one in fresh water.
Specialized Eye Anatomy and Light Organs
The photophores of krill have fascinated biologists since the nineteenth century, but recent work on the Northern krill, Meganyctiphanes norvegica, has revealed something unexpected about the eyestalk photophore in particular. Rather than being a standalone light organ, it is innervated by a curved row of about 32 highly modified ommatidia, the individual optical units of the compound eye, that have been repurposed from vision to light emission.3Arthropod Structure & Development. The eyestalk photophore of Northern krill Meganyctiphanes norvegica (M. Sars) (Euphausiacea) re-investigated: Innervation by specialized ommatidia of the compound eye These photophore-associated ommatidia lack the corneal lenses that regular ommatidia use to focus light. Their crystalline cones are nearly spherical rather than bullet-shaped, and their light-sensitive rhabdoms are less than half the length of those in neighboring visual ommatidia. A dense pigment shield separates them from the rest of the compound eye.
The practical result is that the krill eye does double duty: regular ommatidia gather light for vision while a small subset channels and controls bioluminescence. Scientists believe the blue-green glow may help krill match the faint downwelling light from the surface, making them harder for predators below to spot against the sky. This strategy, called counter-illumination, is common in deep-sea animals but unusual in organisms that also spend time near the surface.
The Escape Flip
When a krill detects a threat, it executes what researchers call the caridoid escape response, a maneuver shared broadly among crustaceans but performed with particular speed in euphausiaceans. The animal rapidly flexes its abdomen and flips its tail fan, launching itself backward. In Antarctic krill, a single flexion-and-flip cycle takes about 42 milliseconds and propels the animal to a peak speed of roughly 57 centimeters per second, which works out to more than 17 body lengths per second.4PubMed Central. Hydrodynamics of the fast-start caridoid escape response in Antarctic krill, Euphausia superba The tail flip during the abdominal closure is a major contributor to thrust, and the animal sheds a complex chain of vortex rings in its wake as it accelerates.
That speed sounds modest in absolute terms, but scale matters. At 17 body lengths per second, krill are proportionally faster in their escape burst than many fish. The maneuver is also energetically cheap compared to sustained swimming, since it relies on a single explosive muscle contraction rather than continuous effort.
Swarming Behavior
Krill are famous for forming dense aggregations, sometimes called swarms or schools, that can stretch for kilometers and contain millions of individuals. These are not random clumps. Research on Antarctic krill has shown that individuals actively align with their nearest neighbors and adjust both their speed and direction relative to the positions of the animals around them, indicating that social coordination, not just passive drifting, drives swarm structure.5PubMed Central. Self-organization and information transfer in Antarctic krill swarms What makes krill swarms distinctive among collectively moving animals is that the strongest information flow and movement adjustments happen in the vertical dimension rather than horizontally, a pattern not seen in fish schools or bird flocks.
The shape of a krill swarm is not random either. Analysis of both krill swarms and fish schools has found a common emergent geometry that balances two competing pressures: staying packed tightly enough to reduce each individual’s exposure to predators while remaining loose enough for everyone to get enough oxygen from the surrounding water.6Current Biology. Shapes of Krill Swarms and Fish Schools Emerge as Aggregation Members Avoid Predators and Access Oxygen Despite these regularities, krill distribution across the ocean is highly patchy. Acoustic surveys in the western Indian sector of the Southern Ocean found that over 85 percent of the surveyed distance had krill densities below one gram per square meter, with most of the biomass concentrated in a small fraction of the area.7ICES Journal of Marine Science. Biomass distribution and swarm characteristics of Antarctic krill (Euphausia superba) in the Western Indian sector of the Southern Ocean
Diet and the Food Web
Most euphausiaceans are omnivorous filter-feeders. They use their thoracic limbs to create a feeding basket that strains phytoplankton, small zooplankton, and detritus from the water. Antarctic krill are often described as herbivores because their diet in summer is dominated by diatoms and other single-celled algae, but winter paints a different picture. Studies of larvae off the western Antarctic Peninsula found that krill in winter had very little plant material in their guts and were likely opportunistic scavengers, eating whatever they could find: microzooplankton, larval invertebrates, detritus, and whatever scarce phytoplankton remained.8Deep Sea Research Part II: Topical Studies in Oceanography. Overwintering Growth and Development of Larval Euphausia superba: An Interannual Comparison under Varying Environmental Conditions West of the Antarctic Peninsula
Ice algae, the microalgae that grow on the underside of sea ice, become important as light levels increase in spring. Even in years with relatively low sea-ice coverage dominated by newly formed pancake ice, juvenile krill can meet and exceed their minimum energy needs by grazing on ice algae. The excess energy goes directly into growth and reproductive development.9ICES Journal of Marine Science. The contribution of ice algae to the winter energy budget of juvenile Antarctic krill in years with contrasting sea ice conditions In the broader Scotia Sea ecosystem, the phytoplankton production that krill consume makes them a crucial link between primary producers and the large predators, the penguins, seals, whales, and seabirds, that depend on krill as a primary food source.10PubMed Central. Spatial and temporal operation of the Scotia Sea ecosystem: a review of large-scale links in a krill centred food web
Shrinking to Survive
One of the more remarkable adaptations of Antarctic krill is their ability to literally shrink when food is scarce. In laboratory experiments, krill kept without food for 211 days lost between about 32 and 56 percent of their initial wet weight while remaining alive, and they did so by reducing their actual body size rather than simply losing stored fat.11Journal of Experimental Marine Biology and Ecology. Body shrinkage as a possible over-wintering mechanism of the Antarctic krill, Euphausia superba Dana Body shrinkage is thought to function as an energy-conservation strategy during the long, dark Antarctic winter when food can be nearly nonexistent. Most animals that endure starvation draw down fat reserves and muscle mass while their skeletal structures remain the same size. Krill apparently go further, reabsorbing structural tissue during each molt cycle and emerging from the molt physically smaller. It is an unusual survival trick in the animal kingdom.
Diel Vertical Migration and the Carbon Pump
Every day, euphausiaceans carry out one of the largest coordinated animal movements on the planet. At dusk, they swim upward toward surface waters to feed under the relative safety of darkness. At dawn, they descend again, sometimes hundreds of meters, to avoid visual predators. This behavior, called diel vertical migration, has been studied in detail in Northern krill. The classic pattern involves ascent around dusk, nighttime feeding near the surface, and descent at dawn, driven primarily by light cues. Daytime depth is shallower on cloudy days and deeper in clear water, and krill swim deeper on moonlit nights.12Advances in Marine Biology. Diel Vertical Migration Behaviour of the Northern Krill (Meganyctiphanes norvegica Sars)
Under ice cover, the migration still continues, but ambient light changes can shift the depth bands considerably. In a Norwegian fjord, Northern krill maintained their daily migration cycle throughout winter, with the highest daytime concentrations around 80 to 100 meters depth. When snowfall accumulated on the ice and dimmed the light further, the upper daytime distribution rose about 20 meters before sinking again as conditions changed.13PubMed Central. Vertical distribution and diel vertical migration of krill beneath snow-covered ice and in ice-free waters
This daily commute has big consequences for global carbon cycling. Krill feed on carbon-rich phytoplankton at the surface and then carry that carbon to depth in their bodies, their fecal pellets, and their shed exoskeletons. In the north Scotia Sea, fecal pellets accounted for about half of the annual particulate organic carbon sinking from the surface, and shed exoskeletons, known as exuviae, contributed close to 38 percent. Together these krill-derived materials dominated the biological carbon pump in the region.14Nature Communications. Continuous moulting by Antarctic krill drives major pulses of carbon export in the north Scotia Sea, Southern Ocean Northern krill play a similar role in the North Atlantic, where they occupy shelf and slope waters from subtropical boundaries northward and contribute to carbon transport through the sinking of fecal material into deeper layers.
A Genome Built for Extremes
The Antarctic krill genome is astonishing for its sheer size. At about 48 billion base pairs, it is roughly 16 times larger than the human genome, making it one of the largest animal genomes ever sequenced.15PubMed. The enormous repetitive Antarctic krill genome reveals environmental adaptations and population insights Most of that bulk comes not from extra genes but from the expansion of transposable elements, segments of DNA that copy and paste themselves throughout the genome over evolutionary time. These “jumping genes” have proliferated in the intergenic regions between protein-coding sequences, inflating the genome without proportionally increasing its functional gene count. Researchers believe the expanded genome may play a role in the species’ ability to regulate its biology across extreme seasonal shifts in light, temperature, and food availability, though the exact mechanisms are still being worked out.
Parasites and Hidden Costs
Krill are not free from parasites, and the consequences for their populations are larger than most people would guess. A recent study of Antarctic krill found that both epibiotic ciliates, single-celled organisms that attach to the outside of the animal, and Apicomplexa parasites, which infect internal tissues, were associated with slower growth rates. Krill carrying heavier Apicomplexa loads showed a step-change drop in growth rate once infection exceeded about 38 percent of detectable reads on their shed molts. Scaling these effects to the whole Southern Ocean population, researchers estimated that ciliates might reduce krill biomass production by roughly 8 percent, while Apicomplexa could be responsible for a reduction of about 15 percent, amounting to roughly 120 million tonnes of biomass effectively lost to parasites each year.16PubMed Central. Antarctic Krill with parasites grow slower than uninfected peers
Those are striking numbers for organisms that receive almost no attention in popular accounts of krill ecology. Parasites do not kill their hosts outright in most cases, but by reducing growth rates they indirectly reduce the amount of krill biomass available to the predators that depend on it.
Microplastic Contamination
Even in the remote Southern Ocean, krill are ingesting microplastics. A study analyzing krill and salps found synthetic polymer particles in more than 77 percent of the specimens examined, with the most common type being polyamide fibers. Juvenile krill showed the highest contamination rate at over 94 percent of individuals sampled.17PubMed Central. Organic or junk food? Microplastic contamination in Antarctic krill and salps Separate research has shown that krill can rapidly fragment microplastic beads through their digestive system, releasing nanoplastics whose toxicological effects remain poorly understood.18PubMed. Nanoplastics affect moulting and faecal pellet sinking in Antarctic krill (Euphausia superba) juveniles Because krill sit near the base of the food web, any contaminants they accumulate can be transferred upward through the chain of animals that eat them.
Climate Projections
Rising ocean temperatures and changing sea-ice patterns pose a direct threat to euphausiacean populations, especially in the Southern Ocean. Modeling work using temperature-dependent krill growth simulations forced by climate projections suggests that by the year 2100, the regions capable of supporting successful krill spawning will shrink considerably. Localized areas along the western Weddell Sea, parts of the Indian Antarctic sector, and the Amundsen and Bellingshausen Seas may remain viable spawning habitats, but the failure of spawning elsewhere is expected to compound the decline of adult populations with cascading consequences for the Antarctic food web.19Geophysical Research Letters. Projected changes of Antarctic krill habitat by the end of the 21st century
The connection between sea ice and krill recruitment makes this especially worrying. Sea ice provides the physical substrate for ice algae that young krill depend on in their first winter, and it shelters larvae from predators. As ice forms later and retreats earlier, those advantages shrink. A warming ocean also speeds up metabolism, meaning krill burn through their energy reserves faster during winter, precisely when food is hardest to find.
The Krill Fishery and Its Governance Problems
Antarctic krill support the largest fishery by weight in the Southern Ocean, managed by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR). The fishery primarily supplies the aquaculture feed and omega-3 supplement industries. Krill oil is marketed as a source of EPA and DHA omega-3 fatty acids bound to phospholipids, and supplementation trials have shown that high-phospholipid krill oil can raise the combined levels of these fatty acids in red blood cells compared to control groups.20PubMed Central. Supplementation of krill oil with high phospholipid content increases sum of EPA and DHA in erythrocytes compared with low phospholipid krill oil Growing demand from these industries has pushed catches steadily upward after a dip following the collapse of the Soviet Union, which had been a major krill-fishing nation.
The management situation is precarious. CCAMLR had been developing a revised framework to distribute krill catches across time and space, reducing the risk that too much fishing pressure would fall on areas critical to krill predators during sensitive periods. But at its 2024 meeting, the commission failed to renew its spatial catch limits.21Marine Policy. The fishery for Antarctic krill – Conflicts between industrial production, protection of biodiversity, and legal governance The practical consequence is that the interim catch limit of 620,000 tonnes can now be taken from anywhere in the southwest Atlantic at any time, including in areas and seasons critical to penguins, seals, and whales. Voluntary industry measures help spread out catches somewhat, but they are no substitute for binding rules. Researchers have argued that the current management does not account for climate variability or population dynamics, and that a new framework integrating variability in krill recruitment and the pathways between spawning and nursery areas is urgently needed.22PubMed Central. Adjusting the management of the Antarctic krill fishery to meet the challenges of the 21st century
Species Beyond Antarctic Krill
Antarctic krill get the most attention, but they are only one of roughly 86 euphausiacean species. Northern krill, Meganyctiphanes norvegica, is the dominant species across the North Atlantic shelf and slope, from subtropical waters up to the boundary with Arctic water masses, and extends into parts of the Mediterranean. Northern krill serve the same dual ecological role in their range that Antarctic krill serve in theirs: a key prey item for groundfish, squid, baleen whales, and seabirds, and a significant contributor to the biological carbon pump through the sinking of fecal material. Other notable species include Euphausia pacifica in the North Pacific, which is commercially harvested off Japan and British Columbia, and Thysanoessa species found in both temperate and polar seas. The various species range in adult size from under a centimeter to about six centimeters, with Antarctic krill among the largest.
Despite their ecological and commercial importance, most euphausiacean species have received far less scientific attention than Euphausia superba. Basic population estimates, life-history data, and genomic information are sparse or nonexistent for the majority of species, leaving significant gaps in our understanding of how the order as a whole responds to changing ocean conditions.