The Southern Ocean, a vast and remote expanse encircling Antarctica, is home to the Antarctic krill. These small, shrimp-like crustaceans, known scientifically as Euphausia superba, exist in vast numbers, forming massive swarms that can stretch for kilometers and even be seen from space. Despite their diminutive size, individual krill collectively represent an immense biomass, estimated to be several hundred million metric tons, making them one of the most abundant animal species on the planet. Their presence in this extreme polar environment highlights unique adaptations for life in challenging conditions.
Understanding Antarctic Krill
Antarctic krill are small, reaching about 6 centimeters in length and weighing up to 2 grams. They are transparent, though their shells display a reddish tint from pigment spots, and their digestive systems can appear vivid green due to their diet. These crustaceans possess large black eyes and a hard exoskeleton, which they must shed periodically to grow. Krill can live for an estimated five to ten years in the open ocean.
Krill inhabit the circumpolar waters of the Southern Ocean, extending as far north as the Antarctic Convergence. They exhibit schooling behavior, forming dense aggregations or swarms where densities can reach 10,000 to 30,000 individuals per cubic meter. Their primary diet consists of minute phytoplankton, microscopic ocean plants that use sunlight for photosynthesis. In winter, when phytoplankton are scarce, krill can also feed on algae found on the underside of sea ice, or even detritus on the seafloor.
Krill exhibit bioluminescence, emitting light from specialized organs called photophores. This yellow-green light, emitted periodically, is thought to assist in predator evasion by disrupting their silhouette and aid in schooling cohesion.
The Krill’s Role in the Ecosystem
Antarctic krill are considered a keystone species in the Southern Ocean, meaning their presence has a disproportionately large effect on their environment. They occupy a central position in the Antarctic food web, serving as the primary food source for many marine animals. Whales, seals, penguins, squid, and numerous seabirds depend on krill for food. Krill can account for up to 96% of the calories consumed by certain seabirds and mammals in the region.
Beyond their role as a food source, krill contribute to nutrient cycling and carbon sequestration, a process known as the “krill pump.” As krill consume phytoplankton, they ingest carbon absorbed by phytoplankton through photosynthesis. Krill then excrete dense, carbon-rich fecal pellets that sink rapidly to the ocean floor, transporting carbon from surface waters to the deep ocean. This process helps to remove atmospheric carbon dioxide and store it in the deep ocean, helping regulate global climate. Krill also distribute other nutrients, such as iron, through their fecal pellets and when they shed their exoskeletons.
Challenges Facing Krill Populations
Antarctic krill populations face environmental stressors that challenge their survival. Climate change is a major concern, primarily through its impact on sea ice and ocean warming. Krill rely on sea ice for shelter and as a feeding ground, particularly for their larvae, which feed on algae growing on the underside of the ice. A reduction in sea ice can directly affect krill numbers and distribution. The Antarctic Peninsula, a region with a high concentration of krill, is experiencing warming at a faster rate than most other places on Earth, causing krill populations in these areas to shrink and shift southward.
Ocean acidification, a consequence of increased atmospheric carbon dioxide dissolving into the ocean, also negatively affects krill. Research suggests it can reduce krill development and hatching rates of their eggs, potentially leading to population decline. Commercial fishing also adds pressure as an additional stressor on krill populations.
Human Interaction and Krill Harvesting
Commercial krill harvesting targets Antarctic krill for various purposes. A portion of harvested krill is processed into krill oil, rich in omega-3 fatty acids and used in nutritional supplements. Krill meal and oil are also used as feed additives in aquaculture, livestock, and pet food. The scale of the krill harvesting industry has been increasing due to growing demand.
The Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR), an international body comprising 26 nations, is responsible for managing the krill fishery. CCAMLR implements an ecosystem-based management approach, prioritizing conservation and setting catch limits to ensure the sustainability of krill stocks and their predators. For instance, in Area 48, which includes the Antarctic Peninsula, the total allowable catch is restricted to approximately 1% of the krill biomass, with actual harvests averaging around 0.8%. This represents one of the lowest removal fractions of any major global fishery. Krill fishing companies have also voluntarily established exclusion zones around penguin breeding colonies to protect foraging areas during breeding seasons.