Krill are small, shrimp-like crustaceans found in oceans worldwide. They typically grow to about 6 centimeters long and are a key component of marine ecosystems. The term “krill” originates from the Norwegian word “kril,” meaning “small fry of fish,” often attributed to these small creatures due to their schooling behavior and role as food for larger animals. While there are over 85 species of krill, Antarctic krill (Euphausia superba) is particularly notable for its immense abundance in the Southern Ocean.
The Foundation of the Antarctic Food Web
They primarily feed on microscopic ocean plants known as phytoplankton, which are suspended in the upper water column where sunlight is available for growth. Krill act as a critical link, transferring energy from these primary producers to higher trophic levels. In winter, when phytoplankton are scarce, krill can also consume algae growing on the underside of sea ice, detritus on the seafloor, or other small animals. Their ability to form vast swarms, sometimes so dense they are visible from space, further highlights their ecological significance.
Mammalian Krill Eaters
Many marine mammals rely on krill as a primary food source. Baleen whales, such as blue whales, fin whales, and humpback whales, are prominent examples. These whales are filter feeders, engulfing large volumes of seawater containing dense swarms of krill and then expelling the water through specialized baleen plates in their mouths, trapping the krill inside. A single blue whale, the largest animal on Earth, can consume an estimated 4 tons of krill per day.
Various seal species also depend on krill. The crabeater seal (Lobodon carcinophaga), despite its name, primarily feeds on Antarctic krill, which can constitute over 90% of its diet. This seal has uniquely adapted, multi-lobed teeth that interlock to form a sieve, allowing it to filter krill from the water. Other seals, including leopard seals and Weddell seals, also consume krill, though it may not be their sole food source. Crabeater seals can consume up to 20 kilograms (44 pounds) of krill per day.
Avian and Piscine Krill Eaters
Birds and fish are also significant predators of krill in the Antarctic. Several penguin species, such as Adélie, Chinstrap, and Macaroni penguins, frequently feed on krill. Their diets often lead to a pinkish hue in their droppings, indicating high krill consumption. Other seabirds, including petrels and albatrosses, also include krill in their diet, diving to catch them from the water. For example, snow petrels are shallow divers that feed on krill, fish, and squid.
Fish species, notably the Antarctic silverfish, are among the piscine predators of krill. Krill form a substantial part of the diet for various commercially valuable fish, including salmon, rockfish, and sardines. The abundance of krill allows these fish populations to thrive.
Other Marine Life Relying on Krill
Beyond mammals, birds, and fish, diverse marine life depends on krill. Various species of squid, including the colossal squid (Mesonychoteuthis hamiltoni), are known to consume krill. While colossal squid primarily prey on fish and other squid, they are part of the broader group of cephalopods that feed on krill. Smaller invertebrates also consume krill as a substantial part of their diet.
The Interconnectedness of Krill and Their Predators
The health and abundance of krill populations directly influence the survival and well-being of these animals. Fluctuations in krill numbers can have far-reaching effects throughout the ecosystem, impacting the reproductive success and population dynamics of their predators. Krill’s role extends to nutrient cycling, as they consume phytoplankton and excrete carbon-rich waste that sinks to the seafloor, contributing to carbon storage in the deep ocean.