Krill are small, shrimp-like crustaceans found in all of the world’s oceans. The largest populations, such as the Antarctic krill (Euphausia superba), inhabit the Southern Ocean. Despite typically measuring about two inches in length, they possess a massive collective biomass, estimated in the hundreds of millions of tons. These swarming organisms are foundational to marine ecosystems, acting as a direct link between the ocean’s primary producers and its largest predators. Understanding their diet is central to understanding ocean health.
The Primary Diet of Krill
The primary and most consistent food source for krill is phytoplankton, which are microscopic, single-celled marine plants that drift in the water. Krill are considered herbivores for a significant portion of their lives because they graze heavily on these tiny organisms. This dietary preference links krill directly to the primary productivity of the oceans, effectively converting microscopic plant matter into a form digestible by larger animals.
A major component of this phytoplankton diet, especially for Antarctic krill, consists of diatoms. Diatoms are a type of algae encased in a silica shell, and they are a preferred food source due to their size and nutritional value. Krill actively select these larger cells over smaller phytoplankton species, which suggests a degree of feeding selectivity. In polar regions, krill also feed on ice algae that grow on the underside of sea ice, providing a crucial food source during the dark winter months when water column phytoplankton is scarce.
Filter Feeding and Opportunistic Consumption
Krill primarily feed through a mechanism known as filter feeding, using specialized appendages called thoracopods. These six pairs of legs are finely bristled and interlock to form an incredibly fine, basket-like net. As the krill swims, this feeding basket filters microscopic particles from the water, which are then scraped and directed into the mouth. This efficient system allows krill to continuously harvest the minute phytoplankton cells from the open water.
Krill are not strictly herbivores, however, and are best described as opportunistic omnivores. They do consume zooplankton, which are tiny marine animals, including organisms like copepods and fish larvae. The consumption of zooplankton provides a rich source of protein and becomes more frequent when the primary food source, phytoplankton, is less abundant. Some krill species, such as the Northern krill, are known to actively hunt larger zooplankton.
This ability to switch their diet is a key adaptation that allows krill to survive periods of low primary productivity, such as during the winter or when migrating to deeper waters. They also consume detritus, which is dead organic matter, further demonstrating their flexibility in scavenging for nutrients. Specific studies confirm that animals like copepods and polychaetes are a deliberate part of their varied diet.
Krill’s Central Place in the Ocean Food Web
The massive biomass of krill and their omnivorous diet position them as a central, low-level link in the marine food web. They transfer the energy from primary producers, like phytoplankton, to a huge range of higher-level consumers. This transfer is so significant that krill are a near-exclusive food source for some of the largest animals on the planet.
A great number of marine animals depend on krill for survival, including:
- Baleen whales
- Seals
- Penguins
- Squid
- Many species of fish
A single blue whale can consume up to four tons of krill in a single day, highlighting the massive quantities involved in this energy transfer. When krill populations decline, entire ecosystems can suffer, with predators experiencing breeding failures or shifting migration patterns. Krill also play a role in the ocean’s carbon cycle by consuming carbon-rich phytoplankton and transferring that carbon to the deep sea when they excrete waste or sink after death.