Krill, the small, shrimp-like marine crustaceans, form one of the largest global animal biomasses, with the Antarctic krill species alone estimated at hundreds of millions of tons. These organisms are a fundamental link in ocean ecosystems worldwide, yet their diet is often oversimplified. While krill rely overwhelmingly on plant matter for energy, their feeding habits are not strictly herbivorous. Krill demonstrate a flexible dietary strategy that shifts based on resource availability throughout the year.
Defining Krill Dietary Habits
Krill are scientifically classified as facultative omnivores, which accurately describes their adaptable feeding behavior. A strict herbivore consumes only plant matter, but krill have the physiological ability to digest both plants and animals. This designation means they will consume animal matter when their primary plant-based food source is scarce.
Their diet is predominantly herbivorous, especially during bloom seasons, but their survival depends on the ability to switch when necessary. This flexibility allows them to navigate the dramatic seasonal shifts in food availability that occur in their polar and oceanic habitats.
The Primary Phytoplankton Diet
The vast majority of krill’s energy comes from primary producers, establishing the basis for their common classification as herbivores. Their main food source is phytoplankton, which are microscopic, single-celled algae that float near the ocean’s surface. Diatoms, a type of phytoplankton, are a particularly favored and nutrient-rich component of the krill diet.
Krill are highly efficient filter feeders, possessing specialized appendages called thoracopods. These six pairs of legs form an intricate, fine-meshed feeding basket used to strain the tiny phytoplankton from the water. This filtering apparatus allows them to harvest minute food particles, sometimes as small as 1 micrometer, from the water column.
Opportunistic and Winter Feeding
The omnivorous side of the krill diet emerges when the seasonal phytoplankton blooms disappear, particularly during the long, dark polar winter. Krill must then switch to opportunistic feeding strategies to sustain themselves. One significant alternative food source is the algae that grow on the underside of sea ice, known as ice algae. Krill actively scrape and feed on this concentrated source of plant matter when the water column above is depleted.
When plant material is scarce, krill turn to zooplankton, such as small copepods, which provide a necessary protein boost. Copepods are a consistent part of the krill diet year-round, with consumption increasing as the seasons progress into winter. They also consume non-living organic materials, including detritus, fecal pellets from other organisms, and bacteria, utilizing any available energy source in a food-poor environment.
Krill’s Role in the Marine Food Web
Krill’s unique dietary position makes them an energy transfer point in the marine food web. By consuming vast quantities of microscopic phytoplankton, they convert this primary production into a concentrated source of fat and protein. This energy is then made available to larger predators that cannot feed directly on the minute algae.
Krill are considered a keystone species, meaning their presence is fundamental to the structure and health of the ecosystem. Nearly all large marine animals in the Southern Ocean, including baleen whales, seals, penguins, and seabirds, rely on krill as a major food source. Without krill, the entire food web would face collapse.