Krill are small crustaceans that inhabit all the world’s oceans, with some of the most concentrated populations found in the cold, nutrient-rich waters of the Southern Ocean. These tiny organisms form immense swarms, sometimes stretching for miles, making them one of the most abundant animal groups on Earth. Understanding their diet is fundamental to comprehending their immense ecological importance and their role as a central component of marine food webs.
The Primary Diet of Krill
Krill are primarily filter feeders, equipped with specialized feeding baskets formed by their thoracic legs. These structures efficiently strain microscopic organisms from the water. Their main food source is phytoplankton, particularly diatoms, single-celled algae thriving in sunlit surface waters.
Diatoms are abundant in polar regions, forming dense blooms that provide a rich food supply. Krill also scrape algae from the underside of sea ice, an important food source during winter when open-water phytoplankton is scarce. This adaptability allows krill to sustain their populations, converting solar energy from phytoplankton into biomass.
Some krill species may also consume zooplankton, such as copepods, or even smaller krill, especially when phytoplankton is less available. Their feeding habits are flexible, allowing them to adapt to varying food conditions. However, their diet primarily consists of microscopic plant-like organisms.
Krill and Seaweed Consumption
While krill are herbivores, macroscopic seaweed is not a significant part of their diet. Their filter-feeding apparatus is adapted to capture tiny, suspended particles, typically ranging from a few micrometers to a millimeter in size. Large pieces of seaweed are too big for krill to ingest.
Krill typically inhabit the open ocean, including pelagic zones and areas beneath sea ice. These environments are generally far from coastal regions where large seaweeds, like kelp, grow attached to the seafloor. Their natural habitat does not frequently expose them to abundant sources of macroscopic seaweed.
Though not a primary food source, krill might incidentally consume very small fragments from decaying seaweed. However, such instances are incidental and contribute negligibly to their nutritional intake. Their specialized feeding mechanisms and typical habitats confirm their diet is overwhelmingly composed of microscopic organisms.
Why Krill’s Diet Matters
Krill’s diet plays an important role in the global marine ecosystem, acting as a key link in the ocean’s food web. By consuming phytoplankton, krill transfer energy from these primary producers to a wide array of larger marine animals. They are a major food source for numerous species, including baleen whales, seals, penguins, and various fish.
Their consumption of phytoplankton also contributes to the ocean’s carbon cycle. When krill feed on carbon-rich phytoplankton, they incorporate this carbon into their bodies. As they move through the water or are consumed by predators, they help transport carbon from surface waters to deeper ocean layers, a process known as the biological pump. This process helps regulate atmospheric carbon dioxide levels.