Krill are small crustaceans found in oceans worldwide, often congregating in vast swarms. These tiny, shrimp-like invertebrates play a significant role in marine ecosystems, connecting microscopic life to larger marine animals. Their feeding habits and ecological contributions make them a subject of considerable interest in understanding ocean food webs. This article will explore the specifics of krill, their dietary preferences, and their broader influence on marine environments.
What Exactly Are Krill?
Krill are marine crustaceans belonging to the order Euphausiacea, encompassing 86 known species. Most adult krill measure 1 to 2 centimeters, though some species can reach 6 or even 15 centimeters. They inhabit all of the world’s oceans, from tropical to polar regions, with Antarctic krill (Euphausia superba) being particularly abundant in the Southern Ocean. These creatures form immense swarms, reaching densities of 10,000 to 30,000 individuals per cubic meter, and can stretch for miles.
The Primary Diet of Krill
Most krill species primarily eat phytoplankton, microscopic marine plants in the ocean’s sunlit upper layers. Phytoplankton form the foundation of the marine food web, performing photosynthesis to convert sunlight into energy. Krill are filter feeders, using specialized thoracic appendages to strain these organisms from the water. These appendages are covered in fine, hair-like structures called setae, which create a mesh-like net to capture food particles. Diatoms, a common single-celled algae, are a favored food source in nutrient-rich waters.
During winter, when open-water phytoplankton is scarce, krill also feed on algae growing on the underside of sea ice. This adaptation allows them to sustain themselves when their primary food source is less available. A single krill can consume up to 10% of its body weight in food each day, highlighting their grazing impact on phytoplankton populations. This continuous consumption links the energy from primary producers directly into the marine food chain.
Are They Strictly Herbivores?
While krill are recognized for their herbivorous diet of phytoplankton, their feeding habits are not exclusively limited to plant matter. They are opportunistic feeders and can consume other organisms, making their diet more omnivorous than strictly herbivorous. Krill consume zooplankton, tiny animals drifting in the water column. This includes smaller crustaceans like copepods, larval stages of other crustaceans, and small fish larvae.
Consuming zooplankton provides krill with protein and other nutrients lacking in phytoplankton alone. This dietary flexibility is evident when phytoplankton is scarce, allowing krill to adapt to varying food availability. Krill also consume detritus, decaying organic matter found in the water column or on the seafloor. Under certain circumstances, particularly when food is scarce or populations are dense, some krill species may resort to cannibalism. This adaptability underscores their capacity to survive in diverse and challenging oceanic conditions.
Krill’s Pivotal Role in Ocean Ecosystems
Krill occupy a position near the bottom of the ocean’s food web, converting primary producers into a food source for a vast array of marine animals. They are a keystone species, meaning their presence and abundance disproportionately affect the entire ecosystem. Large baleen whales, including blue and humpback whales, rely almost exclusively on krill, consuming several tons daily. Other predators that depend on krill include seals, penguins, various seabirds, and numerous fish species.
Their large biomass sustains vast ecosystems, particularly in the Southern Ocean, where Antarctic krill form dense aggregations. Beyond their role as a food source, krill also contribute to the global carbon cycle. By eating phytoplankton, which absorb carbon dioxide, and then excreting carbon and nutrient-rich fecal pellets, krill help sequester carbon to the deep ocean. These fecal pellets constitute a substantial portion of sinking carbon particles, transferring carbon from surface waters to the seafloor and influencing atmospheric carbon levels.