Some large fish species do consume krill, highlighting the diverse feeding strategies found throughout marine environments. While many large fish are predatory, a distinct group has evolved to feed on the ocean’s smallest organisms. This adaptation allows them to thrive by efficiently harvesting abundant, nutrient-rich prey. The methods these fish employ to capture such tiny food items are highly specialized, reflecting unique evolutionary paths.
Large Fish That Consume Krill
Several species of sharks and rays consume krill. The whale shark, the largest known fish, feeds on plankton, including krill. The basking shark, the second-largest fish, also relies on krill and other zooplankton as a primary food source. The megamouth shark predominantly consumes zooplankton such as krill, jellyfish, and shrimp larvae. Manta rays are also filter feeders that include krill in their diet.
Feeding Mechanisms for Krill Consumption
These large fish employ a specialized feeding mechanism known as filter feeding to capture krill. Whale sharks are active feeders that ram filter feed by swimming forward with their mouths open, straining prey from the water. They also gulp water, expelling it through their gills. During this process, water passes over modified gill rakers, which are projections that trap food particles while allowing water to exit. Basking sharks, in contrast, are passive filter feeders, relying on their forward motion to force water through their mouths and over their gill rakers.
The gill rakers in these fish are long, closely spaced, and bristle-like, forming an efficient mesh to capture small prey. These structures prevent food particles from escaping through the gill slits, directing them towards the esophagus. Manta rays similarly open their large, forward-facing mouths, sometimes using cephalic fins to funnel water and plankton inward. The captured organisms are then filtered by specialized gill plates.
Why Krill are a Key Food Source
Krill are a significant food source for these large fish primarily due to their immense abundance and high nutritional value. Antarctic krill, for example, are one of the most abundant animal species on the planet, with an estimated biomass of over 400 million tonnes in the Southern Ocean. They form dense swarms, sometimes reaching densities of 30,000 individuals per cubic meter, making them an easily accessible and concentrated food source. The sheer quantity allows filter feeders to consume vast amounts, meeting their substantial energy requirements.
Krill are also highly nutritious, offering a rich source of protein and omega-3 fatty acids. Their protein content can be as high as 60% to 65% dry weight, providing all nine indispensable amino acids. Additionally, krill contain beneficial compounds like astaxanthin, a powerful antioxidant, and various vitamins and minerals such as vitamins A, E, D, zinc, selenium, and copper. This combination of high caloric density and comprehensive nutritional content makes krill an efficient and vital component of the diet for the large fish that consume them.