Krill, a crustacean resembling a small shrimp, favorite food of whales

Despite their shape that evokes shrimp, the small crustaceans of the order of Euphausiacea so-called “krills” are not. Krill is an animal that fascinates for many reasons, the main ones of which we discuss here.

What exactly is “krill”?

“Krill” is a term of Norwegian origin which means “small fry”, synonymous with “small fish”. There is not a single species but 85 species of crustaceans. However, this animal remains largely unknown. Meganyctiphanes norvegica present in the Arctic, andEuphausia superba, present in Antarctica are the two best known species.

Krill is one of the greatest sources of protein of the planet. Depending on the species, it measures between 6 and 10 cm long, and lives between 5 and 7 years. Its translucent exoskeleton reveals the internal organs. Even though they look like shrimp, the legs of krill are devoid of claws.

Krill are omnivorous, adapting their diet to what they find in their environment, which varies depending on the season. In summer it finds phytoplankton and in winter it feeds on eggs, larvae and zooplankton detritus.

For a long time, it was believed that krills were not found beyond the surface of the seas. However, in 2008, underwater exploration with a British ROV (Remote Operated Vehicle) revealed the presence of Antarctic krill (Euphausia superba) at a depth of 3,000 meters, in search of food. Krill must constantly wiggle its legs to maintain its depth, which is a very significant energy expenditure.

At night, krill emits a special light, thanks to different organs distributed over its body: near the eyes, on the second and seventh legs, and on the sternums. The emitted light is yellow and green in color and lasts two to three seconds. Its role in the life of krill is not yet fully understood. The hypotheses favor a role related to reproduction.

Krill is the favorite crustacean of whales

The krill undoubtedly fascinates first of all because its destiny is associated with that of the largest land animal. Indeed, this small crustacean a few centimeters long constitutes the main food of whales. Carnivorous, an adult whale needs to absorb 730 tons of food each year, composed of krill and plankton. It takes a lot of shellfish to satisfy such an appetite! But this animal is, for the moment, far from being endangered.

This crustacean is indeed present in very large numbers: it lives in swarms grouping together several thousand individuals per cubic meter. The banks can sometimes extend over nearly 450 km² for an estimated weight of 2 million tonnes. What is the secret of this large population? Incredibly rapid multiplication during the summer linked to such a high concentration of krill that the sea is adorned with brown-red hues. Reproduction does not begin until the age of 2.5 years. The female releases her eggs from late spring through mid-summer. Each time, between 100 to 8,000 eggs sink to the bottom of the sea, and this up to 7 times, only when the temperature conditions are optimal. After hatching, the larvae rise to the surface.

Krill is also a very common food in the diet of flamingos. It is the dye it contains, astaxanthin, also playing the role of antioxidant, which colors the feathers of Phoenicopteridae.

The whale filters seawater to get its ration of krill

The whales first locate schools of krill or plankton and then advance with their mouths open, sucking up the water and its contents. Then they slow down, close the mouth and squeeze out the water while keeping the krill and plankton inside the mouth. When the filtering is finished, they swallow them.

The so-called “true” whales constitute the suborder of the mysticetes. Their characteristic is not to have teeth but baleen. These are horn blades, composed of keratin, which line the mouths of cetaceans. Between each blade are hairs, forming a kind of vertical grid which acts as a filter. At birth, calves that feed exclusively on breast milk do not have baleen, or they are very short. Then they develop slowly. It is between 6 and 12 months, depending on the species, that they become operational.

The role of krill in the balance of the planet

Krill plays an essential role in purifying the atmosphere by the so-called “biological pump” phenomenon. The phytoplankton on which it feeds in the summer binds carbon dioxide from the atmosphere. After digestion, krill produces feces that sink 2,000 to 4,000 meters deep. They contain the carbon which is found stored in the seabed over very long periods of the order of a thousand years.

Conversely, it unfortunately concentrates toxic pollutants and heavy metals such as mercury or lead.

Threat to krill

Obviously whales and other animals like seals and penguins are not the only ones interested in krill. Man too quickly identified an important resource: he uses it as food for many farmed fish. The Japanese, Filipinos and Russians consume it. Krill oil is also incorporated into capsules that relieve PMS.

Until 2010, however, extremely harsh sailing conditions, as well as high operating costs, limited fishing. But technical progress has made it possible to reduce losses, strengthening appetites. The Norway became the world’s leading krill fishing nation, helping to unbalance Antarctic ecosystems.

Photo credit: Uwe Kils