Krill

As one of the most abundant animals in the entire food chain, krill are foundational to many of the world’s marine ecosystems. It provides food for hundreds of different species of animals, especially in the otherwise harsh Arctic and Antarctic waters. Krill are also interesting creatures in their own right. These tiny animals emit light from their transparent bodies and hard shells. The name krill derives from a Norwegian word that means small fry of fish, but it is actually a type of crustacean.

Evolution and Origin

Krill have no fossil records, so little is known about their past. However, scientists have studied the DNA of modern krill and compared it with their closest relatives that have fossil remains. Using the molecular clock methods, scientists determined that the last common ancestor of the Euphausiidae lived in the lower Cretaceous period, about 130 million years ago. From there, two lineages survived the extinctions, 65 million years ago, and the modern genera were established before the end of the Palaeogene era, 23 million years ago.

All of this to say, this crustacean has been around for a long time and has been a vital part of the ecology of the world’s oceans.

3 Incredible Krill Facts!

• The krill isn’t a social species in the same sense as many birds and mammals. Nevertheless, they travel together in massive groups, called swarms, for protection. These swarms regularly migrate between the deeper waters in the daytime and shallower waters in the nighttime. Some swarms are so large that they’re visible on satellite images.
• The krill move from place to place by floating along the ocean currents. When they encounter a predator, the krill can make a hasty escape by swimming rapidly backward at a pace of around 10 body lengths per second. This is a trick known as lobstering.
• Krill waste is a critical component of the planet’s carbon cycle.

Scientific Name

Krill belong to the order Euphausiacea. This comes from the Latin and Greek term euphausia, meaning light or illumination. The name was probably given because of their bioluminescent glow. They belong to the class of Malacostraca, which contains some 40,000 species of crustaceans. The defining feature of the Malacostraca and all crustaceans, in general, is the presence of a hard shell composed of a carbohydrate material called chitin.

Species

The krill is a large order composed of some 86 species divided into two broad families. The family of Euphausiidae contains almost every single known species of krill. The family of Bentheuphausia contains only a single species. Here is just a small sample:

• Antarctic Krill (Euphausia superba): Despite living in the inhospitable waters of the extreme south, this is perhaps the most abundant animal species on the planet.
• Ice Krill (Euphausia crystallorophias): Living right off the coast of Antarctica, the ice or crystal krill is the farthest south of any krill species.
• Northern Krill (Meganyctiphanes norvegica): This species is endemic to the northern parts of the Atlantic Ocean.
• Arctic Krill (Thysanoessa raschii): Measuring no more than an inch in length, this species is an important prey item for shearwaters, marine mammals, and some plankton-eating fish.
• Deep Sea Krill (Bentheuphausia amblyops): This is the only species in the Bentheuphausiidae family, living in deep waters below 3,300 ft. This species is considered the most primitive extant krill.

Appearance

Covered in a hard shell, this animal is a crustacean to its core. It has a long body divided into three sections (the cephalon, thorax, and abdomen) with a pair of antennae, 10 swimming legs, plus external gills to take in oxygen. It is also among the smaller species of crustacean, measuring no more than 2.4 inches long — about the same size as a paper clip — and weighing a fraction of an ounce. They have transparent bodies that give off a rather bright glow. The light is a product of internal organs called photophores. It is not exactly known what purpose this light serves, but it might be related to camouflage or social signaling.

Krill vs. Shrimp

The krill is sometimes mistaken for a shrimp because of the similarities between their long, segmented bodies. Shrimp, like krill, have a segmented body, but they often have a more robust exoskeleton and can be less transparent. Some shrimp species are larger and more colorful than krill, but there is considerable overlap in size and appearance. The largest shrimp can even grow up to a foot in length.

Distribution, Population, and Habitat

These crustaceans are versatile and adaptable animals that inhabit just about every single major saltwater body on the planet between the Arctic and Antarctica. This includes both coastal and deepwater regions. The total worldwide krill population is truly staggering. It is estimated that the entire biomass of the Antarctic krill alone (meaning the total mass of every member of the species put together) is between 125 million and six billion tons, among the largest in the animal kingdom. It is the equivalent of trillions of individuals.

However, this impressive number hides some worrying trends. Some scientists have estimated that the population of this species may have fallen by some 80% since the 1970s due to climate change, disease, and overfishing (though it is not yet threatened).

Predators and Prey

These animals are an essential link in the marine food chain. It connects the microscopic marine organisms at the bottom of the chain to the much larger predators at the top. The vast majority of them are herbivorous or omnivorous, feeding on small algae or microscopic animals that happen to pass by. A few species are exclusively carnivorous and supplement their diet with fish larvae. Krill feed by filtering out edible material with their small appendages. They passively suck up the vast quantities of small food in the water.

Higher up the food chain, the krill is perhaps the most widely consumed animal in the entire marine ecosystem. It is an essential prey for seals, birds (especially penguins), whales, and all manner of fish. Therefore, any disruption in the abundance of these animals can have larger ramifications for the food chain. Threats include marine pollution, climate change, and alterations to the habitat or abundance of prey. The krill are caught frequently in some fishery locations, which may drive down numbers as well.

Reproduction and Lifespan

These crustaceans have a distinct breeding season that varies based on location and climate. After the male deposits his sperm sac near the female’s genitals, she lays thousands of eggs throughout the entire breeding season, often spaced out into multiple broods. In total, these eggs weigh the equivalent of a third of her mass. Depending on the species, the female will either release the eggs directly into the water or carry them in a specialized sac for the duration of the gestation period.

After hatching from the eggs, the young will pass through several larval stages. In the early stages, the undeveloped krill lack the appropriate feeding apparatus and survive almost exclusively on the egg yolk. In later stages, they develop a mouth and digestive system for consuming plankton. Each stage requires them to replace their entire exoskeleton through a series of molts. Life expectancy depends on where the species is located. Krill that occupy warm tropical or subtropical waters typically live for about one to two years, whereas polar species can live as long as five to seven years if they can successfully evade predators.

Fishing and Cooking

Worldwide, krill consumption is still a relatively niche phenomenon — at least compared to the closely related shrimp — but it is an important source of seafood in Russia, Spain, Japan, and the Philippines. These countries each developed a large-scale fishery in the mid-20th century to catch vast amounts of these animals. The Antarctic krill fishery is preferred most of all due to the abundance of these species and the ease with which they’re caught. Other uses for krill include aquarium food, pet or livestock food, fishing bait, and nutritional supplements.

Fish Oil vs. Krill Oil

Krill oil is a very nutritious supplement that includes high levels of protein and omega-3 fatty acids. These fatty acids are associated with lower risks of cardiovascular disease. Not enough research has been conducted to tease out the difference between fish oil and krill oil, but the omega-3 fatty acid is largely the same in both of them.