Krill

Krill are tiny sea creatures that belong to a group called crustaceans, and they live in all the world's oceans. They are very small and are a crucial part of the ocean's food chain. Krill eat tiny plants and animals floating in the water, called phytoplankton and zooplankton, and they, in turn, are a main food source for many larger sea animals.

In the Southern Ocean, one kind of krill, called Antarctic krill, is especially important. There is an estimated 379 million tonnes of this krill, making it one of the most abundant animals on Earth. Many animals, like whales, seals, penguins, seabirds, squid, and fish, rely on krill for food.

People also catch krill for various uses. They are used to feed farm animals and fish in tanks, as bait for fishing, and in medicines. In some countries, krill are eaten as food. In Japan, they are called okiami, and in Spain and the Philippines, they are known as camarones or alamang. Krill are especially important for large whales, such as the blue whale, which depend on them for survival.

Taxonomy

Krill are tiny sea creatures that belong to a group called crustaceans, which also includes shrimp and lobsters. They are part of a larger group called arthropods, which have a hard outer shell and jointed legs.

There are two main families of krill. The larger family, Euphausiidae, has 85 different species, with the genus Euphausia having the most species. The other family, Bentheuphausiidae, has just one species that lives in very deep water. Some well-known krill species important for fishing include Antarctic krill, Pacific krill, and Northern krill.

Main article: Phylogeny of Malacostraca

See also: Eucarida § phylogeny

Distribution

Krill are found in all the world's oceans, though some species live only in certain areas. One species, Bentheuphausia amblyops, lives deep in the ocean everywhere.

Species from the group Thysanoessa live in both the Atlantic and Pacific oceans. The Pacific Ocean is home to Euphausia pacifica. In the north, krill live from the Mediterranean Sea upward.

Some krill live close to coastlines. Four types from the group Nyctiphanes are very common where warm ocean currents rise near the surface, such as off California, and along the Humboldt, Benguela, and Canarias currents. Another coastline species is E. crystallorophias, found only around Antarctica.

Certain krill exist only in one place. Nyctiphanes capensis lives just in the Benguela Current, E. mucronata in the Humboldt Current, and six types of Euphausia are native to the Southern Ocean.

In the Antarctic, seven krill species are known — one from Thysanoessa and six from Euphausia. The Antarctic krill (Euphausia superba) usually live as deep as 100 m (330 ft), while ice krill (Euphausia crystallorophias) can go as deep as 4,000 m (13,100 ft), though they often stay between 300–600 m (1,000–2,000 ft). Both types live south of 55° S, with E. crystallorophias being most common south of 74° S and where ice packs form. Other krill in the Southern Ocean include E. frigida, E. longirostris, E. triacantha, and E. vallentini.

Anatomy and morphology

Krill are crustaceans with a hard outer shell, called a chitinous exoskeleton. Their bodies have three main parts: a combined head and thorax, an abdomen with ten swimming legs, and a tail fan. Most krill have a clear outer shell.

Krill have complex compound eyes and two antennae, along with several pairs of legs used for feeding and cleaning themselves. They also have special swimming legs, called "swimmerets," similar to those of a lobster or freshwater crayfish. Most krill are small, usually about 1–2 centimetres (0.4–0.8 in) long, but some can grow much larger. Many krill can produce their own light using special organs, which may help them hide in the deep ocean.

Ecology

Krill are small sea creatures that play an important role in the ocean's food chain. Many krill eat by filtering tiny plants and animals from the water using special tools on their bodies. While most krill eat plants, some also eat small animals.

Many larger sea animals, such as fish, penguins, seals, and whales, depend on krill for food. If the number of krill drops too low, it can affect many other animals in the ocean. Changes in the environment, like warmer temperatures, can also threaten krill populations.

Krill have been observed digesting tiny pieces of plastic smaller than 5 mm, breaking them down and releasing them back into the water.

See also: Carbon sequestration and biological pump

Life history and behavior

Krill go through many stages as they grow. After they hatch, they start as tiny larvae called nauplius, then become pseudometanauplius, metanauplius, calyptopsis, and finally furcilia. Each stage helps them grow bigger and develop new parts of their bodies. As they grow, they shed their hard outer shell, called moulting, to make room for their bigger bodies.

During mating time, male krill place a small packet of cells near the female. Females can carry many eggs, sometimes up to a third of their body weight. Krill have different ways of laying eggs. Some release them into the water, while others carry them until they hatch.

Krill often group together in large numbers, called swarms, to stay safe from predators. They also move up and down in the water each day. During the day, they stay deeper to hide, and at night they move closer to the surface to find food. This helps them avoid being eaten and saves energy. When danger comes, they can quickly flip backward to escape.

Biogeochemical cycles

Further information: Marine biogeochemical cycles

The Antarctic krill is an important species in the Southern Ocean. It helps cycle nutrients and provides food for penguins, baleen whales, and blue whales.

Human uses

See also: Krill fishery

Harvesting history

People have used krill as food for a long time, maybe even since the 1800s. In Japan, they called it okiami. Big krill fishing started in the late 1960s and early 1970s, mainly in Antarctic waters and around Japan. Japan and the Soviet Union (now Russia and Ukraine) were the biggest krill fishers. The amount of krill caught was very high in 1983, but now rules help make sure we don’t catch too much.

In 1993, krill fishing went down because Russia stopped fishing krill and rules were made to protect Antarctic krill. These rules set limits on how much can be caught each year to keep the krill population healthy. After a review in 2011, these rules stayed the same.

The amount of krill caught each year in Antarctica is now about 100,000 tonnes, which is much less than the allowed limit. This is mostly because it costs a lot and there are legal issues. Japan catches about 70,000 tonnes each year. Even though krill are found everywhere, Antarctic krill are preferred because they are easier to catch and there are lots of them.

In 2018, it was announced that most krill fishing companies would stop fishing in big areas around the Antarctic Peninsula from 2020. This includes areas near penguin breeding spots to help protect them.

Human consumption

See also: Shrimp paste

Even though there may be a huge amount of krill in Antarctica, more and more is being caught for humans. From 2010 to 2014, the amount caught went up by 39%, to 294,000 tonnes. The main countries catching krill are Norway (which catches about 56% of the total), the Republic of Korea (about 19%), and China (about 18%).

Krill are rich in protein and omega-3 fatty acids. These are used to make food for people, supplements like oil capsules, food for animals, and even pet food. Krill taste salty and a bit like fish, but stronger than shrimp. To make them easier to eat, the hard outside shell is removed.

Antarctic krill can store a lot of fats during the summer and use these during the winter when there is less food. Scientists are also looking at krill meal as a good source of healthy fats for growing animals.

In 2011, the US Food and Drug Administration said that krill oil was safe to eat.

Krill and other small shrimp-like animals are most commonly eaten in Southeast Asia. There, they are often fermented (left out to change over time) with their shells still on, and then ground up to make shrimp paste. This paste can be stir-fried with rice or used to add flavor to many dishes. The liquid from fermenting them is also used to make fish sauce.

Bio-inspired robotics

Krill are great swimmers in a special area called the intermediate Reynolds number area. Because there aren’t many solutions for robots that move underwater, scientists have looked at how krill swim to help design new underwater robots. They study krill’s movement to find better ways to make robots move through water.