Krill are tiny shrimp-like, semi-transparent crustaceans measuring about one inch long and weighing a fraction of an ounce. They are in the same class as crayfish, lobsters, woodlice, shrimp, and crabs.
Krill is a general term used to describe about 86 species of crustaceans found in open oceans. They belong to the group of crustaceans called Euphausiids and the order of crustaceans Euphausiacea.
They live in the open sea and congregate in amazingly huge swarms. In most cases, their dense swarms are more than 10,000 individuals per cubic meter (1 cubic yard).
Krill are too small and weak to swim. They are plankton, that cannot swim against currents and tides because of their size, weakness, or immobility. However, like other phytoplankton and zooplankton (two types of plankton), these animals occupy a special place in the global food chain.
Appearance
Like other crustaceans, they are covered in an exoskeleton that supports and protects its body.
They have two main body parts. The first one is the cephalothorax, composed of the fused thorax and head. The next one is the pleon (abdomen).
The external skeleton is transparent in most species. Compound eyes are another conspicuous feature of this zooplankton. Species with a screening pigment are also found, which helps them adapt to different lighting conditions.
Krill also have two antennae and plenty of thoracic legs attached to the thorax.
The exact number of these legs varies depending on the species and genera. However, since these sea animals are decapods, they have ten swimming legs that look the same as those of freshwater crayfish or lobster.
Some adults measure between 0.4 inches and 0.8 inches. However, some species can grow to up to 5.9 inches.
Unlike shrimp and other crustaceans, their distinctive feature is the presence of externally visible gills for breathing.
Habitat
They are found worldwide, but different species are predominant in different oceans or parts of the sea. For example, the Antarctic krill is dominant in the Southern Ocean. Estimates show that it makes up a biomass of approximately 379,000,000 tonnes in this area.
These planktons depend on currents and tides to move. So, krill populations in different parts of the ocean always change depending on the direction of these forces.
Diet
Most krill are renowned filter feeders able to use their thoracopods to filter out the right food types from the water. They usually feed on the surface of the water at night and sink deeper in the water column during the day.
Some krill feed on phytoplankton. Others are omnivorous, meaning they can survive on both plants and animals. There are also predatory species. They prey mainly on larvae and zooplankton.
In winter, these small shrimp-like crustaceans have to use other food sources such as algae growing on the underside of the pack ice, detritus on the sea floor or the other animals in the water. They can survive for long periods (up to 200 days) without food. They shrink in length as they starve.
Reproduction
The krill breeding season varies depending on the species, climate, and location.
When a male is ready to reproduce, it deposits sperm near the genitals of the female. That’s enough to assist the females in laying fertilized eggs throughout the breeding season. Some species release the eggs into the water, but others carry them safely in their specialized sacs until the gestation period is over.
Each egg is about half a millimeter across. From near the surface, the developing eggs sink into the deep ocean for seven days down to 1000 meters, where the eggs hatch. The ‘nauplius’ larvae then begin the long swim back to the surface. Juveniles eat algae on the underside of the sea ice. The larval stages change shape many times over several months until adulthood.
Adults can live for more than five years in the open ocean.
Threats
Krill are a critical link in the ocean’s food chain. They connect phytoplankton at the bottom of the chain to larger predators at the top.
Small animals like squid, seabirds, penguins, seals, fish, and large ones like baleen whales feed on the swarms of krill. Since most animals depend on krill for food, a minor disturbance in their abundance can harm the entire ecosystem.
Krill are not endangered, but over-dependence on them could lead to severe problems in the future.
The impact of climate change and marine pollution threaten these animals. Scientists believe the fact that they are caught frequently in some fisheries worldwide is another potential threat. Currently, krill is an essential food source in the Philippines, Spain, Japan, Russia, and other places.
More Amazing Facts about Krill
- There are over 85 species of krill, and each of them has a few unique features.
- Krill float along ocean currents and waves because they are too small and weak to swim.
- Their population has dropped by about 80% since the 1970s because of overfishing, disease, and climate change.
- It is estimated that Antarctic krill live for 5 to 10 years.
- Krill oil and fish oil contains rich omega-3 fatty acids and high levels of proteins. Experts believe that these fatty acids can reduce the risk of cardiovascular disease.
- They are a major food source for salmon. Krill pigments also give salmon flesh its characteristic pink color.
- Antarctic krill play an important role in this process, as they eat vast quantities of phytoplankton and excrete the atmospheric carbon remains in the form of fecal pellets that sink to the seafloor.
- Its meat and oil are also used in aquaculture and animal feed.
FAQs
What is the role of krill in the food chain?
Their role in the food chain is to provide food for many oceanic life forms. They feed on phytoplankton, a type of plankton, but drift near the ocean’s surface to get enough sunlight and carbon dioxide. These single-celled plants use these things to make their food.
They are the primary food source for baleen whales, birds, fish, and many other living things.
Can humans eat krill?
Yes, humans eat krill in many parts of the world. Most people consider them the best source of proteins, Omega-3 fatty acids, vitamin A, and other valuable nutrients. Due to their increased harvesting for human consumption and pollution-related activities, their population has dropped by about 80% since the 1970s. However, these sea animals are not considered as threatened as of today.
Are krill intelligent?
If you observe krill over a short time, you’ll notice that they are intelligent. They know their strengths and weaknesses and can take advantage of that to survive for up to 10 years. Krill travel together in swarms of more than 10,000 individuals per cubic meter for protection. This behavior inspired the computer program for devising better traffic control systems known as the krill herd algorithm.
Is krill a fish or shrimp?
These sea animals look like shrimp, but they have externally visible gills for taking in air that sets them apart. Both of them are crustaceans, and that’s why they look alike. Also, remember that the number of people who feed on shrimp seems higher than krill.
