Krill are small, aquatic, shrimp-like invertebrates living in all of the oceans in the world. The temperature of these waters varies greatly including both the warm waters of the South Pacific and the near freezing waters around Antarctica. Krill have black compound eyes. A close up of one of these eyes is shown below. Krill also have bright colors including blue, green, orange, and red. These small animals are made up of two families which, combined, currently contain eighty-five species. One of the most abundant species of krill is the Antarctic krill which (surprise!) lives in the ocean around Antarctica. Scientists estimate that there are over 1.1 trillion pounds or 550 million tons (500 million tonnes) of Antarctic krill in the  world. This does not include the other eighty-four species of krill! Antarctic krill may have the largest biomass of any multi-cellular animal! The biomass of this species is almost twice that of humans! This link shows the same image of a krill as I have above, but if you click on different parts of the krill’s body, the picture there will change to a close up of that body section.


Krill eye, close up


I said earlier that krill were small, and with so many species of krill, their size varies from species to species. On average an adult krill will be about two and a half inches (6 cm) long. This is the size of Antarctic krill. Some smaller species of krill only grow up to .3 inches (8 mm) while large ones can be up to 5.5 inches (14 cm). The average weight of all of the species of krill is one gram. This is about the weight of a paperclip!


Because they are small, krill need something really small to eat. The diet of these animals consists mainly of algae, specifically algae that grows under sea ice. Another main food source of these animals is phytoplankton, which is plankton consisting mainly of algae. Zooplankton is occasionally eaten by these animals, and this is plankton consisting of animals and protozoa. One of the reasons Antarctic krill are so numerous is the fact that The waters around Antarctica are rich with plankton and have a lot of ice for ice algae to grow on. If at any time there is not enough food, and individual krill can start digesting its own body tissue without harming itself! Because of this, they will shrink, shedding their skin as if they were growing, but instead the new skin is smaller. Due to this skill, krill can survive up to 200 days without food!


While krill have many threats, there are so many krill that their population is fairly stable. In 2010, the world krill catch was about 231,000 tons (210,000 tonnes).  The krill catch for the years before 2010 was much lower than this. The catches in the 1980’s and 90’s were much higher, being 330,000 to 550,000 tons (300,000 to 500,000 tonnes) each year! In the future, total krill catches are expected to reach 680,000 tons (620,000 tones). The total 2010 krill catch translates to approximately 200 billion krill! If you think this is a lot, there is another threat to krill that consumes almost one hundred times this number each year: blue whales. A single blue whale can eat up to four tons (3.6 tonnes) of krill in a single day! There are approximately ten to twenty-five thousand blue whales in the world. If we said, for this example, that there were 15,000 blue whales and each of them ate 4 tons of krill each day, that would end up being 21,900,000 tons (19,900,000 tonnes) each year. This ends up being almost twenty trillion krill eaten just by blue whales each year!  Many whales, seals, birds, and fish depend of krill as their main source of food. If krill went extinct, which currently seems highly unlikely, many of these animals would also go extinct.

Human uses for krill

Humans use the krill they catch in a variety of ways. A large portion of the world catch is used as food, especially in Japan. Some krill are used as food in aquariums, either in personal aquariums or in aquariums used as attractions. Some krill are also used as bait for fishing or even in medicine. For an in-depth article about the uses of krill, click here.

Young krill

Krill spawn in the summer, and each female lays thousands of eggs within the top 350 feet (106 m) of water. These eggs will then sink down to at most 1.2 miles OLYMPUS DIGITAL CAMERA(2 km) below the ocean surface. There the eggs will hatch into young krill, called nauplii. This picture shows a nauplii swimming out of the egg on the right of the picture. The bar at the top represents one hundred micrometers, the equivalent of just one tenth of a millimeter! This nauplii is about as long as five normal sheets of paper are thick! During this stage, the young krill will start journeying up the ocean, closer to the surface. During this long migration, the young krill pass through many stages, and in the following spring, they will become juvenile krill, looking like a miniature version of the adults. At two years of age, the young krill are able to mate. The average life span for krill is two to six years while some individuals can reach up to ten. While this is not long compared to humans, it is pretty good when you consider how severely hunted they are.


Krill have several means of self-defense depending on what animal has them on the menu. Krill almost always travel in groups, making their large numbers confusing or frightening for smaller predators. When larger animals such as whales come along, the groups may decide to split up, maybe making the whale decide to go for a larger meal than an individual krill. If an individual happens to be chased by a predator, it can shed its skin as a decoy to trick the other animal into eating the old skin instead of the more important parts of the krill’s body.

Krill groups

The groups krill stay in are HUGE, with numbers in the billions! During the day, the krill swarms will migrate up to the ocean’s surface to feed. At night, they will go back down into the deep ocean where there are not as many predators. They will stay here until the next day when they will repeat this process.

Don’t forget to scroll down and leave your guess about what the next animal is!



Photo credits:

  • Krill – Uwe Kils
  • Krill eye – Uwe Kils
  • Krill hatching – Dr. Jaime Gómez-Gutiérrez
  • Mystery animal – Jeff Kubina
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