Introducing Antarctic Krill

Antarctic krill (Figure 1) are small sea creatures that grow no bigger than a human thumb but can do amazing things. For example, they glow in the dark and they can shrink or grow by replacing their skeletons, which are on the outside of their bodies. Those abilities are impressive, but their most amazing achievement is that they have developed a huge population. There are about 800 trillion Antarctic krill [1], or 100,000 for every human being! These enormous numbers make Antarctic krill very important for other animals, for human beings, and for the planet (Figure 2), as you will see if you keep reading!

Antarctic krill belong to a group of animals called crustaceans, which includes crabs, shrimps, lobsters, and 86 species of krill. The other krill species are important in their own ways, but Antarctic krill is such an important species that many people simply call it “krill”, as we do in the rest of this article. This species of krill lives in the cold Southern Ocean that surrounds the frozen continent of Antarctica. They mainly feed on tiny plants called phytoplankton, which they filter out of the water using their feeding basket (Figure 1A), but they can also eat other small animals.

Lots of Krill Means Lots of Predators and Lots of Poo

We know of almost two hundred species of predators that feed on krill (Figure 2B) [2], but there are probably many more. These species include blue whales, which are the biggest animals that have ever lived, and most of the other baleen whales that live south of the equator. Other krill predators include most penguin species, several types of seals, flying birds such as albatrosses, fish, jellyfish, and squid. Many people think of krill as penguin or whale food, but in fact, the billions of fish in the Southern Ocean eat more krill than all the other predators eat. Most of these predators feed on other species as well as krill, but they still manage to eat up to 390 billion kg of krill per year. That is more than all the meat eaten by humans. Without krill, the animals that depend on them would be scarce or extinct.

The world would also be hotter without krill. Carbon, in the form of carbon dioxide gas (CO₂), drives climate change when it builds up in the air (Figure 2C). Krill help to reduce the build-up of CO₂ by sending carbon down to deep waters, far away from the air. This process starts when phytoplankton use CO₂ to build their cells. Krill eat the phytoplankton, and some of the carbon comes back out of them, packed into pellets of feces (also known as poo). Huge numbers of krill produce huge numbers of pellets, which sink rapidly down to deep waters, taking the carbon with them (for more information about krill’s importance for Earth’s climate, see this Frontiers for Young Minds article). The skeletons that krill shed when they change size, which also contain carbon, sink to deep waters too [3]. Each atom of carbon that krill send to deep waters is one less atom causing climate change.

Krill are important for people, too. It is easy to see how we benefit from krill’s effect on CO₂, and we see the importance of the Southern Ocean ecosystem every time we notice a picture of a penguin. There is also a krill fishing industry that provides hundreds of jobs. Krill is the main species caught in the Southern Ocean, and this catch is mainly used to feed farmed fish and other animals. However, some people think that krill could become a valuable food for humans in the future.

Krill Are Explorers

Krill are often described as plankton, but two things set them apart from other animals that drift on ocean currents. The first is their size: krill can grow much bigger than most other plankton species. The second is their ability to swim against currents, which gives them more control over where they go. Swimming allows krill to form large, tightly packed groups called swarms. The biggest swarms can cover areas up to 100 km² and contain over 10 billion krill. Krill swarm together to defend themselves against predators. Although whales can scoop up great mouthfuls of krill from a swarm, more krill escape than get eaten. Most predators have much smaller mouths and eat only one krill at a time, meaning that any individual krill in a swarm has a very high chance of escape.

Individual krill can travel thousands of kilometers during their lifetimes. As a result, adult krill are found throughout an area of 19 million km² (5% of the world’s ocean). Their habitats stretch from the ice-covered waters that surround the Antarctic continent to the ice-free edge of the Southern Ocean, up to 2,000 km further north, and from coastal areas to remote waters in the middle of the ocean (Figure 3).

Krill also travel up and down. They mainly live near the ocean surface where they can find the most food, but adults can also swim down to depths of 3,000 m, way beyond the reach of whales, seals, and penguins. Krill eggs sink to a depth of about 500 m before they hatch, so young krill start their lives by traveling back to the surface to feed.

Mysterious Krill

Studying krill is not easy. It usually requires a ship that can withstand storms and sea ice, and special equipment to observe or catch krill. Even then, it is only possible to study a small part of the vast area where krill live. Scientists are exploring new technologies to provide more information, but in the meantime, there are a lot of unsolved mysteries about krill. We worked with other krill experts to identify the main mysteries for scientists to solve [4].

One important mystery is that the number of young krill changes from year to year, but we do not know what causes this change. There is no clear relationship between the number of adults and the number of offspring. Certain factors might affect whether young krill live long enough to become adults. For example, hiding from predators under sea ice and feeding on the phytoplankton that grow there allows some young krill to survive their first winter, but others survive in places without sea ice. Solving the mystery of young krill numbers will help us to understand whether the krill population can continue to replace the immense numbers eaten by predators every year, along with the smaller numbers caught by the fishing industry.

Another krill mystery is where young krill come from. Krill lay their eggs in surface waters. The eggs sink and hatch far below the surface. This can only happen in places with warm, deep water that allows the young krill to develop quickly and return to the surface to feed before they starve. Adults must be able to reach these egg-laying places in high numbers, and young krill must be able to reach safe habitats to survive their first winter. There are very few places with all these qualities. Detective work has identified some suitable places, and confirming whether krill really do produce offspring in these places is the next step toward protecting them.

The biggest mystery concerns the future. The Southern Ocean is getting warmer, and winters have become less icy. How will these changes affect krill populations and populations of predators that depend on them? Will warmer temperatures and less ice reduce the size of the krill population and make parts of the Southern Ocean uninhabitable for krill? There are signs that changes have already begun, and that krill have become less common in the northern part of their habitat where waters are warmest [5].

Antarctic krill is an important species, but climate change threatens the things that make it important: its population size and ability to live in most of the Southern Ocean. Krill fishing could make the situation worse. We need to solve some of the mysteries about krill so that the people responsible for protecting the ecosystem where krill live have the information necessary to do their job. People from all over the world can help, including scientists, people in the fishing industry, and people responsible for protecting the ecosystem. Krill scientists have recently formed an organization called the Krill Expert Group, to help everyone work together. We hope that through teamwork we will solve the mysteries and help to protect this important species.

Glossary

Crustaceans: ↑ A group of animals with jointed limbs and skeletons on the outside of their bodies that mainly live in water. Examples include crabs, shrimps, lobsters and krill.

Phytoplankton: ↑ Small plant-like living things that drift in water and are the main source of food in ocean ecosystems.

Baleen Whales: ↑ Whales that feed by filtering small animals out of seawater. Examples include blue and humpback whales.

Climate Change: ↑ Changes to weather and oceans, including warming, that happen over many years. Human activities that burn oil, coal, and natural gas cause dangerous changes.

Feces: ↑ Material that comes out of an animal and contains the waste that is left over from feeding. Also known as poo.

Ecosystem: ↑ The plants, animals, and other living things, together with their habitat, that are found in a particular place and are connected by activities like eating each other.

Plankton: ↑ Small living things (bacteria, phytoplankton, and animals) that float in water. They are mainly unable to swim against currents but may change depth to control their movement.

Habitat: ↑ The places where a particular type of living thing is found.

Acknowledgments

We thank the Board of the Scientific Committee on Antarctic Research Krill Expert Group for advice and support. We also thank Maera Hill and Joe and Megan Tate for reading drafts of the article and Ruth Hatfield for writing tips. This work was funded by the Natural Environment Research Council (NERC) British Antarctic Survey (BAS) ALI-Science Southern Ocean Ecosystems project.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.