9 things you need to know about krill and why they are essential to the health of the ocean

Watercolour painting of krill bioluminescence. (c) Wikimedia commons
Watercolour painting of krill bioluminescence. (c) Wikimedia commons

Imperial’s Dr Emma Cavan, lead author of a recent Nature Communications paper on the role of krill in influencing the environment, and the University of Tasmania’s Professor Steve Nicol, author of ‘The Curious Life of Krill’, blog on why krill are so much more than they seem.

Krill – a crustacean that looks a bit like a prawn or shrimp – may seem like insignificant creatures, but a particular species called Antarctic krill (Euphausia superba) play a huge role supporting life in the ocean.

A recent review has highlighted the importance of these creatures in improving ocean health. When krill feed they excrete nutrients into the ocean. This essentially fertilises the oceans and allows phytoplankton, ocean plants that support the base of Southern Ocean food webs, to thrive. Krill also influence global levels of atmospheric carbon dioxide. Their poo is full of carbon, and sinks quickly, locking carbon in the deep ocean for years or even decades. Surprising? Here are 9 more things you need to know about these wonderful creatures.

Diagram adapted from image in Nature comms paper showing that Krill can release nutrients and produce fast sinking, carbon-rich poo pellets. The large size of krill, their high biomass in swarms and extensive vertical migrations over 100s of metres each day amplify their impact on their environment.
Krill can release nutrients and produce fast sinking, carbon-rich poo pellets. The large size of krill, their high biomass in swarms and extensive vertical migrations over 100s of metres each day amplify their impact on their environment. (c) McCork Studios, adapted from the image in Nature Communications.

1. Krill poo pellets are a nutritious delicacy – for other krill.

Krill poo pellets are large, carbon-rich and sink quickly to the deep ocean locking carbon away from the atmosphere for long periods of time. However, not all poo makes it to the deep ocean, as krill will also eat the carbon-rich and nutritious faeces produced by other krill!

2. Krill are astronomically abundant.

Antarctic krill could be the world’s most abundant multi-celled animal (metazoan). They contribute 0.05 gigatons of carbon to global biomass – that’s similar to other prominent species, such as humans or cows. This makes krill a top contender for the title of the world’s most abundant metazoan animal. And without a doubt, krill have the greatest biomass of any single marine metazoan species. This fact alone makes them globally important.

3. They are bigger than you think.

Many people – including scientists – imagine krill to be microscopic or tiny. However, they often astonish those who view them up close because of how big they are. Antarctic krill grow up to 6 cm and can weigh 1 gram. This makes them average-sized (by weight) for animals that live in the ocean.

A close up of Antarctic Krill
Antarctic krill (Euphausia superba) in the research aquarium at the Australian Antarctic Division in Tasmania. Each krill here is about 5 cm in length. (c) Stephen Nicol

4. Krill swarms are the largest aggregations of animal life.

Krill live in three-dimensional swarms or schools that can stretch for up to 20 km and can contain up to three million tonnes of animals – that’s 30 trillion individuals. Such swarms are likely to be the largest aggregation of animal life on Earth. The density of swarms and their sheer size makes them ecologically (and economically) of vast importance. This is because large whales can feed on the swarms and conserve energy by eating tens to hundreds of thousands of krill in just one mouthful. Similarly, the fishery can catch millions of swarming krill in just one hour.

5. Krill have a vast geographic range.

Estimates of the total area of ocean inhabited by Antarctic krill extend up to 32 million square kilometres – that’s 10% of the planet’s ocean area or roughly the size of Africa. Antarctic krill also inhabit all water depths, from the surface to the seafloor many kilometres below. As a result, studying krill is fraught with difficulty. It is impossible to survey the whole habitat at one time, particularly as large areas of krill habitat can be under ice.

Krill swarm under sea-ice in Antarctica
Krill swarm under sea ice in Antarctica. (c) Donna Patterson via Flickr

6. Krill are an astonishingly good food source.

Blue whales are the largest animals that have ever lived on the planet, weighing around 90 tonnes – that’s about the weight of a Boeing passenger aircraft! Most blue whales living in the Southern Ocean feed on one single food item – Antarctic krill. The entire population of Southern Ocean blue whales, estimated to be 200,000 individuals before commercial whaling commenced (17th century), maintains itself by feeding in Antarctic waters on Antarctic krill for a maximum of only 6 months a year. This sustains them for the rest of the year, when they migrate thousands of kilometres north to breed, during which time they don’t feed at all. In addition to this summer feeding frenzy by blue whales, Antarctic krill sustain many other species of whales during the summer, and provide year-round food for millions of penguins, seabirds, seals, fish and squid.

7. Krill have been fished extensively for the last 40 years.

Vitamin tablets with a label saying 'high strength deep sea krill oil'
(c) Emma Cavan

Krill are used in pharmaceuticals and animal feed. They have been fished extensively – and largely sustainably – for the past 40 years. Currently the krill fishery catches around 0.4 million tonnes a year (about 0.5 % of the krill population), which is far less than the internationally agreed catch limits of 8.7 million tonnes a year. This makes the fishery unique in global terms as it is currently very sustainable. However, scientists don’t yet know the effect that harvesting krill could have on atmospheric carbon and ocean chemistry. Scientists need to work with the krill fishery to conduct more research on how fishing may influence the ocean health.

8. Krill can glow in the dark!

All krill, including Antarctic krill, are bioluminescent – meaning they can produce light! They have specially adapted organs in their bodies which are responsible for producing light. Possible reasons for light production could be to camouflage themselves or to communicate with each other.

9. The krill genome is 12 times the size of ours.

Recent studies have revealed that the krill genome (the amount of genetic information in each cell) is roughly twelve times the size of the human genome – no one is quite sure what the implications of this finding are, but it is an incredible fact on its own. Other intriguing biological features include that they are heavier than seawater and must stay swimming to remain in the open ocean. This is an extraordinarily expensive lifestyle considering they live in an environment where food is remarkably scarce for much of the year. Krill can also be remarkably long-lived – they have been shown to survive in the laboratory for up to 12 years.

Find out more about the Dr Cavan’s research

The importance of Antarctic krill in biogeochemical cycles, by Emma Cavan et al., is published in Nature Communications.

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