Krill: magicians of the Southern Ocean

Underwater view looking up to krill swam under ice and back lit blue water
A swarm of adult and larval krill. (Photo: Alfred Wegener Institute; Ulrich Freier)

Beautiful creatures

With their large black eyes and transparent bodies, krill are beautiful creatures that have an almost extraterrestrial appearance. They also seem to be endowed with magical powers in that they can transform themselves from adults into ‘younger’ creatures.

Krill is a general term used to describe about 85 species of crustaceans found in open oceans. They belong to the group of crustaceans called euphausiids and look like smaller versions of familiar crustaceans such as prawns or lobsters. They range in size from small tropical species of less than a centimetre in length to little known deep sea giants that can reach 14 centimetres.

Krill are mostly transparent, although their shells have a bright red tinge from small pigment spots. Their digestive system is usually visible and this is often a vivid green from the pigment of microscopic plants they have eaten.

Krill usually feed on the surface of the water at night and often sink deeper in the water column in the daytime. The primary food of krill is phytoplankton, which are microscopic ocean plants suspended in the upper water column where light is sufficient to allow for growth.

Five species

There are five species of krill found in Antarctic waters. The most dominant of these species is Antarctic krill, Euphausia superba. Antarctic krill is one of the most abundant and successful animal species on the planet. They are frequently found in such abundance that they colour the sea a reddish brown. E superba only grow to about 60mm long and weigh more than a gram, but there is estimated to be about 500 million tonnes of Antarctic krill in the Southern Ocean.

Growing and shrinking

Krill are crustacean ‘magicians', in that they can continue to change from adults into juveniles. It is estimated that Antarctic krill live for 5–10 years, but determining the age of the animals presents quite a problem for scientists. This is because crustaceans usually grow by moulting their hard shell (exoskeleton), expanding the new one and then growing into it. When the exoskeleton becomes tight again, the moulting and growing process starts once more. In most crustaceans the moulting tends to slow down as the animal grows older, and stops altogether in adulthood. This means that scientists can usually tell the age of an animal from its size. On average, the larger the creature the older it is.

Antarctic krill are an exception to this rule. Because they live in the cold, dark Southern Ocean, they must survive the winter months when food is scarce. They do this very successfully. In fact, laboratory studies have shown that Antarctic krill can survive more than 200 days of starvation. They don't build up fat reserves so they must rely on another survival mechanism. Krill retain the ability to moult for life and use the ability to continue growing and reducing their body size to help them survive. ‘Downsizing’ enables Antarctic krill to use their own body proteins as a source of fuel. All species of krill seem to share this adaptation.

At the end of summer adult krill begin to lose their sexual characteristics. After a series of moults they again resemble two-year-old juveniles, giving no indication that they were once adults. In spring, adults once more begin to develop sexual characteristics and become mature before the spawning season. Antarctic krill are thought to lay a number of broods of eggs, with as many as 8000 eggs per brood in a season that may last as long as 5 months. Downsizing and the production of multiple broods make measurements of age on size alone impossible.

Massive swarms

Antarctic krill don’t occur randomly but aggregate in schools or swarms, where the density of the animal can be as high as 30,000 individuals per cubic metre. These swarms occur in larger groupings or patches. Scientists are still determining the social structure of the swarms since it seems that some krill swarms may be made up of entirely of juveniles, while other swarms may consist of all females or all males.

Krill are the major food source for whales, penguins and seals. Although it has been the largest fishery in the Southern Ocean for the last 25 years, krill and its impact on other species, have received little scientific attention until recently.

Krill are fished commercially by Ukraine, Poland and Japan and are used for human consumption and aquaculture. International concerns about how the potential overfishing of krill might affect other Antarctic wildlife has led to the signing of an innovative treaty, the Convention for the Conservation of Antarctic Marine Living Resources (CCAMLR). The international agreement on the krill fishery is part of the Antarctic Treaty System, and came into effect in 1982. The Convention takes an ecosystem approach to fisheries management and recognises the place of krill at the centre of the Antarctic food web. The agreement has set limits to catches which take into account the needs of the other animals in the ecosystem. These catch limits are conservative, even though scientists still do not know the full extent of the krill resource or have a complete understanding of the biology of these remarkable crustaceans.

Written by Wendy Rockliffe and Steve Nicol, Australian Antarctic Division