IELTS Academic Reading Practice Test 13: Passage 1

In the cold waters surrounding Antarctica lives an animal that few people have ever seen, but on which a large part of the region's wildlife depends. The Antarctic krill is a shrimp-like crustacean that grows to about six centimetres in length and weighs no more than two grams. Individually, it is unremarkable. Collectively, however, it is one of the most abundant animals on Earth. Estimates of the total weight of all the Antarctic krill in the Southern Ocean vary widely, because the animals are patchily distributed and difficult to count, but most run to hundreds of millions of tonnes, a figure comparable to the combined weight of all the people on the planet.

Krill are almost transparent, although their shells have a reddish tinge, and they possess small organs that can produce light, the purpose of which is still not fully understood. They feed mainly on phytoplankton, the microscopic algae that drift near the surface of the sea. To catch them, a krill uses its front legs, which are lined with fine hairs and together form a kind of basket. As the animal swims forward, water is pushed through this basket, and the algae trapped inside are then moved towards the mouth. In winter, when there is little light and phytoplankton is scarce, krill turn to other sources of food, including the algae that grow on the underside of the sea ice.

Krill are also remarkably good at surviving without food. In laboratory experiments, individuals have lived for many months without eating. Like other crustaceans, they grow by shedding their hard outer shell, a process known as moulting, but unlike most animals, they can continue to moult when food is short, producing a smaller shell each time. In other words, a starving krill can actually shrink, using its own body as a store of energy. Antarctic krill are also long-lived for such small creatures, with a lifespan in the wild that is thought to be around five or six years.

One of the krill's most striking habits is its tendency to gather in enormous swarms, which can contain thousands of individuals in a single cubic metre of water and may stretch for several kilometres. Swarming probably offers some protection from predators, since an individual is less likely to be caught when surrounded by others, but it also makes krill an easy target for animals that feed by swallowing large volumes of water at once. In many areas, swarms sink to deeper water during the day and rise towards the surface at night, a pattern that may help krill to avoid predators that hunt by sight.

Breeding takes place during the southern summer. A female may release several thousand eggs at a time, and she can do this more than once in a single season. The eggs sink as they develop, often to a depth of several hundred metres or more, before hatching. The larvae then begin a long swim back towards the surface, where they must find food quickly. Young krill depend heavily on sea ice during their first winter, sheltering beneath it and feeding on the algae that grow there. For this reason, scientists regard the extent of winter sea ice as one of the most important influences on how many young krill survive.

A remarkable number of animals rely on krill for food. Baleen whales such as blue whales and humpbacks filter huge quantities of them from the water; a single blue whale may eat several tonnes of krill in a day during the feeding season. Krill also make up a large part of the diet of penguins, many seabirds, squid and fish. Even the crabeater seal, despite its name, feeds almost entirely on krill, straining them from the water through teeth with an unusually complex shape. Krill also play a part in regulating the planet's climate. The algae they eat absorb carbon dioxide at the surface, and when krill produce waste pellets, which sink rapidly, some of that carbon is carried down to the deep ocean, where it may remain for centuries.

Since the 1970s, krill have also been harvested commercially, mainly for use as food for farmed fish and in dietary supplements. The fishery is managed by an international body, the Commission for the Conservation of Antarctic Marine Living Resources, which sets catch limits that are intended to take account of the needs of predators as well as the size of the krill population itself. At present, the annual catch is only a small fraction of the estimated total stock. However, concern has grown that fishing is concentrated in a few areas close to penguin colonies, and that warming waters and shrinking sea ice in parts of the Southern Ocean may already be reducing krill numbers in some regions.