Krill

I

Introduction

Krill, common name for small, shrimplike crustaceans that often live as plankton in the open oceans. Krill can swim in vast swarms that reach several miles across. They are found worldwide but are most abundant in the polar regions and where cold currents create upwellings of nutrients. These small crustaceans feed on phytoplankton such as diatoms and algae. Krill are an important link in the marine food web, especially in Antarctic waters, and are a major food source for baleen whales, as well as for many birds. Threats to krill include effects of global warming and large-scale harvesting by the fishing industry.

II

Description

Scientists have identified about 90 species of krill, which range in length from 8 to 70 mm (0.3 to 2.8 in). Krill have two pairs of long antennae and two compound eyes on stalks. They have at least 11 pairs of legs, with each leg split into two branches at the tip. Six (or sometimes eight) pairs of legs on the thorax have feathery structures used like a net for collecting tiny phytoplankton from seawater. Five pairs of legs on the abdomen are pleopods or swimmerets that have small paddles used for swimming.

Except for one deep-water species, krill are bioluminescent and have photophores (light-producing organs) on parts of their bodies that can emit a strong blue-green light that may help the creatures congregate and spawn.

III

Reproduction and Growth

Most species of krill spawn their eggs directly into the ocean, although some species carry their eggs in a brood pouch. A single female may release thousands of eggs at a time and may spawn multiple times in a season.

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The larvae usually float as plankton. As krill grow, they molt their exoskeletons. Adult krill have the unusual ability to shrink in size if food is scarce. Krill can live as long as six years, and in the cold Antarctic waters, up to 10 years.

IV

Feeding and Role as Food Source

In many regions, species of krill rise closer to the surface at night to feed and descend deeper into the water during the day. In polar regions, adult krill spend winter months at lower depths, and feed near the surface on phytoplankton in the spring and summer. Because of their ability to swim freely, most adult krill are not truly zooplankton—animal plankton that can only move with currents. Instead, krill are sometimes called part of the micro-nekton—tiny animals that swim independent of currents.

Krill in the Antarctic feed on algae that grow on sea ice, the frozen sea water that forms in the winter and partially melts during the summer. The algae live in brine channels that form as seawater freezes. During winter, larval krill may feed on ice algae and take refuge from predators under the sea ice. When the sea ice begins to melt in spring, adult krill feed on the huge quantities of algae in preparation for reproduction.

Krill are eaten by fish, birds, and especially baleen whales such as blue whales, humpback whales, fin whales, and minke whales. Baleen whales can expand their throats to take in enormous gulps of seawater containing swarms of krill. The whales force the water through baleen filter plates that line their upper jaws using their giant tongues. A blue whale may consume as much as 2,000 kg (4,400 lb) of krill in one feeding. Shoals (swarms) of the crustaceans have densities of 20 kg per cu m (35 lb per cu yd); some species remain near the surface and others are found as deep as 2,000 m (6,500 ft).