Found in every ocean, krill form an important part of the diet of whales, seals, fish, squid and many other marine species, playing a vital role in the global ecosystem. These crustaceans, also called euphausiid shrimps, are considered the most populous species on Earth. They form swarms of millions near the ocean surface. In an effort to help preserve this food resource, the United States has banned krill fishing along the Pacific coastline.
Krill spend their entire lives floating in oceans. After hatching from eggs they pass through larval and juvenile stages before becoming adults, molting their shells and swelling larger while their new shells are soft. During molting they add appendages and segments. Unlike many shelled animals, krill may shrink when food is scarce, making it difficult to determine age -- they are thought to live 10 years. Krill feed on phytoplankton, which are microscopic plantlike organisms, and algae that grow under sea ice.
Measuring 2.5 inches long and weighing 0.7 ounce, krill look similar to shrimp, with large, black eyes, segmented tails and featherlike legs, which are used for swimming. Salmon flesh turns pink from eating their red-pigmented shells. Below their heads are modified legs that function like a basket, collecting food as they swim along. Since the krill is transparent, its digestive system is visible. Due to the krill's plant diet, it usually looks green.
Krill swarm in concentrations of up to 100,000 per cubic yard, which gives individual krill some protection from smaller predators. Larger animals, such as whales, scoop up thousands at a time. Krill tend to swarm in spring and summer, feeding at the ocean surface during the day and returning to deeper, colder regions at night to conserve energy. When approached by predators, krill react by scattering in all directions. Groups of krill can simultaneously molt their shells, possibly to confuse pursuers.
Since the 1970s, for as yet undetermined reasons, krill numbers worldwide have fallen. More recent Antarctic sea ice loss adds to the decline. Along the California coast during 2005 and 2006, unusual weather patterns reduced local krill populations, severely impacting animals dependent on them for food. Seabirds abandoned their nests, unable to feed their young, and blue whales didn't enter the famine-stricken waters. There is little doubt that ecosystems reliant on krill will be seriously affected by further declines in krill populations.
- whale shark image by Earl Robbins from Fotolia.com