How Do Krill Move?

The blue whale is the world's largest animal, and depends on the tiny krill to survive.

Krill are minute crustaceans that inhabit every ocean in the world. Although they average only 4 to 5 cm in length, krill provide the basis for an entire food web. Numerous animals, from tiny fish to enormous whales, feed primarily on krill.

Locomotion

Like other crustaceans and insects, krill have three main body parts: a head, a thorax and an abdomen. The thorax bears specialized legs called perepods, which are used to capture food from the water. The abdomen (think of it like a shrimp's "tail") also has appendages, and these are known as pleopods. The pleopods are setose, or hairy, and the krill use these as paddles for swimming.

Motivation

In a 1986 article in "Nature," J.Z. Gliwicz explains that animals like krill move for two main reasons: to find food and to avoid being eaten. Krill feed on phytoplankton, microscopic organisms that float freely in the ocean, especially near the surface. In turn, krill are eaten by a vast array of ocean life including fish, birds, seals and whales, so predation is a constant threat.

Migration

To accomplish both food-finding and predation avoidance, krill undergo a vertical migration. This means that every day krill journey from deep water to the surface, then back to the deep. The reason for this is that both predators and prey are most common at the surface. Krill hide in deep water during the daytime when predators are most active, then rise up under the cover of darkness to capture their food. According to a 1999 article in "Marine Ecology Progress Series" by G.A. Tarling, J. Cuzin-Roudy and F. Buchholz, one particular species of krill can travel to depths of up to 800 m.

Swarming

In addition to migration, krill exhibit a behavior called swarming. This means that krill gather together in large, tight groups. These swarms may have densities of up to 60,000 krill per cubic meter, and cover hundreds of square kilometers of ocean. Although krill swarming is not completely understood, it is generally believed that this behavior may help to confuse small predators.

References

• "Invertebrate Zoology": Euphausiacea; E.E. Rupert, R.S. Fox and R.D. Barnes; 2004
• Cool Antarctica: Antarctic Krill
• "Nature": Predation and the Evolution of Vertical Migration in Zooplankton; J.Z. Gliwicz; 1986
• "Marine Ecology Progress Series": Vertical Migration Behavior in the Northern Krill (Meganyctiphanes norvegica) is Influenced by Moult and Reproductive Processes; G.A. Tarling, J. Cuzin-Roudy and F. Buchholz; 1999