KRILL: Euphausiacea

PHYSICAL CHARACTERISTICS

Krill are shrimplike in appearance and measure 1.57 to 5.9 inches (40 to 150 millimeters) in length. Their red-spotted bodies are transparent, with their internal organs visible from the outside. They have two pairs of antennae, both of them branched, or biramous (BY-ray-mus). Their large compound eyes are set on stalks. Each compound eye has many individual lenses. The head and segmented thorax are closely joined together, forming a region known as the cephalothorax (SEH-feh-lo-THOR-acks). The cephalothorax is covered by a shieldlike carapace that does not cover the feathery gills located on the bases of some legs.

The thorax has eight pairs of biramous limbs. Thoracic (thuh-RAE-sik) limb pairs six through eight form a netlike structure that is used to strain food out of the water. Krill do not have any thoracic limbs associated with the mouth. The abdomen has six segments, plus a flaplike tail segment, or telson. The first five segments each have a pair of paddlelike limbs, or pleopods (PLEE-oh-pawds). These special pleopods are used for swimming and are also called swimmerets (SWI-meh-rehts).

Many species are called "light-shrimp" because they have light-producing tissues in their eyestalks, legs, and abdomen. These displays of yellowish green or blue light are probably used to locate mates or confuse predators.

GEOGRAPHIC RANGE

Krill are found in all oceans.

HABITAT

Krill swim in water off the coast, out in the open ocean, and around polar ice. Most krill feed and mate close to the surface, but a few species live at depths of up to 16,404 feet (5,000 meters).

DIET

Krill eat free-floating organisms known as plankton, which are microscopic plants and animals.

BEHAVIOR AND REPRODUCTION

Krill filter their food from the water using their bristly thoracic legs like a basket. Water is squeezed through the basket, leaving the plankton behind. The food is moved toward the mouth with the other legs. Krill grow by molting, or shedding their external skeletons. They molt and grow throughout their lives. When food is scarce, their bodies actually shrink with each molt. Smaller bodies require less food (energy) to maintain. When threatened, krill sometimes molt instantly, leaving a empty external skeleton behind as a decoy.

Eighteen species of krill form massive, shapeless swarms that sometimes stretch the length of several city blocks. These swarms usually spend the day at lower depths to avoid being eaten by other animals. Many fish, sea birds, and marine mammals regularly prey on krill. Krill rise to the surface of the ocean to feed. Although krill can use their swimmerets to move around for short distances, they are mostly dependent on ocean currents to cover large distances. They also have the ability to adjust the buoyancy (BOI-en-see) of their bodies so they can rise or sink to different levels in the water.

Krill require both males and females to reproduce. Reproduction only takes place when there is plenty of food. The male produces sperm packets and uses his legs to transfer them to the opening of the female's reproductive organs. The female stores the sperm in a special pouch until she is ready to lay her eggs. The eggs are fertilized as they leave her body.

After hatching, krill pass through several juvenile stages in a few months. They hatch as larvae (LAR-vee) that have only antennae and mouthparts as appendages. The first pair of antennae, or antennules, is used for swimming. This stage is followed by larval stages that have thoracic limbs used for swimming. These are followed by a larval stage in which the antennae are no longer used for swimming. With each molt the larvae increase in size and add more body segments and appendages until they reach adulthood.

Adult krill lose their male and female characteristics after the summer mating season and return to a more juvenilelike form. They regain their adult characteristics in spring in preparation for the new mating season. Krill live between two and 10 years, depending on the species.

KRILL AND PEOPLE

Krill are fished commercially and are used as feed for fish, especially on farms that raise salmon and yellowtail tuna. In Japan and other parts of the world, the protein-rich bodies of krill are processed and added to foods for human consumption.

THE KRILLING FIELDS

Blue whales measure up to 100 feet (meters) in length and weigh in at 300,000 pounds (136,080 kilograms). Hungry whales swim through swarms of krill, which are only one one-thousandth of their size, with mouths and throats wide open, swallowing as much as 50 tons (45.36 metric tons) of water in one gulp. The krill are screened out by comblike plates and swallowed. Blue whales may eat 3 to 4 tons (2.72 to 3.63 metric tons), or 40,000,000 krill every day.

CONSERVATION STATUS

No species of krill are listed by the World Conservation Union (IUCN) as endangered or threatened. Still, over-fishing and climate change could threaten krill populations and the animals that depend on them.

NORTH PACIFIC KRILL (Euphausia pacifica): SPECIES ACCOUNTS

Physical characteristics: North Pacific krill measure about 0.75 inches (19 millimeters) and weigh about 0.003 ounces (0.1 grams).

Geographic range: They are found in the North Pacific Ocean, from North America to Japan.

Habitat: North Pacific krill live in open water at the edge of the continental shelf or just beyond. They are found at the surface to depths of about 984.24 feet (300 meters).

Diet: They eat plankton.

Behavior and reproduction: North Pacific krill rise to the surface to feed at night. They sink to lower levels during the day and seldom feed there, even if food is abundant. Males take part in one breeding season and live about two years. Females live longer and take part in two breeding seasons.

North Pacific krill and people: This species is fished commercially in the United States, Canada, and Japan.

Conservation status: North Pacific krill are not considered endangered or threatened. ∎

ANTARCTIC KRILL (Euphausia superba): SPECIES ACCOUNTS

Physical characteristics: Adult Antarctic krill measure up to 2.5 inches (65 millimeters) in length and weigh up to 0.07 ounces (2 g). Their gut is green due to their diet of free-floating, microscopic plant organisms or phytoplankton.

Geographic range: They live in the oceans surrounding Antarctica.

Habitat: Antarctic krill are found from the surface to depths of 1,640 feet (500 meters). Their massive swarms are found under and around the edges of sea ice.

Diet: They eat mostly phytoplankton, but will also feed on small crustaceans.

Behavior and reproduction: Antarctic krill swarms may reach densities of 30,000 krill per 35 cubic feet (0.99 cubic meters). Some swarms are made up almost entirely of a single sex or age group. They rise to the surface at night to feed and sink to lower depths during the day.

Females lay several batches of eggs during the 5-month breeding season. Each batch may have up to 10,000 eggs. The eggs sink to the bottom. The larvae spend 10 days rising back to the surface to feed and develop. They reach maximum size in three to five years and may live for a total of seven years.

Antarctic krill and people: This krill species is fished commercially by several countries. Nearly 100,000 tons (90,000 metric tons) are caught each year. They are processed as food for humans, domestic animals, farm-raised fish, and sport fishing bait.

Conservation status: Antaractic krill are not considered endangered or threatened. However, krill fishing limits were set in 1982 by the Convention on the Conservation of Antarctic Marine Living Resources. Part of the Antarctic Treaty System, the Convention is intended to encourage the recovery of whale populations. ∎

FOR MORE INFORMATION

Books:

Brusca, Richard C., and Gary J. Brusca. Invertebrates. Sunderland, MA: Sinauer Associates, 2003.

Periodicals:

Brierly, A. S., et al. "Antarctic Krill Under Sea Ice: Elevated Abundance in a Narrow Band Just South of Ice Edge." Science 295 (March 8, 2002): 1890-1892.

Hamner, W. M. "Krill-Untapped Bounty From the Sea?" National Geographic 165, no. 5 (May 1984): 626-643.

Nakagawa, Y., Y. Endo, and H. Sugisaki. "Feeding Rhythm and Vertical Migration of the Euphausiid Euphausia pacifica in Coastal Waters of North-eastern Japan During Fall." Journal of Plankton Research 25, no. 6 (2003): 633-644.

Web sites:

Krill.http://www.enchantedlearning.com/subjects/invertebrates/crustacean/Krillprintout.shtml (accessed on February 17, 2005).