Krill: Euphausiacea

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.