Crustaceans are animals with hard shells. Crabs, lobsters, shrimp, and wood lice are among some of the best-known crustaceans.

Where crustaceans live

Crustaceans are found throughout the world, mainly in water. There are freshwater and saltwater crustaceans. For example, some species live in the open waters of lakes and oceans. Other species live at the bottom of the sea, in places as deep as 32,808 feet (10,000 meters). Some can be found in rocky, sandy, and muddy areas. Some are so small that they live in the spaces between sand grains. Others tunnel in the leaves of seaweeds or into wooden structures.

Crustaceans inhabit even high mountain lakes at altitudes of 16,404 feet (5,000 meters). A number of crabs are amphibious, meaning that they live in both land and water. The most terrestrial, or land-inhabiting, crustaceans are the wood lice. Most of them live in damp places, though a few species can survive even in deserts. Some crustaceans are found among damp leaves on forest floors, particularly in the tropics.

Physical features

The basic crustacean body consists of a number of segments. At the front end of the body there is a region called the acron, which is the head on most crustaceans. The tail ends of crustacean bodies differ greatly in form. In many crustaceans the tail is short, but in some it may be as long as the rest of the body.

The shell is a common crustacean feature. This shell is occasionally shed because its hard structure does not allow growth. Adult crustaceans have two pairs of antennae. Crustaceans may also have other features such as legs or gills.

The largest crustaceans include the American lobster, which can reach a weight of 44 pounds (20 kilograms), and the giant Japanese spider crab, which has legs that can span up to 12 feet (3.7 meters). At the other end of the scale, some water fleas reach lengths of less than 0.009 inch (0.25 millimeter).

Life cycle

In crustaceans the male and female sexes are normally, but not always, separate. The marine crustacean called a barnacle has male and female reproductive organs in one individual. Some types of shrimp change their sex during their lifetime. They begin life as males but change into females after about 13 months.

Some crustaceans lay their eggs freely in the water. Others carry the eggs attached to part of their body until the eggs are ready to hatch. In some crustaceans, the young resemble adults when they hatch from the eggs. This trait is found in many species of crabs and crayfish.

Uses

Crustaceans are valuable sources of food. People in many parts of the world capture shrimp, prawns, lobsters, crayfish, and crabs. In Japan, barnacles are allowed to settle and grow on bamboo stakes. Later these barnacles are scraped off and crushed for use as fertilizer.