What Is the Meaning of Crustacean?

Crabs are common crustaceans.

Crustaceans are among the largest groups of animals on the planet. These common marine creatures, called arthropods, make up about 50,000 species that range widely in size, shape and environment. From shrimp to crabs to lobsters to barnacles to crayfish to krill, crustaceans date back hundreds of millions of years, predating the dinosaurs, and have several things in common that put them in their immense group.

Physical Structure

Crustaceans are made up of numerous body segments, the head (cephalon), the thorax and the abdomen (pleon). Occasionally, however, the head and thorax will be fused into one segment known collectively as the cephalothorax. All of these body segments may be covered by a large hard shell known as a carapace, with all of the animals' meaty parts protected by a hard exoskeleton. Some crustaceans have antenna, giving them an insect-like appearance, while others such as shrimp and most lobsters have tail fans that look like paddles; these appendages allow them to move quickly through the water. Most crustaceans also have several legs, usually at least six, although many such as lobsters and shrimp have more legs. Nearly all crustaceans have two eyes. Crustaceans also vary widely in size, with most krill measuring only an inch or so long yet spider crabs measuring up to 13 feet in diameter.

Ecosystem

A majority of crustaceans are free-swimming marine organisms, meaning that they inhabit saltwater, freshwater and brackish water environments from major rivers, oceans, bays and seas to lakes, ponds and streams. Crabs and lobsters very seldom swim and instead walk along the seafloor, usually in the shallows, and live among rocks and reefs. Some crustaceans can be found in very deep waters, crawling along the bottom. Barnacles use an organic cement to attach their carapaces to the rocks or the bottom of boats while krill travel in enormous schools resembling red clouds that are often feasted on by hungry fish or whales.

Parasitic Crustaceans

Some crustaceans are parasitic, meaning that they live on or in a larger organism and extract nutrients from it. These include sea lice, whale lice, tongue worms, and fish lice, most of which are simply referred to as "crustacean lice" collectively.

Terrestrial Life

Some crustaceans have adapted to terrestrial life. Hermit crabs are a classic example of crustaceans that can live entirely out of the water. These crabs are often sold as pets to children in pet stores and can be kept in small enclosures with rocky or sandy gravel. Other crustaceans that have adapted to a terrestrial lifestyle include land crabs such as the enormous coconut crabs and woodlice.

Molting

In order to grow, every crustacean must shed its hard exoskeleton and grow a larger one. When a crustacean outgrows its shell, it will go through a molting process where it may be very vulnerable; it has to sit still for several hours while it wriggles free of its hard carapace, and then when it is free, its body will be soft for several days before a new exoskeleton grows. Hermit crabs, because they live in shells, must not only molt their carapace but find a larger shell to inhabit, forcing them to leave the safety of their mobile homes.

References

• Crustacea
• Biodiversity Explorer: Crustacea
• NHM: Cephalothorax
• Teara: Crabs, Crayfish, and Other Crustaceans
• Aquarium: Japanese Spider Crabs