What Are Crustaceans?
The word “crustacean” comes from the Latin crusta meaning a hard covering shell or crust. Apart from crabs, lobsters and shrimps, there are thousands of different crustaceans. They live in the sea, except for a few species such as the common woodlouse.
Crustaceans differ greatly in size and shape. Many of them pass through remarkable changes of form (metamorphoses) before reaching the adult stage. All of them, however, have bodies and limbs which are divided into segments. All, too, are covered with a tough, hard shell, which is pliable at the joints, so that the creature can move its limbs. This coat, or cuticle, cannot grow to fit its wearer.
As its owner grows, it is split and cast off. A new coat, still soft and pliable, has been forming underneath, and may take several days before it hardens into a truly protective shell. During this time the creature is defenseless and may fall an easy prey to any enemy.
Vast numbers of crustaceans live in the oceans. They provide food for many kinds of fish and also for the largest living mammals, the whales. The majority of crustaceans are aquatic, living in either marine or freshwater environments, but a few groups have adapted to life on land, such as terrestrial crabs, terrestrial hermit crabs, and woodlice. Marine crustaceans are as ubiquitous in the oceans as insects are on land.
The majority of crustaceans are also motile, moving about independently, although a few taxonomic units are parasitic and live attached to their hosts (including sea lice, fish lice, whale lice, tongue worms, and Cymothoa exigua, all of which may be referred to as “crustacean lice”), and adult barnacles live a sessile life – they are attached headfirst to the substrate and cannot move independently.
Some branchiurans are able to withstand rapid changes of salinity and will also switch hosts from marine to non-marine species. Krill are the bottom layer and the most important part of the food chain in Antarctic animal communities. Some crustaceans are significant invasive species, such as the Chinese mitten crab, Eriocheir sinensis and the Asian shore crab, Hemigrapsus sanguineus.