The Crustacea is the largest subphylum of the phylum Arthropoda. The Crustacea includes the lobsters, crabs, copepods, shrimps and barnacles. Its members are characterised by a head with two pairs of sensitive antennae and three pairs of specialised mouthpart appendages which process food before it enters the mouth. They often have heavily armoured carapaces with a rigid exoskeleton made of chitin and other proteins. To grow, crustaceans must moult or shed their exoskeleton and produce a new one.
The Crustacea are a highly diverse group, including mobile and sessile forms that have a juvenile planktonic life stage. The majority of crustaceans are mobile and often travel great distances whilst feeding. They are generally scavenging organisms but some species are carnivorous and use their clawed appendages to crush and tear apart their prey. Highly specialised species such as barnacles are immobile and live attached to hard substrate within an enclosed series of calcium carbonate plates. Crustaceans are nearly as diverse as polychaete species on cold-water coral reefs.