Decapod in the context of "Swimmeret"

Crustaceans (from Latin word "crustacea" meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (/krəˈsteɪʃə/), a large, diverse group of mainly aquatic arthropods including the more familiar decapods (shrimps, prawns, crabs, lobsters and crayfish), seed shrimps, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, opossum shrimps, amphipods and mantis shrimp. The crustacean group can be treated as a subphylum under the clade Mandibulata. It is now well accepted that the hexapods (insects and entognathans) emerged deep in the crustacean group, with the completed pan-group referred to as Pancrustacea. The three classes Cephalocarida, Branchiopoda and Remipedia are more closely related to the hexapods than they are to any of the other crustaceans (oligostracans and multicrustaceans).

The 67,000 described species range in size from Stygotantulus stocki at 0.1 mm (0.004 in), to the Japanese spider crab with a leg span of up to 3.8 m (12.5 ft) and a mass of 20 kg (44 lb). Like other arthropods, crustaceans have an exoskeleton, which they moult to grow. They are distinguished from other groups of arthropods, such as insects, myriapods and chelicerates, by the possession of biramous (two-parted) limbs, and by their larval forms, such as the nauplius stage of branchiopods and copepods.

A shrimp (pl.: shrimp (US) or shrimps (UK)) is a common name typically used for crustaceans with an elongated body and a primarily swimming mode of locomotion – usually decapods belonging to the Caridea or Dendrobranchiata, although some crustaceans outside of this order are also referred to as "shrimp". Any small crustacean may also be referred to as "shrimp", regardless of resemblance.

More narrow definitions may be restricted to Caridea, to smaller species of either of the aforementioned groups, or only the marine species. Under a broader definition, shrimp may be synonymous with prawn, covering stalk-eyed swimming crustaceans with long, narrow muscular tails (abdomens), long whiskers (antennae), and slender, biramous legs. They swim forward by paddling the swimmerets on the underside of their abdomens, although their escape response is typically repeated flicks with the tail, driving them backwards very quickly ("lobstering"). Crabs and lobsters have strong walking legs, whereas shrimp typically have thin, fragile legs which they use primarily for perching.

Crabs are decapod crustaceans, either the Brachyura (the "true crabs") or various groups within the closely related Anomura, characterised by having a heavily armoured shell, their tail segments concealed under the body, the ability to run sideways, and the habit of hiding in rocky crevices. They do not form a single natural group or clade, but have convergently evolved multiple times from the ancestral decapod body plan through the process of carcinisation. As a group they are thus polyphyletic.

Crabs vary in size from the pea crab, a few millimeters wide, to the Japanese spider crab, with a leg span up to 4 m (13 ft). Many crabs are free-living marine omnivores; others are specialist herbivores or carnivores, while some are parasitic. A substantial number of species are adapted to freshwater or other non-marine habitats.

Brachyura (from Ancient Greek βραχύς (brakhús), meaning "short", and οὐρά (ourá), meaning "tail"), is an infraorder of decapod crustaceans comprising the true crabs. They typically have a very short projecting tail-like abdomen, usually hidden entirely under the thorax. Their exoskeleton is often thickened and hard. They generally have five pairs of legs, and they have "pincers" or "claws" on the ends of the frontmost pair, scientifically termed the chelae. They are present in all the world's oceans, in freshwater, and on land, often hiding themselves in small crevices or burrowing into sediment. True crabs are omnivores, feeding on a variety of food, including a significant proportion of algae, as well as detritus and other invertebrates. True crabs are widely consumed by humans as food, with over 1.5 million tonnes caught annually.

True crabs first appeared in the fossil record during the Jurassic period, around 200 million years ago, achieving great diversity by the Cretaceous period; around 7,000 extant species in 96 families are known. The true crabs are the largest of the crab decapod groups with carcinised body forms. Smaller groups include the porcelain crabs, king crabs, and hairy stone crabs which have independently evolved similar forms and lifestyles.

Anomura (sometimes Anomala) is a group of decapod crustaceans, including hermit crabs and others. Several groups such as the king crabs and porcelain crabs within the Anomura have independently undergone carcinisation, acquiring the armoured crab body plan with a concealed tail. It is the sister group to the "true crabs" or Brachyura.