Crustacean: biology, diversity, ecology, and human importance

Anatomy and physiology

Crustaceans share the typical arthropod features of a hard exoskeleton, segmented body, and jointed appendages. The body is commonly divided into head, thorax and abdomen, although many taxa show fusion of regions. They characteristically bear two pairs of antennae and mandible-equipped mouths; the mouthparts include mandibles and other modified appendages. Eyes are often compound (eyes) and, in aquatic species, respiration usually occurs through gills. Some terrestrial lineages, such as certain land crabs, have evolved respiratory modifications that serve a similar function to lungs. Growth is achieved by moulting the exoskeleton periodically.

Life cycle and reproduction

Most crustaceans have separate sexes, though some species are hermaphroditic. Females commonly brood eggs which hatch into free-living larval stages, often planktonic and morphologically distinct from the adult. Larval development may include several recognizable stages before metamorphosis into a juvenile. In many groups the young disperse widely as plankton; in others the juvenile settles quickly and adopts a benthic or sessile lifestyle, as happens with barnacles. Several crustaceans are parasitic in habit — for example some species known as fish lice attach to hosts — and these species can show highly modified life cycles.

Size and form across crustaceans vary dramatically. Tiny ectoparasites and planktonic copepods may measure a fraction of a millimetre, while large decapods reach impressive dimensions: the Japanese spider crab has one of the longest leg spans among living arthropods. Other common groups include isopods, amphipods, and the abundant marine copepods. A short list of representative taxa illustrates the group’s variety:

• Crabs — typically benthic, often with broad carapaces.
• Lobsters — large decapods prized in fisheries.
• Crayfish — freshwater relatives of lobsters.
• Shrimp and prawns — important swimmers and fishery species.
• Krill — small, swarming crustaceans crucial to marine food webs.
• Barnacles — sessile, attached to hard substrata.

Ecology, behaviour and habitats

Most crustaceans are aquatic, and the majority are marine, occupying habitats from intertidal zones to the deep sea. Many are benthic, walking on or burrowing into sediments; others can swim and form important pelagic populations. Some groups have colonised land: terrestrial isopods (woodlice) and several species of land-dwelling crabs show adaptations to life away from water. Barnacles famously become fixed to rocks and other surfaces on the sea shore, while planktonic crustaceans such as krill and copepods represent a huge biomass that sustains many fish, birds and whales.

Human use, fisheries and conservation

Crustaceans are economically significant. More than ten million tonnes are harvested or produced by aquaculture annually, with most of that yield comprising shrimp and prawns — products of global fishery and farming industries. Beyond food, crustaceans serve as model organisms in research, as indicators of environmental change, and as vectors or hosts in disease cycles. Overexploitation, habitat loss and climate-driven changes to ocean chemistry threaten many populations, prompting management and conservation measures worldwide.

From an evolutionary perspective crustaceans preserve an informative fossil record that helps reconstruct the early diversification of arthropods, tracing back to the Cambrian. Debates about higher-level classification continue, but the fossil and molecular evidence together underline the long and varied history of this successful group of animals within the broader arthropod and superphylum discussions among the world’s animal phyla.