crustaceans

Crustaceans are a diverse group of mainly aquatic arthropods that form the subphylum Crustacea within the phylum Arthropoda. They include decapods such as shrimp, crabs, and lobsters; isopods and amphipods; copepods and ostracods; barnacles and krill, among others. Most crustaceans have a rigid exoskeleton, a segmented body, and paired appendages that are often biramous. They typically possess two pairs of antennae and specialized mouthparts. The body is divided into a cephalothorax and an abdomen in many species, sometimes shielded by a carapace. Respiration varies: gills in most aquatic forms; some terrestrial crustaceans exchange gases across moist surfaces or through specialized structures. The circulatory system is open, with a heart pumping hemolymph.

Life cycles are diverse and usually involve sexual reproduction. Many crustaceans have larval stages such as the nauplius, zoea, or nauplius-like forms, while others undergo direct development. Reproduction strategies range from free-spawning to brooding in some isopods and barnacles.

Habitat and ecology: crustaceans occupy nearly every aquatic environment and, in some cases, moist terrestrial habitats. They play key roles as scavengers, detritivores, herbivores, and predators, and form important prey for many species. Barnacles are sessile filter feeders; krill are major planktonic grazers; decapods include prominent commercial species.

Economic and conservation context: crustaceans support large global fisheries and aquaculture. They face pressures from overfishing, habitat degradation, and climate change. Biodiversity within Crustacea is substantial, but many groups remain poorly understood.