Crustacean

Crustaceans are a large, diverse group of arthropods constituting the subphylum Crustacea. They include crabs, lobsters, shrimp, crayfish, barnacles, copepods, amphipods, isopods, ostracods, and krill. Most crustaceans inhabit aquatic environments—marine, freshwater, and brackish waters—though a few groups are terrestrial or live in moist habitats.

Anatomy and development: Crustaceans typically have a segmented body with a head and thorax often fused into a cephalothorax, and an abdomen. They possess a chitinous exoskeleton that they molt to grow. Many have biramous appendages—two-branched limbs specialized for feeding, locomotion, and respiration. Respiration mechanisms vary: gills in aquatic forms, cutaneous or branchial structures in others. The nervous and circulatory systems are typical of arthropods, with an open circulatory system. Reproduction is usually sexual, with separate sexes, and many groups exhibit complex life cycles that include free-living larval stages such as the nauplius or zoea, while others show direct development.

Classification and diversity: Crustaceans are traditionally divided into several classes, including Malacostraca (the largest group, containing decapods such as crabs, lobsters, and shrimp, as well as krill and many isopods), Branchiopoda (water fleas, tadpole shrimps, brine shrimps), and Maxillopoda (barnacles, copepods, ostracods). The lesser-known groups Remipedia and Cephalocarida are also included in some classifications.

Ecology and human relevance: Crustaceans occupy a wide range of ecological roles, including grazers, detritivores, and prey for fishes, birds, and mammals. They are important in many aquatic ecosystems and underpin commercial fisheries and aquaculture. Some species are pests or parasites, and others are studied as models in physiology, development, and evolution.