Crustaceans are a diverse group of invertebrates, encompassing familiar creatures like crabs, lobsters, and shrimp, alongside less known species such as barnacles and copepods. These arthropods predominantly inhabit aquatic environments, thriving in marine and freshwater ecosystems, though some have adapted to terrestrial life. Understanding the unique characteristics that define crustaceans helps clarify their ecological success.
Key Physical Characteristics
A defining feature of crustaceans is their hard, protective exoskeleton, composed of chitin and often reinforced with calcium carbonate. This rigid outer covering provides support and defense. Because the exoskeleton does not grow, crustaceans must periodically shed it through molting to increase in size.
Crustaceans exhibit a segmented body plan, divided into two main regions: a cephalothorax and an abdomen. The cephalothorax is a fused head and thorax, covered by a single, large shell called a carapace. Each body segment, or somite, has the potential to bear a pair of appendages.
Their appendages are distinctive, often biramous, meaning they have two main branches. Crustaceans possess two pairs of antennae, which are sensory organs. Beyond these, they have specialized mouthparts like mandibles and maxillae, used for chewing and handling food. Other biramous appendages, such as pereiopods (walking legs) and pleopods (swimmerets), are adapted for locomotion and feeding.
Life Cycle and Respiration
Crustaceans breathe using gills. These gills are located on their appendages or within specialized gill chambers beneath the carapace. Oxygen from the water diffuses across the gill surfaces into the crustacean’s circulatory system. Some smaller crustaceans can exchange gases directly through their body surface.
Molting, or ecdysis, is important for crustacean growth. As the animal grows, a new, soft exoskeleton forms beneath the old one. The crustacean then takes in water or air, causing the old shell to split, allowing the animal to wriggle out. This leaves the new, soft exoskeleton vulnerable until it hardens.
The life cycles of crustaceans involve distinct larval stages that differ from the adult form. A characteristic larval stage for crustaceans is the nauplius larva. This tiny, planktonic larva possesses three pairs of appendages (antennules, antennae, and mandibles) used for swimming and feeding, and has a single, median eye. As the nauplius grows, it undergoes a series of molts, developing more segments and appendages before transforming into a juvenile and eventually an adult.