Crustaceans are a diverse group of arthropods, including familiar creatures like crabs, lobsters, and shrimp, as well as barnacles and copepods. They inhabit a wide array of environments, predominantly aquatic, found in marine and freshwater ecosystems, with some species adapting to terrestrial life. Their physical characteristics allow them to thrive in these varied habitats.
Defining External Features
Crustaceans are recognized by their hard, protective outer covering, known as an exoskeleton. This rigid casing provides structural support and defense. Since it does not grow, crustaceans must periodically shed their old exoskeleton through molting (ecdysis). They emerge with a new, larger, temporarily soft shell that then hardens.
The head region features several sensory and feeding appendages. Two prominent pairs of antennae, often called antennules and antennae, are equipped with specialized sensors that detect touch, water movement, vibrations, and chemical cues, such as smell and taste. Many crustaceans also possess compound eyes, typically on movable stalks, providing a wide field of vision. These eyes are composed of numerous individual light-sensing units called ommatidia.
Gas exchange in aquatic crustaceans primarily occurs through gills, often located under the carapace or on modified appendages. Water currents, created by other appendages, ensure a constant flow of oxygenated water over the gill surfaces. Specialized mouthparts, including a pair of mandibles and two pairs of maxillae, are adapted for processing food. Mandibles are used for biting and grinding, while maxillae assist in manipulating food particles, filtering, and grooming.
Segmented Bodies and Specialized Limbs
A crustacean’s body is typically divided into distinct regions, or tagmata: a head, thorax, and abdomen. In many species, the head and thorax fuse to form a cephalothorax, often covered by a single, large carapace. The abdomen, or pleon, typically consists of six segments and ends in a tail fan, which includes the telson and uropods.
Crustaceans have multiple pairs of jointed appendages, with each body segment potentially bearing a pair. Many limbs are biramous (two-branched), though some are uniramous (single-branched). These limbs are highly specialized for various functions. Walking legs, or pereiopods, are on the thorax for surface locomotion. Swimming legs, or pleopods, are typically on the abdomen, facilitating movement through water and often aiding in egg brooding in females.
Beyond locomotion, crustaceans have appendages modified for other important tasks. Large claws, called chelae or chelipeds, are common on the walking legs of many species. They are used for defense, capturing prey, and handling food. The tail fan, formed by the uropods and telson, assists in rapid backward escape movements. The versatility of these appendages allows crustaceans to adapt to diverse ecological roles, enabling functions like sensing, gas exchange, and reproduction.
Diversity in Appearance and Environment
The appearance of crustaceans varies widely across the approximately 67,000 described species. This group includes microscopic copepods, less than a millimeter in length, and the massive Japanese spider crab, with a leg span exceeding 3.7 meters. Body shapes range from flattened crabs to elongated lobsters and shrimp, and even the sessile, shelled structures of barnacles.
Crustacean coloration exhibits a wide spectrum of hues and patterns. This can be influenced by diet, genetics, and environmental factors, often serving as camouflage to blend with surroundings and avoid predators. Some species can even rapidly change their coloration intensity by expanding or contracting specialized pigment structures.
Variations in appearance are linked to specific habitat adaptations. Deep-sea crustaceans, for example, may possess bioluminescent organs or pressure-resistant exoskeletons to survive in challenging, low-light environments. Terrestrial crustaceans, such as woodlice, have developed modifications to their respiratory systems and exoskeletons to prevent desiccation. This diversity in form and function highlights the adaptability of crustaceans to nearly every aquatic and some terrestrial niches.