Crustaceans are a diverse group of invertebrate animals belonging to the phylum Arthropoda, which also includes insects and spiders. This subphylum encompasses over 67,000 described species found across the globe. They exhibit an impressive range in size, from microscopic parasites measuring just 0.1 millimeters to the Japanese spider crab, with a leg span of up to 4.3 meters. While primarily aquatic, living in both marine and freshwater environments, some crustaceans have adapted to life on land. These creatures play diverse roles in ecosystems, from being a fundamental part of food webs to acting as scavengers.
Defining Characteristics
Crustaceans possess several distinctive anatomical features. Their bodies are segmented, divided into three main regions: the head, thorax, and abdomen. In many species, the head and thorax are fused, forming a cephalothorax, often covered by a protective shield called a carapace. The exoskeleton, a rigid outer covering made of chitin and calcium carbonate, provides support and protection.
Crustaceans have jointed appendages. They have two pairs of antennae on the head, which serve sensory functions. They also possess specialized mouthparts, including mandibles for chewing and maxillae for food manipulation. Their limbs are biramous, meaning they are branched into two parts, a feature that distinguishes them from other arthropods. Respiration in most aquatic crustaceans occurs through gills for gas exchange.
Diverse Forms and Habitats
The crustacean subphylum exhibits diverse forms, each adapted to specific habitats. Crabs, known for their broad, flattened bodies and prominent claws, are found in marine and some terrestrial environments, including intertidal zones and deep-sea ecosystems. Lobsters, with elongated bodies and powerful claws, inhabit rocky seafloors in marine waters. Shrimp, smaller and more agile, swim freely in marine and freshwater habitats, contributing significantly to aquatic food chains.
Barnacles are unique, spending their adult lives sessile, meaning they are attached headfirst to surfaces like rocks, ships, or even whales. These marine filter feeders use specialized feathery appendages to strain microscopic particles from the water. Copepods are tiny, planktonic crustaceans, less than 2 millimeters long, abundant in both marine and freshwater environments. They serve as an important food source, forming a primary link between microscopic algae and larger aquatic organisms. Krill are small, planktonic crustaceans, prevalent in polar oceans where they form vast swarms, providing a major food source for whales, seals, and seabirds.
Life Cycle and Growth
Crustaceans undergo a unique growth process involving shedding their rigid exoskeleton, known as molting or ecdysis. Since the exoskeleton does not grow with the animal, molting allows the crustacean to increase in size and replace damaged parts. This hormonally regulated process involves reabsorbing minerals from the old shell and forming a new, soft exoskeleton underneath. After shedding, the new exoskeleton is initially soft and pliable, requiring the animal to absorb water to expand before it hardens, leaving them temporarily vulnerable.
Many crustaceans exhibit complex life cycles involving multiple larval stages that look very different from the adult form. The earliest and most characteristic larval stage is the nauplius, a minute, free-swimming larva with a simple, unsegmented body and three pairs of appendages used for movement and feeding. Following the nauplius stage, many species progress through further larval forms, such as the zoea, which has a more developed cephalothorax and distinct spines on its carapace. These larval stages undergo significant transformations, or metamorphosis, through successive molts before gradually developing into a form resembling the adult.