Crustaceans are a diverse group of invertebrates in the phylum Arthropoda. This group includes over 45,000 described species found globally in various aquatic environments, with a few adapted to land. They are found across oceans, lakes, and rivers, playing a significant role in many ecosystems.
Key Features of Crustaceans
A defining characteristic of crustaceans is their hard outer covering, known as an exoskeleton. This exoskeleton, primarily composed of chitin and calcium carbonate, provides both protection and structural support. As crustaceans grow, they must periodically shed this rigid outer layer in a process called molting, allowing for the formation of a new, larger exoskeleton.
The body of a crustacean is segmented, typically organized into three regions: the head (cephalon), thorax (pereon), and abdomen (pleon). In many species, the head and thorax are fused to form a cephalothorax, which is often covered by a single, large carapace. Each body segment can bear a pair of jointed appendages.
These specialized appendages include two pairs of antennae in front of the mouth, primarily used for sensory perception. Near the mouth, crustaceans possess mandibles for chewing food and maxillae to assist with feeding. Further back, various pairs of legs serve purposes such as walking, swimming (pleopods or swimmerets), and even assisting with feeding. Respiration in most crustaceans occurs through gills, typically located within the thoracic cavity or on appendages. Some terrestrial species have adapted their gills or developed specialized structures, like pleopodal lungs, for air breathing.
Types and Environments
The crustacean group encompasses a wide array of forms, from familiar species like crabs, lobsters, and shrimp to lesser-known organisms such as barnacles, copepods, and krill. Crayfish are common examples found in freshwater environments, while isopods, including pill bugs and woodlice, are among the few crustacean groups that have successfully adapted to terrestrial life. These land-dwelling species often inhabit damp areas, such as under rocks or decaying wood.
Most crustaceans are aquatic, thriving in both marine and freshwater habitats. Marine crustaceans can be found across all ocean depths, from shallow coastal areas and coral reefs to the deep sea. Freshwater species inhabit lakes, rivers, and streams. Their ability to inhabit diverse aquatic conditions, including hypersaline environments, demonstrates their adaptability.
Role in Ecosystems
Crustaceans play a key role in various ecosystems, contributing to food webs and nutrient cycling. Many smaller planktonic crustaceans, such as copepods and krill, serve as a fundamental food source for a wide range of marine animals, including fish, seabirds, and large mammals like whales.
Beyond being a food source, crustaceans also function as scavengers and decomposers. By consuming decaying organic matter, they help recycle nutrients back into the environment. Many larger crustaceans, particularly crabs, lobsters, and shrimp, are economically important for humans. They are a valuable food commodity globally, supporting extensive fisheries and aquaculture industries.