Crustaceans, encompassing familiar species like crabs, lobsters, and shrimp, as well as many lesser-known forms such as barnacles and copepods, belong to the subphylum Crustacea within the phylum Arthropoda, which also includes insects, spiders, and myriapods. With an estimated 50,000 to 67,000 species worldwide, crustaceans have colonized a vast range of environments and represent one of the most dominant animal groups in ocean ecosystems. Their importance extends beyond ecological roles, making them a significant component of global biodiversity and human economies.
Defining Physical Characteristics
The defining traits of a crustacean include its segmented body and unique set of appendages, which differentiate it from other arthropods. All crustaceans are protected by a hard external skeleton, known as an exoskeleton, which is primarily composed of chitin and often strengthened by calcium salts. This rigid covering acts like armor and does not grow with the animal, requiring the crustacean to periodically shed it in a process called molting to allow for growth.
The body is typically divided into three main regions: the head, thorax, and abdomen. The head and thorax are frequently fused together to form a cephalothorax. The most distinctive feature of the group is the presence of two pairs of antennae, which function as sensory organs, contrasting with the single pair found in insects.
Crustaceans possess jointed appendages modified for different functions. Specialized mouthparts include mandibles for chewing and maxillae to help position food. The limbs on the thorax, called pereiopods, are used for walking, while abdominal appendages, or pleopods, are adapted for swimming or carrying eggs. Many larger species, such as crabs and lobsters, have the first pair of pereiopods modified into large pincers or claws, known as chelipeds, used for defense, feeding, and communication.
Diversity in Habitat and Form
The subphylum Crustacea has adapted to nearly every type of aquatic and some terrestrial habitats. The majority of species are found in marine environments, inhabiting everything from the open water column as plankton to the deepest ocean trenches and coastal tidal zones. Freshwater habitats, including rivers, lakes, and even temporary pools, also host a variety of crustaceans like crayfish and water fleas.
While most are aquatic, a few groups have successfully transitioned to land, with the most terrestrial being woodlice, also known as pill bugs or sow bugs. These land-dwelling species still require moist conditions to breathe, as they rely on modified gill-like structures for respiration.
Size variation is immense, spanning from microscopic species that are less than 0.25 millimeters long, such as certain copepods and ostracods, to the giant Japanese spider crab, whose legs can span up to 3.7 meters. The group also includes sessile forms, like barnacles, which attach permanently to surfaces and use specialized appendages to filter food from the water.
Major Groups and Relevance
The more than 50,000 crustacean species are generally divided into several major groups, including Malacostraca, Maxillopoda, and Branchiopoda. Malacostraca is the largest and most familiar class, containing all the decapods like crabs, lobsters, shrimp, and crayfish. Maxillopoda includes smaller, ecologically significant forms such as copepods and barnacles.
Crustaceans serve as a significant link in aquatic food chains. Microscopic forms, such as copepods and krill (a type of euphausiid), graze on phytoplankton and are a food source for larger organisms, including fish, seabirds, and baleen whales. Bottom-dwelling crustaceans also play a role as scavengers, consuming detritus and dead organisms, which contributes to nutrient recycling within their habitats.
Beyond their ecological function, many crustaceans have economic importance to humans. Large-scale commercial fishing and aquaculture harvest species like shrimp, prawns, lobsters, and crabs. Over 7.9 million tons of crustaceans are harvested annually for human consumption.