Crustaceans are a massive group of animals within the arthropod family, the same broad category that includes insects and spiders. They’re defined by their hard exoskeletons, jointed limbs, and segmented bodies. Most live in water, from tiny plankton to large lobsters, but some species have successfully colonized land. The group contains over 70,000 known species, with the decapods alone (crabs, shrimp, lobsters, and crayfish) accounting for more than 17,000 described species as of 2022.
Where Crustaceans Fit in the Animal Kingdom
Crustaceans belong to the subphylum Crustacea within the phylum Arthropoda. That places them alongside insects, arachnids, and myriapods (centipedes and millipedes) as fellow arthropods, all sharing the signature traits of an external skeleton and jointed legs. Within Crustacea, there are 12 recognized classes, ranging from the familiar decapods (the “ten-legged” group containing crabs, lobsters, and shrimp) to less well-known groups like copepods, barnacles, and krill.
The diversity is staggering. Crustaceans include animals as small as water fleas barely visible to the naked eye, barnacles cemented to rocks and whale skin, and Japanese spider crabs with leg spans exceeding 12 feet.
Body Structure and Limb Diversity
A crustacean’s body is built from repeating segments grouped into three main regions: the head, the thorax, and the abdomen, plus a tail section called the telson. In many species, the head and front part of the thorax fuse together into a single structure called the cephalothorax, which is the broad, shield-like front section you see on a crab or lobster. The entire body is covered by an exoskeleton made largely of chitin, a tough, flexible material that serves as both armor and structural support.
What really sets crustaceans apart is the remarkable variety of their limbs. Even on a single animal, appendages can be specialized for completely different tasks. A lobster, for example, carries feeding legs near its mouth, walking legs along its thorax, swimming legs on its abdomen, grasping claws, and even specialized reproductive appendages. Across the broader group, crustacean limbs have evolved for walking, swimming, eating, grooming, brooding young, scraping surfaces, and sensing the environment.
The mantis shrimp holds one of the most extreme examples of limb specialization in the animal kingdom. Its raptorial striking appendage can deploy in as little as 2 milliseconds to club or spear prey, generating forces powerful enough to crack aquarium glass.
How Crustaceans Grow
Because their exoskeleton is rigid, crustaceans can’t grow gradually the way soft-bodied animals do. Instead, they grow through molting, a process called ecdysis where the old shell is shed and a new, larger one forms. This happens in three stages: pre-ecdysis, when the body prepares by beginning to build a soft new shell underneath the old one; ecdysis itself, when the animal wriggles out of the old exoskeleton; and post-ecdysis, when the new shell hardens.
The process is triggered by steroid hormones that the animal synthesizes from dietary cholesterol. During the brief window after shedding, crustaceans are soft and vulnerable to predators. Many species hide during this period, and some, like certain crabs, eat their old shell to reclaim the calcium and minerals. Molting continues throughout a crustacean’s life, though it slows considerably as the animal reaches full size.
Senses and Vision
Crustaceans have evolved several types of light-detecting organs. Compound eyes are the most recognizable, built from hundreds or thousands of tiny individual units that together create a mosaic image of the world. But compound eyes aren’t the only option. Some crustaceans also have simpler larval eyes, frontal organs, or even light-sensitive cells inside their brains. Deep-sea and cave-dwelling species may lack photoreceptors entirely.
Compound eyes adapt to changing light conditions through pigment movements and optical adjustments that fine-tune performance for both bright daylight and dim conditions. Many crustaceans can also detect polarized light, which helps with navigation, finding prey, and communication. The mantis shrimp has one of the most complex visual systems known in any animal, with 16 types of color receptors compared to the three that humans use.
Beyond vision, crustaceans are highly sensitive to chemical signals in their environment. Antennae and specialized sensory hairs detect dissolved chemicals in water, functioning much like a combined sense of smell and taste. The most land-adapted crustaceans have developed advanced olfactory systems that work in air rather than water.
Life on Land
While crustaceans are overwhelmingly aquatic, several groups have made the transition to land. The most successful are woodlice (also called pill bugs or roly-polies), which are isopod crustaceans found in gardens, forests, and even deserts worldwide. Land-dwelling amphipods and some decapods, like coconut crabs, have also adapted to terrestrial life.
These land crustaceans have evolved a suite of adaptations to cope with the challenge of drying out. Many have developed more efficient ways to process food on land, including slow intake rates, longer digestion times, and the ability to break down tough plant material like cellulose. Some microscopic crustaceans survive in habitats with only tiny amounts of moisture trapped between soil particles, and certain species can enter a dormant, cyst-like state to survive prolonged droughts. Land crustaceans thrive in a surprising range of environments, from tropical forests to high-altitude Arctic regions and extremely dry, acidic habitats.
Role in Marine Ecosystems
Crustaceans are foundational to ocean food webs. Krill, small shrimp-like crustaceans typically under two inches long, form some of the largest single-species aggregations in the animal kingdom. Antarctic krill in particular are a critical food source for whales, seals, penguins, and seabirds. Their swarms are so dense and widespread that they function as a major link between microscopic algae at the base of the food chain and the ocean’s largest predators.
Krill also play a less visible but equally important role in ocean chemistry. Through their daily vertical migrations (rising to the surface at night to feed and sinking deeper during the day), they transport nutrients between different ocean layers. Their large, fast-sinking fecal pellets move carbon from surface waters to the deep ocean, contributing to what scientists call the biological carbon pump. This single species influences nutrient cycling on a scale large enough that fishery management now considers their biogeochemical impact alongside their value as food for other animals.
Copepods, another crustacean group, are even more abundant than krill in many parts of the ocean and serve a similar role as the primary food for small fish, which in turn feed larger fish. In freshwater systems, water fleas and crayfish fill comparable ecological roles.
Economic Value
The global crustacean market is valued at roughly $19 billion as of 2025, with projections reaching around $26 billion by 2032. Shrimp account for the largest share of commercial crustacean harvest, followed by crabs and lobsters. Aquaculture, particularly shrimp farming in Southeast Asia and Latin America, has grown rapidly over the past few decades and now produces a significant portion of the world’s crustacean supply.
Beyond direct harvest, crustaceans support massive secondary economies in processing, distribution, and restaurant industries. Crayfish farming is a multibillion-dollar industry in China, and lobster fisheries are economically vital to coastal communities in New England and Atlantic Canada.
Shellfish Allergies
Crustacean allergy is one of the most common food allergies worldwide, and unlike many childhood food allergies, it typically persists into adulthood. The primary trigger is tropomyosin, a muscle protein found in crustacean flesh. This same protein also occurs in house dust mites and cockroaches, which is why people with shellfish allergies sometimes react to dust mite exposure as well.
Prevalence varies significantly by region and diet. In Southeast Asian countries where shellfish consumption is high, rates among children and teenagers reach around 5% in the Philippines and Singapore. In China, reported rates are lower, ranging from about 0.2% to 0.4%, while Hong Kong and Taiwan report rates around 1%. Allergic reactions range from mild hives and tingling in the mouth to severe anaphylaxis. If you react to one type of crustacean (shrimp, for instance), you’re likely to react to others, since tropomyosin is structurally similar across crabs, lobsters, and crayfish.