Crustaceans are a diverse group of invertebrate animals that inhabit nearly every aquatic environment and some terrestrial habitats. They belong to the phylum Arthropoda, which also includes insects and spiders. With over 67,000 described species, crustaceans range from microscopic organisms floating in the ocean to massive creatures crawling on the seafloor. Many animals people encounter, from dinner plates to garden soil, are crustaceans, often without realizing their shared classification.
What Defines a Crustacean
Crustaceans are distinguished from other arthropods primarily by their possession of two pairs of antennae. These appendages, located on the head, are used for sensory functions like touch and detecting chemicals in the water. Their bodies are made up of segments, typically grouped into a head, thorax, and abdomen. In many familiar species, the head and thorax segments are fused, forming a single functional unit called the cephalothorax, which is often covered by a protective shield known as the carapace.
The body of a crustacean is protected by a hard, external skeleton, or exoskeleton, made of chitin. This rigid structure does not grow with the animal, requiring crustaceans to shed their old shells in a process called molting to increase in size. Most crustaceans possess at least five pairs of jointed appendages attached to their body segments. The majority are aquatic and breathe using gills, although a few terrestrial species have evolved different methods for gas exchange.
The Large and Edible Examples
The most commonly recognized and commercially important crustaceans belong to the order Decapoda, meaning “ten legs.” This group is defined by having five pairs of thoracic legs, or pereiopods, used for walking. In many Decapods, the first pair of these walking legs is modified into enlarged claws or pincers, known as chelae, which are used for defense and manipulating food.
The Decapods include nearly 15,000 species and contain the largest crustaceans, such as crabs, lobsters, and shrimp. Crabs are characterized by their short, broad bodies and tucked-under abdomens. Lobsters and crayfish feature elongated, cylindrical bodies and prominent tails. Shrimp and prawns are generally smaller and possess slender bodies, adapted more for swimming than walking, and are harvested globally for consumption.
Unexpected Crustacean Examples
Beyond the familiar edible species, the subphylum Crustacea includes a surprising range of life forms that look nothing like their larger relatives. Barnacles are perhaps the most surprising inclusion, as they spend their adult lives permanently attached to surfaces like rocks or whales. They are protected by hard, calcareous plates, leading many to mistake them for mollusks. Despite their sessile lifestyle, barnacles are crustaceans that lie on their backs and use specialized feathery legs to filter-feed plankton from the water.
On the microscopic end of the scale, copepods are among the most numerous animals on Earth, with many species measuring less than a millimeter in length. Along with krill, which are small, shrimplike euphausiids, these planktonic crustaceans form the base of many aquatic food webs, providing sustenance for countless larger marine creatures. The group also contains the only truly terrestrial crustaceans, the woodlice, also known as pill bugs or roly-polies. These isopods are adapted to life on land but must remain in damp environments to prevent desiccation, a biological link to their aquatic ancestry.