The term “crab” is widely used for many crustaceans, but the scientific designation “true crab” is reserved for the infraorder Brachyura. This name is derived from Greek words meaning “short tail.” Brachyurans represent a dominant lineage of decapod crustaceans, having evolved a distinct and robust body plan. Their evolutionary success is evident in the roughly 7,300 species found across virtually every aquatic and many terrestrial habitats worldwide.
Defining the Brachyura Order
True crabs are classified primarily by the morphology of their abdominal segment, known as the pleon. Unlike lobsters or shrimp, which have long, projecting abdomens, the pleon of a true crab is short, flattened, and permanently tucked beneath the broad cephalothorax. This anatomical distinction separates them from “false crabs,” such as hermit crabs or king crabs, which belong to the infraorder Anomura. While Anomurans often exhibit a crab-like shape, a phenomenon called carcinization, they retain a flexible, often asymmetrical, and less-reduced abdomen. True crabs are also characterized by having four pairs of well-developed walking legs, plus a fifth pair modified into large claws, or chelipeds. Many false crabs, in contrast, only have three pairs of visible walking legs, with the fourth pair being highly reduced or hidden beneath the carapace.
Essential Biological Mechanics
The compressed, wide body and hard outer shell, or carapace, provide the structural foundation for the crab’s functional biology. The heavily mineralized chitin of the carapace acts as a rigid exoskeleton, offering protection to the internal organs. This armored structure also serves as a strong anchor point for the powerful muscles operating the legs and claws. Locomotion is often characterized by their signature sideways walk. This lateral movement is facilitated by the wide separation and hinging of the leg bases, allowing for efficient scuttling and quick evasion into crevices.
The first pair of legs, the chelipeds, are specialized tools for defense, social signaling, and food processing. These claws are often sexually dimorphic, with males possessing much larger claws used for attracting mates and aggressive display, as seen in fiddler crabs. Respiration is achieved through gills housed in a chamber on the underside of the carapace. While aquatic species constantly circulate water over these gills, terrestrial crabs have adapted by stiffening their gills and developing elaborated linings in the branchial chamber to facilitate breathing air.
Global Range and Behavioral Diversity
The Brachyura infraorder is highly diverse, inhabiting virtually every aquatic environment, from the deep sea to tropical freshwater rivers. Many species are strictly marine, but others have successfully invaded freshwater and terrestrial habitats, a transition that required significant physiological adaptations. Freshwater crabs, for example, have evolved to complete their entire life cycle without returning to the ocean, representing over 1,400 species globally.
Land-dwelling species, such as the Christmas Island red crab, have developed specialized gills and circulatory systems to maximize oxygen uptake from the air. These terrestrial adaptations still require them to maintain moisture, often achieved by remaining near damp soil or returning to water to replenish their branchial chambers. Their behaviors are equally varied, ranging from complex mating rituals, such as the waving display of the male fiddler crab, to extensive burrowing habits for shelter and defense. Some crabs are considered keystone species, actively shaping their ecosystems through their feeding and burrowing activities.