Crabs are among the most successful and varied animals, inhabiting the world’s oceans, coastlines, and freshwater streams. These crustaceans have an effective body plan that has evolved repeatedly across geological time. Their remarkable adaptability allows them to colonize a vast range of environments, from deep ocean trenches to tropical forests. This biological diversity is a testament to the evolutionary success of this group, which is characterized by hard shells and ten jointed legs.
Defining a True Crab and Counting the Species
Determining the exact number of crab species requires distinguishing between a “true crab” and other crab-like creatures. True crabs belong to the infraorder Brachyura, a taxonomic group within the order Decapoda. The defining physical feature of a Brachyuran is its short, flattened abdomen, or pleon, which is tucked completely underneath the cephalothorax. This characteristic broad shape has led to the current estimate of over 7,600 described species of true crabs globally.
The count of true crabs is often confused by “false crabs,” which are not members of the Brachyura infraorder. These false crabs, such as hermit crabs, king crabs, and porcelain crabs, belong to the separate infraorder Anomura. Their crab-like appearance is an example of convergent evolution, a process known as carcinization, where different lineages independently evolve the same body plan. A key difference is that true crabs have four visible pairs of walking legs, while many Anomurans appear to have only three because the fourth pair is reduced and often hidden near the gills.
Major Ecological Groupings
True crab species are broadly categorized by the major habitats they occupy, reflecting distinct evolutionary adaptations. The vast majority of species are marine, living entirely in saltwater environments, from the intertidal zone to the deep sea. These crabs display incredible variety, including fast-swimming species like the blue crab and deep-dwelling bottom scavengers. Their gills are structured to extract oxygen from water and regulate the salt balance in a hyper-saline environment.
A smaller, yet numerous, group has successfully transitioned to freshwater, with over 800 species inhabiting rivers and lakes. These freshwater crabs, such as those in the family Potamonautidae, have evolved physiological mechanisms to retain salt against the osmotic gradient of low-salinity water. Molecular analyses suggest this migration occurred on multiple independent occasions, with some lineages separating from marine relatives as far back as 135 million years ago.
The third major grouping is terrestrial and semi-terrestrial crabs, representing a remarkable feat of adaptation. They have conquered land at least 17 separate times throughout their evolutionary history. Species like the ghost crab and the coconut crab, the largest land arthropod, have developed specialized respiratory structures to breathe air. They modify their branchial chambers into lung-like organs that must be kept moist, allowing them to spend most of their lives away from the water, only returning to the sea to release eggs.
Extreme Diversity in Form and Function
The physical forms and behaviors across the infraorder Brachyura illustrate an astonishing range of biological solutions for survival. Size variation spans a massive spectrum, from the millimeter-wide pea crab, which lives as a parasite inside bivalve mollusks, to the Japanese Spider Crab. The Japanese Spider Crab is the largest arthropod by leg span, measuring up to 12 feet (3.8 meters) from claw tip to claw tip, allowing it to forage effectively on the deep seafloor.
Many crabs have evolved highly specialized appendages that dictate their functional role in the ecosystem. For instance, male fiddler crabs possess one disproportionately massive claw, or cheliped, sometimes making up half their total body weight. This oversized claw is primarily a signaling device, used in courtship displays and territorial defense, rather than for feeding.
Specialized Feeding and Defense
Conversely, some species, like porcelain crabs, have evolved feather-like mouthparts to filter tiny plankton from the water column. Other crabs, such as certain carrier crabs, actively use their form for survival by attaching sponges or debris to their shells, creating a living camouflage to hide from predators.