Crabs are a highly successful and diverse group of decapod crustaceans found in nearly every aquatic and semi-terrestrial environment globally. Over 7,000 species exist, instantly recognizable by their broad, flattened bodies and characteristic sideways walk. Their hard external skeleton and specialized appendages allow them to occupy various ecological niches, from the deep ocean floor to tropical beaches.
Physical Features of True Crabs
The body of a true crab is defined by a thick, mineralized chitin exoskeleton that forms a protective shell, covering the fused head and thorax with a shield-like carapace. Crabs are decapods, possessing five pairs of thoracic appendages, or ten legs. Beneath the carapace, crabs respire using gills housed in specialized branchial chambers.
The front pair of these appendages is modified into powerful claws, known as chelae or chelipeds, used for defense, capturing food, and communication. The remaining four pairs are used as walking legs. A distinguishing feature is their significantly reduced abdomen, which is tucked tightly beneath the cephalothorax, giving them their compact shape. Compound eyes are mounted on movable stalks, providing a wide field of vision.
Taxonomy and the Concept of Crab-Like Animals
True crabs belong to the infraorder Brachyura, a classification within the order Decapoda. The name Brachyura translates to “short-tail,” referring to the highly reduced and folded abdomen that is inconspicuous from above. The most consistent anatomical trait separating true crabs from other related crustaceans is the structure of their walking legs. True crabs possess four pairs of well-developed, visible walking legs.
This distinction helps differentiate them from members of the infraorder Anomura, often called “false crabs,” such as hermit crabs and king crabs. Anomurans only have three pairs of clearly visible walking legs. Their fourth pair is significantly smaller and often hidden or used for specialized tasks like cleaning the gills. The wide array of animals that look like crabs is due to carcinization, an evolutionary phenomenon where the crab-like body form has evolved independently multiple times across different decapod lineages.
Reproduction and the Molting Process
Crabs grow by repeatedly shedding their hard exoskeleton in a process called molting, or ecdysis. Before a molt, a new, soft shell forms beneath the old one. The crab extracts its entire body, including legs and eyestalks, from the split old shell. Following the molt, the crab rapidly absorbs water or air to inflate its new soft shell, increasing its size before the shell hardens.
This post-molt phase leaves the crab in a soft-shell state, making it highly vulnerable to predators until the new shell is fully hardened. Reproduction in many aquatic species is linked to this cycle, as mating often occurs immediately after the female molts while her body is still soft. The female releases pheromones to attract a male, who may guard her during this vulnerable period.
Once fertilization occurs, the female stores the sperm and later extrudes her eggs, attaching them to small appendages on her abdomen. This mass of developing eggs creates a spongy cluster beneath her body, leading to the common name of being “berried.” She carries and ventilates the eggs until they hatch into planktonic larvae. These larvae drift in the water column before settling and developing into juvenile crabs.
Habitat Distribution and Feeding Habits
Crabs are highly adaptable, occupying environments worldwide from the deepest marine trenches to high-altitude freshwater lakes. The majority of species are found in marine habitats like coastlines, estuaries, and the continental shelf. Some species, such as ghost crabs, have adapted to terrestrial life but must still return to the water to release their larvae.
Their diet is diverse, with most crabs functioning as omnivorous scavengers. They consume organic matter, including algae, detritus, mollusks, and worms. Specialized feeders, such as porcelain crabs, filter plankton using feathery mouthparts. Crabs use their strong chelipeds for grasping and tearing food, while mandibles and maxillipeds manipulate the pieces closer to the mouth.