Many marine creatures have evolved to look strikingly similar despite belonging to entirely different biological groups. This visual overlap is particularly evident in crustaceans, where the familiar, armored body plan of the crab is mimicked by several unrelated species. The term “false crab” refers to any organism that possesses the broad, flattened body and prominent claws typically associated with a crab but is not taxonomically classified as one. This confusion stems from convergent evolution, where different species independently develop similar physical traits in response to shared environmental pressures.
Defining the True Crab
A “true crab” belongs exclusively to the infraorder Brachyura, a scientific classification that includes nearly 7,000 recognized species worldwide. The defining feature separating this group is the morphology of the abdomen, known as the pleon, which is dramatically reduced and flattened. This short segment is completely folded and tucked tightly beneath the cephalothorax (the fused head and thorax). This tucked abdomen gives the true crab its characteristic compact, wide, and armored appearance.
True crabs, like all decapods, possess ten appendages. They have four pairs of walking legs and a front pair of legs modified into claws, or chelipeds. These four pairs of walking legs are used for locomotion, contributing to the animal’s signature sideways gait. The tucked abdomen and four pairs of visible walking limbs are the two most reliable external markers for placing a crustacean within the Brachyura infraorder.
The Most Common Imposters
The majority of false crabs are found within the infraorder Anomura, a sister group to the true crabs that includes the most convincing mimics. Some members of this group have independently evolved the classic crab shape through a phenomenon called carcinisation. This evolutionary trend toward a crab-like form explains the striking resemblance.
The King Crabs (Lithodidae) are among the largest and most commercially recognized false crabs, often mistaken for true crabs due to their heavily calcified, spiny shells and impressive size. Genetic evidence suggests King Crabs evolved from ancestors resembling hermit crabs, which are also Anomurans. Porcelain Crabs (Porcellanidae) are another common imposter, superficially resembling true crabs with their large claws and flattened bodies, an adaptation that allows them to hide in rock crevices. Hermit Crabs, the most diverse group of Anomurans, are less crab-like but still fall into this classification, distinguished by their soft, asymmetrical abdomens protected inside discarded mollusk shells.
Key Differences in Anatomy
The most telling anatomical distinction between a true crab (Brachyura) and a false crab (Anomura) lies in the arrangement of their legs. All decapods possess five pairs of thoracic appendages. True crabs display four pairs of fully functional, visible walking legs. Conversely, false crabs typically only display three pairs of visible walking legs (six legs total), plus the claws.
This difference occurs because the fifth pair of legs in most Anomurans is significantly reduced in size. These tiny appendages are often tucked tightly beneath the carapace, sometimes hidden within the gill chamber, and are primarily used for cleaning the gills rather than for walking. Another structural difference is found in the abdomen, or pleon. In true crabs, the abdomen is a small, symmetrical plate completely hidden beneath the cephalothorax. In many Anomurans, even those that look crab-like, the abdomen is often asymmetrical, segmented, or not fully tucked, revealing itself as distinct from their true crab counterparts.