The term “crab” is often used loosely to describe any crustacean with a broad, flat body and prominent claws, leading to confusion in the common naming of marine life. This popular usage overlooks the precise biological classifications that define a true crab versus other crab-like creatures. Distinguishing between a true crab and a false crab requires moving beyond superficial appearance and examining specific anatomical markers, such as the structure of their legs and their abdomen.
The Defining Characteristics of True Crabs
True crabs belong exclusively to the infraorder Brachyura, a name that literally means “short tail,” which is a reference to their most distinguishing feature. This group of decapods includes over 7,000 species, such as the Dungeness crab, Blue crab, and the Japanese Spider crab. The body of a true crab is characterized by a hard, broad, and fused upper shell called the carapace, which provides rigid protection for the head, thorax, and internal organs.
A key marker of Brachyura is their highly reduced abdomen, or pleon, which is symmetrically folded tightly underneath the cephalothorax. This tucked-in abdomen is so small and reduced that it is often completely hidden from view when looking at the crab from above. The pleon is used differently between the sexes, serving as a protective chamber for carrying eggs in females, while remaining narrow and triangular in males. True crabs also possess a full set of four pairs of functional walking legs, in addition to their single pair of claws, or chelipeds.
Identifying the False Crabs of the Sea
The creatures commonly misidentified as crabs are generally classified in the infraorder Anomura, which is the sister group to the true crabs. Anomura translates to “different tail,” reflecting the wide variety of abdominal structures within this group, which includes species that are genetically closer to lobsters. This infraorder is highly diverse and encompasses many animals that have convergently evolved a crab-like shape.
High-profile examples of these false crabs include the King Crabs, such as the Alaskan King Crab, which are actually thought to have evolved from hermit crab ancestors. Porcelain crabs, which are small and have a flattened body, are another example and are more closely related to squat lobsters. Hermit crabs also fall into this group, distinguished by their long, soft, and unprotected abdomens that compel them to seek shelter in discarded mollusk shells.
The Critical Difference: Legs and Abdomen Structure
The most reliable way to distinguish a true crab from a false crab is by counting their visible walking legs and examining the structure of their abdomen. True crabs (Brachyura) are decapods, meaning they have ten limbs, typically composed of one pair of claws and four pairs of visible walking legs, totaling five functional pairs. The limbs are fully developed, hinged for a characteristic sideways gait, and widely spaced beneath the broad carapace.
False crabs (Anomura) also possess ten limbs, but they have a distinct structural difference in their leg count. In many Anomurans, the fifth and final pair of walking legs is significantly reduced in size and is often tucked up under the carapace, making them appear to have only three pairs of visible walking legs plus the claws. This reduced pair is frequently used for specialized tasks, such as cleaning the gills beneath the shell, rather than for locomotion.
The abdomen, or pleon, provides the second differentiator, particularly when looking at the underside of the animal. In true crabs, the pleon is symmetrical and firmly pressed against the underside of the carapace, forming a solid, protective plate. Conversely, in false crabs like King Crabs, the abdomen is generally less tightly tucked and often retains some asymmetry or visible segmentation, hinting at their evolutionary history.
Even the highly crab-like porcelain crabs can be differentiated by their abdomen. While their pleon is folded beneath them, it is typically softer or less uniformly calcified than the rigid, fused abdominal plate of the Brachyura. This difference in the calcification and tuck of the abdomen speaks to the distinct ancestral lineages of the two groups.
Why Do They Look So Similar?
The remarkable visual similarity between true and false crabs is an outcome of a process in evolutionary biology known as convergent evolution. This phenomenon is termed “carcinization,” which describes the independent evolution of a crab-like body plan from non-crab ancestors. Essentially, the broad, flattened body, hard carapace, and tucked abdomen is an extremely successful body form for survival in marine environments.
This body shape provides a low center of gravity, which is advantageous for walking on the ocean floor and resisting wave action. A compact, armored body also offers superior protection from predators and allows the animal to seek shelter in small crevices and under rocks. The “crab shape” has therefore been selected for multiple times across different lineages of decapod crustaceans, leading to the common confusion between the genetically distinct true crabs and the various groups of false crabs.