The answer to whether hermit crabs are actually crabs is a clear no, from a scientific perspective. Despite sharing the common name, hermit crabs are not considered “true crabs” in the biological sense. The confusion stems from a long history of common names being applied based on superficial appearance rather than genetic lineage. Taxonomists separate these ten-legged crustaceans into distinct groups within the order Decapoda due to significant differences in their body structure and evolutionary paths.
The Taxonomic Truth: Hermit Crabs are Anomurans
Hermit crabs belong to the infraorder Anomura, which translates from Greek to mean “differently-tailed.” This designation immediately sets them apart from true crabs. The Anomura infraorder includes approximately 2,500 species and is considered the sister taxon to the true crabs.
The infraorder Anomura is a diverse group that also encompasses organisms like king crabs and porcelain crabs, which are often called “false crabs.” Their crab-like appearance is an example of convergent evolution, known as carcinization. This process occurs when unrelated species independently evolve similar body shapes. Hermit crabs specifically fall into the superfamily Paguroidea within the Anomura.
Defining the True Crabs (Infraorder Brachyura)
The crustaceans considered “true crabs” belong to the infraorder Brachyura. This scientific name translates to “short-tailed,” which is a defining characteristic of the group. Brachyura is the largest and most diverse group of all decapod crustaceans, containing around 7,000 species, including familiar types like the blue crab and stone crab.
True crabs have a robust and compact body plan. They possess a wide, flattened, and heavily calcified upper shell called the carapace, which covers the fused head and thorax (cephalothorax). Their abdomen is reduced to a small, flat plate that is tightly tucked and folded underneath the cephalothorax. This tucked abdomen contributes to their centralized body weight and characteristic sideways gait.
Anatomical Differences That Separate the Species
The most significant distinctions between true crabs and hermit crabs lie in the structure of their abdomen and the number of visible walking legs. True crabs have a uniformly hard exoskeleton across their entire body, including the small, folded abdomen. In contrast, the hermit crab’s abdomen is soft, elongated, and often coiled to fit into a spiral shell.
This soft, non-calcified abdomen is extremely vulnerable to predators. Hermit crabs have thus evolved the necessary behavior to occupy the empty shells of gastropod mollusks for protection. The shell is a biological necessity for survival, a requirement that true crabs do not share. True crabs are fully armored by their hard, protective carapace and are free-living, not relying on scavenged housing.
A clear anatomical difference is the apparent count of their walking legs. Decapods, by definition, have ten legs, but their arrangement differs between the two groups. True crabs typically have five visible pairs of appendages: four pairs of well-developed walking legs and the first pair modified into claws.
Hermit crabs, however, only appear to have three pairs of walking legs, or six total visible legs, plus the claws. The fourth and fifth pairs of legs are significantly reduced in size and are often tucked up inside the shell. These specialized, small rear legs are used to grip the central column of the gastropod shell, helping the hermit crab secure itself firmly inside its mobile home.