Hermit crabs, despite their common name and crab-like appearance, are not considered “true crabs” in the biological sense. The widespread confusion stems from superficial similarities, but a closer look at their anatomy and evolutionary lineage reveals significant differences that place them in separate biological classifications.
The Defining Features of True Crabs
True crabs belong to the infraorder Brachyura, a scientific classification meaning “short-tailed.” These crustaceans exhibit a body plan characterized by a robust and entirely calcified exoskeleton. This hard outer shell, known as a carapace, fully covers their fused head and thorax, called the cephalothorax.
A distinguishing feature of true crabs is their abdomen, which is significantly reduced, flattened, and tucked tightly underneath the cephalothorax. This compact body shape contributes to their characteristic sideways walking. True crabs also possess five pairs of legs. The foremost pair typically consists of large claws, known as chelipeds, while the subsequent four pairs are used for walking.
The Unique Biology of Hermit Crabs
Hermit crabs differ significantly from “true crabs.” Their most notable difference lies in their abdomen, which is soft, asymmetrical, and spirally curved, lacking the hard, calcified exoskeleton found in true crabs. This vulnerability necessitates their reliance on scavenged mollusk shells for protection, into which they can fully retract their bodies.
To firmly grasp and maneuver within their borrowed shells, hermit crabs have specialized appendages at the tip of their abdomen called uropods. Though decapods, only eight of their legs are visible and used for walking. Their fourth and fifth pairs of legs are significantly reduced and remain hidden inside the shell, serving to secure the crab and assist with gill grooming. As hermit crabs grow, they must periodically search for and switch to larger, more suitable shells.
Deciphering the Crustacean Family Tree
The classification of crustaceans clarifies why hermit crabs are not true crabs, despite both being decapods. True crabs are categorized under the infraorder Brachyura. In contrast, hermit crabs belong to the infraorder Anomura, a diverse group that also includes king crabs and porcelain crabs.
While Brachyura and Anomura are closely related, their evolutionary paths have led to distinct anatomical differences. The varied forms within Anomura, including some that strikingly resemble true crabs, result from carcinization, where different lineages independently evolve a crab-like body shape. This convergence highlights that superficial resemblances do not always indicate a direct taxonomic relationship, underscoring the precise criteria used in biological classification.