King crabs, despite their common name and crab-like appearance, are not classified as “true crabs” in the scientific sense. This distinction arises from fundamental differences in their biological classification and anatomical features.
Defining True Crabs
True crabs belong to the infraorder Brachyura, a scientific grouping characterized by specific anatomical arrangements. A defining feature of true crabs is their possession of five pairs of legs, totaling ten limbs. This includes two large pincers, known as chelipeds, and four pairs of walking legs. These legs are typically positioned to facilitate their characteristic sideways walking motion.
Another distinguishing characteristic of true crabs is their abdomen. This abdomen is significantly reduced, flattened, and tucked tightly underneath the cephalothorax, the fused head and thorax. True crabs also possess a rigid and calcified carapace, a dorsal shell that covers their vital organs and provides protection.
King Crabs and Their Unique Traits
King crabs are not members of the Brachyura infraorder. Instead, they belong to the family Lithodidae, part of the infraorder Anomura, a group that also includes hermit crabs. A primary distinction of king crabs is their leg count; they typically have only four visible pairs of legs. Their fifth pair of legs is often greatly reduced in size and tucked away beneath their carapace, making it difficult to see.
The abdomen of a king crab also differs significantly from that of a true crab. While folded under, it is not as tightly tucked or symmetrical as a true crab’s abdomen. King crabs also have distinct larval stages that differentiate them from true crabs.
Evolutionary Resemblance
The striking resemblance between king crabs and true crabs, despite their separate classifications, can be explained by convergent evolution. This occurs when unrelated species independently evolve similar traits or body forms due to similar environmental pressures or lifestyles.
The “crab-like” body plan, characterized by a hard, broad carapace and tucked abdomen, is a highly successful adaptation for bottom-dwelling crustaceans. This body shape, known as carcinization, offers advantages such as protection from predators and efficient movement across the seafloor. Both true crabs and king crabs have independently developed this robust, compact body form due to the evolutionary benefits it provides. Their similar appearance is a result of adapting to comparable ecological niches, rather than a close evolutionary relationship. This highlights how superficial similarities can be misleading when determining biological classifications.
Other Crab Impostors
Beyond king crabs, several other crustaceans are often mistaken for true crabs due to their similar appearances. Hermit crabs, also belonging to the Anomura infraorder, are well-known for their soft, spiraled abdomens, which they protect by inhabiting discarded mollusk shells. Porcelain crabs, another Anomura example, are characterized by their flattened bodies and a long, folded tail, not as tightly tucked as a true crab’s. Even horseshoe crabs, despite their name, are not crustaceans; they are chelicerates, more closely related to spiders and scorpions. These diverse examples illustrate how various arthropods have independently evolved “crab-like” morphologies, underscoring the effectiveness of this body plan in different marine environments.