Many creatures commonly called “crabs” are not classified as “true crabs” by scientists. This distinction often surprises people, as some recognized crustaceans, like the king crab, are “false crabs.” Understanding these differences requires a closer look at their biological characteristics.
The True Crab Blueprint
True crabs belong to the infraorder Brachyura, known for its “short-tailed” members. These crustaceans have five pairs of functional legs, totaling ten limbs. This includes their front claws (chelipeds) and four pairs of walking legs, all visible and actively used for movement.
Their abdomen, often called a “tail,” is short, symmetrical, and flattened. It is tucked neatly beneath the broad, shell-covered body (cephalothorax). Familiar species like the Dungeness crab or blue crab exemplify this body plan, with all ten limbs clearly presented and a hidden tail.
Defining Characteristics of False Crabs
False crabs are classified within a different infraorder called Anomura, which translates to “differently-tailed.” While decapods (possessing ten limbs), they display only four visible pairs of walking legs. Their fifth pair is significantly reduced, often tucked within gill chambers or held against the carapace, primarily used for cleaning.
The abdomen of false crabs differs from true crabs. Instead of being short and neatly tucked, it can be asymmetrical, elongated, or not fully concealed. The term “false” refers purely to this scientific classification, not suggesting they are lesser crustaceans. Many false crabs have evolved crab-like appearances independently through carcinisation, demonstrating convergent evolution.
Common Examples of False Crabs
King crabs are an example of false crabs, despite their strong resemblance to true crabs. These large crustaceans belong to the same lineage as hermit crabs and exhibit the hidden fifth pair of legs. Their body may also display an asymmetrical structure, a trait shared with their hermit crab relatives.
Hermit crabs are another group of false crabs, easily identified by their unique adaptation of living in discarded snail shells. They possess a soft, vulnerable, and often coiled abdomen that lacks the hard, calcified exoskeleton found in true crabs, making external shell protection a necessity. As they grow, hermit crabs must continuously seek out larger empty shells to accommodate their increasing size.
Porcelain crabs, though appearing delicate with their flattened bodies, are also false crabs. They are well-suited for hiding in narrow rock crevices, facilitated by their body shape. These crabs visibly use only three pairs of walking legs, as their fourth pair is reduced and kept close to their bodies. Porcelain crabs are known for their ability to shed limbs when threatened, a defense mechanism called autotomy, allowing them to escape from predators.