King crabs are a popular seafood, and their appearance often leads people to assume they are true crabs. However, despite their crab-like shape, king crabs are not classified as true crabs in the biological sense. This article delves into the distinct biological characteristics that define true crabs and king crabs, clarifying their places within the broader crustacean family.
Defining “True Crabs”
True crabs belong to the infraorder Brachyura, meaning “short-tailed.” These crustaceans are characterized by a broad, flattened body (carapace) covering their fused head and thorax. Their abdomen is reduced to a thin, flat plate, tucked entirely underneath the cephalothorax and largely out of sight.
True crabs possess five pairs of legs, characteristic of decapods. The first pair forms chelipeds (claws) used for defense and manipulating food. The remaining four pairs are for walking, allowing many species to move with a characteristic sideways gait. This leg arrangement and the tucked, symmetrical abdomen are anatomical markers for identifying a true crab.
The Unique Traits of King Crabs
King crabs exhibit several distinct features that set them apart from true crabs. While they also possess a hard exoskeleton and a protective carapace, their body shape can often appear more triangular or pear-shaped. A noticeable difference lies in their leg count; king crabs typically have only three pairs of visible walking legs, in addition to their two claws, totaling eight visible appendages.
Their fifth pair of legs is significantly reduced in size and often tucked away inside the gill chamber, making them difficult to observe externally. Another distinguishing characteristic is the king crab’s abdomen. Unlike the symmetrically tucked abdomen of true crabs, the king crab’s abdomen is typically asymmetrical and folded to one side, a trait shared with their closer relatives. These anatomical variations contribute to their unique classification within the crustacean lineage.
Their Place in the Crustacean Family Tree
When comparing king crabs to true crabs, the differences in leg count and abdominal structure become clear. This fundamental distinction in their body plan highlights that king crabs do not belong to the infraorder Brachyura, which encompasses true crabs.
King crabs are instead classified under the infraorder Anomura, a diverse group that also includes hermit crabs, porcelain crabs, and squat lobsters. Their crab-like appearance, despite not being true crabs, is a result of a phenomenon known as convergent evolution, or “carcinization.” This process describes how unrelated species can independently evolve similar physical traits due to similar environmental pressures or ecological niches, leading to a crab-like body form multiple times throughout crustacean evolutionary history. Therefore, while king crabs may look like true crabs, their evolutionary history places them in a separate branch of the crustacean family tree.