The Red King Crab is one of the largest and most commercially sought-after crustaceans in the world. Found primarily in the cold waters of the North Pacific and the Bering Sea, this arthropod is instantly recognizable by its spiny, broad body and long legs. Despite its common name and crab-like appearance, the Red King Crab presents a biological puzzle. This giant is not classified as a member of the group containing the animals most people think of as crabs.
Defining True Crabs and False Crabs
The confusion surrounding the Red King Crab’s identity stems from two major classifications within the order Decapoda, which contains all ten-legged crustaceans. The group known as Brachyura contains the “true crabs,” a diverse infraorder whose members share specific anatomical features that define them. True crabs are characterized by a broad, flattened carapace and a very short, symmetrical abdomen that is folded completely underneath the main body.
In contrast, the infraorder Anomura, which translates to “different tail,” groups together crustaceans whose body plans deviate from the true crab form. This group includes animals like squat lobsters, porcelain crabs, and hermit crabs, all sharing a common ancestor with the Red King Crab. Although Anomurans have ten legs, their last pair is often reduced in size and tucked away. This reduction gives them the visual appearance of having only eight main appendages.
The Surprising Evolutionary Link
The Red King Crab (Paralithodes camtschaticus) is firmly placed within the Anomura infraorder, specifically belonging to the family Lithodidae, also known as stone crabs. Molecular and genetic evidence confirms that the closest biological relatives of the Red King Crab are not true crabs at all, but rather certain types of hermit crabs. Scientists have determined that king crabs evolved from an ancestor that was a shell-dwelling hermit crab, making them evolutionary cousins to the creatures that inhabit abandoned gastropod shells.
This transformation from a shell-user to a heavily armored creature is a phenomenon known as carcinization. Carcinization is an example of convergent evolution, where different species independently evolve a similar body shape because that form offers an advantage. For the Red King Crab, this process resulted in a hard, calcified exoskeleton and a broad, low body plan. This structure provides protection and a lowered center of gravity, proving the crab-like shape is highly successful. The king crab’s resemblance to a true crab is superficial, resulting from adaptation to similar selective pressures.
Physical Traits That Reveal Ancestry
The most telling physical trait that separates the Red King Crab from true crabs lies in the number of its visible walking legs. While all decapods have five pairs of appendages, true crabs utilize four pairs for walking, resulting in eight visible walking legs plus the two claws. Red King Crabs, however, appear to have only three pairs of walking legs plus the claws, totaling eight readily visible appendages.
The missing pair is not truly missing; instead, the fifth pair of legs is significantly reduced in size and is often vestigial, meaning it has lost its original function. These small, feather-like legs are typically hidden away beneath the carapace in the gill chamber, where the crab uses them for cleaning its gills. This tucked-away, reduced fifth pair is a hallmark feature of the Anomura infraorder, directly linking the king crab to its hermit crab ancestors.
Another distinguishing feature is the king crab’s abdomen, which is fan-shaped and tucked underneath the thorax. Although folded under, it retains a noticeable asymmetry. It is not fully symmetrical and flattened against the body like the abdomen of a true crab. This anatomical detail confirms its lineage, showing that the Red King Crab is a highly specialized stone crab.