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Welcome to Arthropod Facts! Did you know that king crabs aren't actually crabs? King crabs (family Lithodidae) sit in the hermit crab superfamily Paguroidea in the decapod infraorder Anomura, whereas true crabs comprise the decapod infraorder Brachyura. The current strong and widespread scientific consensus is that king crabs are hermit crabs which originated in the North Pacific about 15 million years ago and evolved a crab-like, calcified exoskeleton through carcinisation.

Evidence for this includes:

• King crabs exhibit the same asymmetries that hermit crabs do, such as the asymmetrical abdomen (underside) found in females but not males.
• Lithodidae can be divided into the two subfamilies Lithodinae and Hapalogastrinae. Respectively, these have a calcified abdomen and an uncalcified/partially calcified abdomen. This suggests that Lithodinae fully calcified whereas Hapalogastrinae only partially calcified.
• Many king crabs (mainly Hapalogastrinae) are exclusively distributed in the North Pacific and are adapted to only living in cold waters.
• Molecular phylogenetics (the study of things like how species differ in their DNA) strongly suggests that Lithodidae is most closely related to the hermit crab family Paguridae.

An easy way to distinguish a king crab from a true crab is to look at the number of visible legs it has. While all decapods have ten legs, the last (hind) pair of legs in king crabs are dramatically reduced in size and usually sit inside the branchial chamber – used for cleaning rather than walking. By contrast, crabs always have their last pair of walking legs, although in the crab superfamily Portunoidea these are flattened and used as paddles for swimming. Thus, when counting the chelipeds (claw-bearing arms) as legs, you'll see eight legs on a king crab (four rows of two) and ten legs on a true crab (five rows of two).

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