King crab

King crabs or stone crabs are marine decapod crustaceans of the family Lithodidae that are found chiefly in deep waters and are adapted to cold environments. They are composed of two subfamilies: Lithodinae, which tend to inhabit deep waters, are globally distributed, and comprise the majority of the family's species diversity; and Hapalogastrinae, which are endemic to the North Pacific and inhabit exclusively shallow waters. King crabs superficially resemble true crabs but are generally understood to be closest to the pagurid hermit crabs. This placement of king crabs among the hermit crabs is supported by several anatomical peculiarities which are present only in king crabs and hermit crabs, making them a prominent example of carcinisation among decapods. Several species of king crabs, especially in Alaskan and southern South American waters, are targeted by commercial fisheries and have been subject to overfishing.

King crabs are believed to have originated during the Early Miocene in shallow North Pacific waters, where most genera – including all Hapalogastrinae – are distributed and where they exhibit a high amount of morphological diversity. Since the late 1800s, carcinologists have suspected that king crabs are hermit crabs who underwent secondary calcification and left their shell.

The king crab family Lithodidae was created and placed among the true crabs in 1819 by zoologist George Samouelle to hold the then-recently-described genus Lithodes. In 2007, examining the monophyly of the decapod infraorder Anomura, carcinologist Patsy McLaughlin and colleagues moved the king crabs from their classification within the hermit crab superfamily Paguroidea into a separate superfamily, Lithodoidea. They furthermore found Lithodoidea to be a sister clade to the mole crab superfamily Hippoidea. This was controversial, as there is strong phylogenetic evidence that king crabs are derived from hermit crabs and closely related to pagurid hermit crabs. In 2023, king crabs were folded back into Paguroidea, with Lithodoidea being considered superseded. The king crabs' relationship to other hermit crabs, as well as the family's internal phylogeny, can be seen in the following two cladograms:

As of May 2025^[update], there are 15 known genera of king crabs across two subfamilies. These include:

King crabs are a morphologically diverse group, distinctive among hermit crabs for their superficial similarity to true crabs.

They have five pairs of legs, called pereopods: the first – frontmost – set are chelipeds whose right side is often larger and more robust than the left; the second, third, and fourth are walking legs tipped with sharp dactyli; and the fifth, used for cleaning, are very small and generally sit inside the branchial chamber. Starting from the carapace, the walking legs can be divided into a coxa, ischiobasis, merus, carpus, propodus, and finally dactylus; the chelipeds lack coxae and are instead composed of an ischiobasis, merus, carpus, and palm which then forks into a movable dactylus and an inflexible pollex.

On their underside, they have a short abdomen – composed of plates or nodules – which is asymmetrical in females. This abdomen (sometimes called a pleon) is folded against the underside of the cephalothorax and is composed of six segments – called somites or pleonites – and a telson. In Hapalogastrinae, this abdomen is soft, while it is hard and calcified in members of Lithodinae. Lithodids lack any sort of uropod seen in some decapods.

King crabs are typically found in deep waters, especially in polar and subpolar regions and near hydrothermal vents and cold seeps. Members of Lithodinae can be found in all five of the world's oceans, namely the Pacific, Atlantic, Indian, Southern, and Arctic, while members of Hapalogastrinae are only found in the North Pacific. Members of Hapalogastrinae exhibit a tolerance for higher temperatures than Lithodinae; whereas Lithodinae tend to live exclusively in deep waters or – less commonly – high-latitude shallow waters, Hapalogastrinae are found only in shallow waters (<100 m (330 ft)). At the deepest, members of the Lithodinae genera Paralomis, Neolithodes, and Lithodes have been found at depths of 4,152 m (13,622 ft), 3,207 m (10,522 ft), and 1,821 m (5,974 ft), respectively.