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; 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. Since the late 1800s, carcinologists have suspected that king crabs are hermit crabs who underwent secondary calcification and left their shell. In 2023, king crabs were folded back into Paguroidea, with Lithodoidea being considered superseded.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:
, there are 15 known genera of king crabs across two subfamilies. These include:
| Glyptolithodes | Faxon, 1895
| Lithodes | Latreille, 1806
| Neolithodes | A. Milne-Edwards & Bouvier, 1894
| Paralithodes | Brandt, 1848
| Paralomis | White, 1856
| Phyllolithodes | Brandt, 1848
| Rhinolithodes | Brandt, 1848
| Sculptolithodes | Makarov, 1934
Description
King crabs are a morphologically diverse group, distinctive among hermit crabs for their superficial similarity to true crabs. Southern, and Arctic, while members of Hapalogastrinae are only found in the North Pacific. Alaskan fisheries additionally target the golden king crab (Lithodes aequispinus). In South America, both the southern king crab (Lithodes santolla) and several species of Paralomis are targeted by commercial fisheries,
Symbionts and parasites
Juveniles of species of king crabs, including Neolithodes diomedeae, use a species (Scotoplanes Sp. A) of sea cucumber (often known as "sea pigs") as hosts and can be found on top of and under Scotoplanes. The Scotoplanes reduce the risk of predation for the N. diomedeae, while the Scotoplanes are not harmed from being hosts, which supports the consensus that the two organisms have a commensal relationship. Endosymbiotic microorganisms of the order Eccrinida have been found in Paralithodes camtschaticus and Lithodes maja, living in their hindgut between molts.
Some species of king crab, including those of the genera Lithodes, Neolithodes, Paralithodes, and likely Echidnocerus, act as hosts to some parasitic species of careproctus fish. The careproctus lays eggs in the gill chamber of the king crab which serves as a well-protected and aerated area for the eggs to reside until they hatch. The barnacle irreversibly sterilizes the crab, and over 50% of some king crab populations are affected.