King Crab

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. In 2023, king crabs were folded back into Paguroidea, with Lithodoidea being considered superseded.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 }}

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 (<). At the deepest, members of the Lithodinae genera Paralomis, Neolithodes, and Lithodes have been found at depths of , , and , respectively.

s are widely fished in Alaska.]] Because of their large size, the taste of their meat, and their status as a delicacy, some species of king crabs are caught and sold as food. 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,

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.

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