King crab

King crabs are marine decapod crustaceans of the family Lithodidae that are chiefly found 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.
The phylogeny of king crabs as hermit crabs who underwent secondary calcification and left their shell has been suspected since the late 1800s. They are believed to have originated during the Early Miocene in shallow North Pacific waters, where most king crab genera – including all Hapalogastrinae – are distributed and where they exhibit a high amount of morphological diversity.
In 2007, the king crabs were moved from their classification among the hermit crabs in the superfamily Paguroidea into a separate superfamily, Lithodoidea. This was not without controversy, as there is widespread consensus in the scientific community that king crabs are derived from hermit crabs and closely related to pagurid hermit crabs; therefore, a separate superfamily in the classification poorly reflected the phylogenetic relationship of this taxon. In 2023, king crabs were folded back into Paguroidea, with Lithodoidea being considered superseded. The king crab's relationship to other hermit crabs as well as the family's internal phylogeny can be seen in the following two cladograms:
As of December 2023, there are 15 known genera of king crabs across two subfamilies. These include:
King crabs are distinctive among hermit crabs for their superficial similarity to true crabs. They are a morphologically diverse group, but they all have in common the functionalities of their five pairs of legs, called pereopods: the first and anteriormost set are chelipeds whose right side is generally noticeably 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. On their underside, they have a short abdomen – composed of calcified plates – 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)).
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. Red (Paralithodes camtschaticus) and blue (Paralithodes platypus) king crabs are heavily targeted by commercial fisheries in Alaska and have been for several decades. However, populations have fluctuated in the past 25 years, and some areas are currently closed due to overfishing. 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, and as a result, the population of L. santolla has seen a dramatic decline.
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. On occasion king crabs have been found to be host to the eggs of multiple species of careproctus simultaneously. King crabs are additionally parasitized by rhizocephalan genus Briarosaccus, a type of barnacle. The barnacle irreversibly sterilizes the crab, and over 50% of some king crab populations are affected.