King crab

King crabs, also known as stone crabs, are marine decapod crustaceans from the family Lithodidae. These animals are highly specialized for life in cold environments. While they look like true crabs, they are actually closely related to pagurid hermit crabs. This relationship is a famous example of carcinisation, a process where an animal evolves to look more like a crab.

King crabs are important to ocean ecosystems and human economies. They are found in deep waters and are often targeted by commercial fisheries.

The anatomy of a king crab is quite complex. They possess five pairs of legs known as pereopods. The first pair are the chelipeds, which are large claws used for grasping. In many species, the right cheliped is larger and more robust than the left. The second, third, and fourth pairs are walking legs that end in sharp points called dactyli. The fifth pair is very small and is used for cleaning. These tiny legs usually sit inside the branchial chamber, which is the area where they breathe.

On their underside, they have a short abdomen called a pleon. This abdomen consists of six segments, called somites, and a telson. In the subfamily Lithodinae, this abdomen is hard and calcified. In the Hapalogastrinae subfamily, the abdomen remains soft.

Scientists divide the Lithodidae family into two distinct subfamilies. The first is Lithodinae, which contains most of the family's species diversity. These crabs are globally distributed and typically inhabit very deep waters. The second subfamily is Hapalogastrinae, which is endemic to the North Pacific. Unlike their deep-sea relatives, Hapalogastrinae live exclusively in shallow waters. They also show a higher tolerance for warmer temperatures than the Lithodinae.

The history of king crab classification has seen much debate. Zoologist George Samouelle established the family Lithodidae in 1819. He created the family to include the genus Lithodes. In 2007, carcinologist Patsy McLaughlin and her colleagues proposed a change. They moved king crabs into a separate superfamily called Lithodoidea. This was controversial because phylogenetic evidence suggested they were still related to hermit crabs. In 2023, scientists resolved this by folding king crabs back into the Paguroidea superfamily.

King crabs inhabit a vast range of ocean depths and regions. Lithodinae members can be found in all five oceans: the Pacific, Atlantic, Indian, Southern, and Arctic. Some species live at extreme depths. For example, members of the genus Lithodes have been found at 2,500 meters. The genus Neolithodes lives at 1,500 meters, and Paralomis lives at 1,000 meters. They are often found near hydrothermal vents and cold seeps.

Human activity significantly impacts king crab populations through commercial fishing. Red and blue king crabs are heavily targeted in Alaskan waters. The golden king crab is also a major target in Alaska. In South America, fisheries focus on the southern king crab and several Paralomis species. However, some populations have faced serious trouble. The southern king crab has seen a dramatic decline due to overfishing. Some fishing areas must be closed entirely to protect the remaining crabs.

King crabs also host many other living things. Juvenile Neolithodes diomedeae live on sea cucumbers called Scotoplanes. This is a commensal relationship, meaning the sea cucumber is not harmed while the crab gets protection from predators. Some species of careproctus fish use the crab's gill chamber to lay eggs. This provides a safe, aerated environment for the eggs to hatch. However, some relationships are more harmful. The rhizocephalan barnacle Briarosaccus is a parasite that can irreversibly sterilize a crab. In some populations, over 50% of the crabs are affected by these barnacles.