King crabs, also called stone crabs, are a superfamily of crab-like decapod crustaceans chiefly found in cold seas. Because of their large size and the taste of their meat, many species are widely caught and sold as food, the most common being the red king crab, Paralithodes camtschaticus.

King crabs are generally thought to be derived from hermit crab-like ancestors, which may explain the asymmetry still found in the adult forms. Although some doubt still exists about this theory, king crabs are the most widely quoted example of carcinisation among the Decapoda. The evidence for this explanation comes from the asymmetry of the king crab's abdomen, which is thought to reflect the asymmetry of hermit crabs, which must fit into a spiral shell. Although formerly classified among the hermit crabs in the superfamily Paguroidea, king crabs are now placed in a separate superfamily, Lithodoidea.

Around 121 species are known, in 10 genera:

Glyptolithodes is found chiefly in the Southern Hemisphere, but extending as far north as California, although all its closest relatives live in the Northern Hemisphere. Its single species, G. cristatipes was originally placed in the genus Rhinolithodes.

A monarch is the person who leads a monarchy. This is a form of government in which a state or polity is ruled or controlled by an individual who typically inherits the throne by birth and rules for life or until abdication. Monarchs may be autocrats (absolute monarchy) or ceremonial heads of state who exercise little or no power or only reserve power, with actual authority vested in a parliament or other body (constitutional monarchy).

The word monarch is derived from the Greek μονάρχης (from μόνος, "one/singular," and ἄρχων, "leader/ruler/chief") through the Latin: monarcha (mono: "one" + arch "chief") which referred to a single, at least nominally, absolute ruler. In current usage the word monarchy generally refers to a traditional system of hereditary rule, as elective monarchies are rare in the modern period (though they do still exist in Vatican City, Andorra and Malaysia).

Most states have at most one monarch at any given time, although a regent may rule when the monarch is a minor, not present, or otherwise incapable of ruling. Two monarchs have ruled simultaneously in some countries, as in the ancient Greek city-state of Sparta or the joint sovereignty of spouses or relatives (e.g. William and Mary of Kingdom of England and Scotland, Peter and Ivan of Russia, Charles and Joanna of Castile, etc.).

True crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (Greek: βραχύς / brachys = short,οὐρά / οura = tail), or where the reduced abdomen is entirely hidden under the thorax. Many other animals with similar names – such as hermit crabs, king crabs, porcelain crabs, horseshoe crabs and crab lice – are not true crabs.

Crabs are generally covered with a thick exoskeleton, and armed with a single pair of chelae (claws). Crabs are found in all of the world's oceans, while many crabs live in fresh water and on land, particularly in tropical regions. Crabs vary in size from the pea crab, a few millimetres wide, to the Japanese spider crab, with a leg span of up to 4 metres (13 ft).

About 850 species of crab are freshwater, terrestrial or semi-terrestrial species; they are found throughout the world's tropical and semi-tropical regions. They were previously thought to be a monophyletic group, but are now believed to represent at least two distinct lineages, one in the Old World and one in the New World.