Porcelain crab

Porcelain crabs are small, usually with body widths less than 15 millimetres (9/16 inch).^[1] They share the general body plan of a squat lobster, but their bodies are more compact and flattened, an adaptation for living and hiding under rocks.^[2] Porcelain crabs are quite fragile animals, and often shed their limbs to escape predators,^[3] hence their name. The lost appendage can grow back over several moults. Porcelain crabs have large chelae (claws), which are used for territorial struggles, but not for catching food.^[1] The fifth pair of pereiopods is reduced and used for cleaning.^[4]

Porcelain crabs are an example of carcinisation, whereby a noncrab-like animal (in this case a relative of a squat lobster) evolves into an animal that resembles a true crab.^[5][6] Porcelain crabs can be distinguished from true crabs by the apparent number of walking legs (three instead of four pairs; the fourth pair is reduced and held against the carapace), and the long antennae originating on the front outside of the eyestalks.^[3] The abdomen of the porcelain crab is long and folded underneath it, free to move.^[3]

Porcelain crabs live in all the world's oceans, except the Arctic Ocean and the Antarctic.^[7] They are common under rocks, and can often be found and observed on rocky beaches and shorelines, startled creatures scurrying away when a stone is lifted.^[8][9] They feed by combing plankton and other organic particles from the water using long setae (feathery hair- or bristle-like structures) on the mouthparts.^[4]

Some of the common species of porcelain crabs in the Caribbean Sea are Petrolisthes quadratus, found in large numbers under rocks in the intertidal, and the red-and-white polka-dotted Porcellana sayana, which lives commensally within the shells inhabited by large hermit crabs. In Hong Kong, Petrolisthes japonicus is common.^[2]

As of 2018^[update], some 4723 extant species of porcelain crab had been described,^{[10][citation needed]} divided among these 30 genera:^[10][11]

The fossil record of porcelain crabs includes species of Pachycheles, Pisidia, Polyonyx, Porcellana, and a further six genera known only from fossils:^[12]

• Annieporcellana Fraaije et al., 2008
• Beripetrolisthes De Angeli & Garassino, 2002
• Eopetrolisthes De Angeli & Garassino, 2002
• Lobipetrolisthes De Angeli & Garassino, 2002
• Longoporcellana Müller & Collins, 1991

The earliest claimed porcelain crab fossil was Jurellana from the Tithonian aged Ernstbrunn Limestone of Austria.^[12] However, it was subsequently determined to be a true crab. With the new oldest porcelain crab being Vibrissalana from the same locality.^[13]