Porcelain crabs are decapod crustaceans in the widespread family Porcellanidae, which superficially resemble true crabs. They have flattened bodies as an adaptation for living in rock crevices. They are delicate, readily losing limbs when attacked, and use their large claws for maintaining territories. They first appeared in the Tithonian age of the Late Jurassic epoch, 145–152 million years ago.
Description Porcelain crabs are small, usually with body widths less than . 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. Porcelain crabs are quite fragile animals, and often shed their limbs to escape predators, 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. The fifth pair of pereiopods is reduced and used for cleaning. 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. The abdomen of the porcelain crab is long and folded underneath it, free to move.
Biogeography and ecology
Porcelain crabs live in all the world's oceans, except the Arctic Ocean and the Antarctic. 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.Lagos ME, J Muñoz, D Contreras & C Cáceres. 2011. Microhabitat segregation and physiological differences in two species of intertidal porcellanid crabs (Genus Petrolisthes) on the Southern coast of Chile. Scientia Marina 75(2): 273-278.Marcelo E. Lagos, Nicole Castillo, Natalia Albarrán-Mélzer, Javier Pinochet, Paulina Gebauer & Mauricio A. Urbina. (2021). Age dependent physiological tolerances explain population dynamics and distribution in the intertidal zone: A study with Porcelain crabs. Marine Environmental Research. 169(2):105343 They feed by combing plankton and other organic particles from the water using long setae (feathery hair- or bristle-like structures) on the mouthparts.
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.
Evolution
Porcelain crabs are an example of convergent evolution commonly known as carcinisation, in which a decapod crustacean evolves to resemble a crab.
Diversity , some 4723 extant species of porcelain crab had been described, divided among these 30 genera:
Aliaporcellana Nakasone & Miyake, 1969 Allopetrolisthes Haig, 1960 Ancylocheles Haig, 1978 Capilliporcellana Haig, 1978 Clastotoechus Haig, 1960 Enosteoides Johnson, 1970 Euceramus Stimpson, 1860 Eulenaios Ng & Nakasone, 1993 Heteropolyonyx Osawa, 2001 Heteroporcellana Haig, 1978 Liopetrolisthes Haig, 1960 Lissoporcellana Haig, 1978 Madarateuchus Harvey, 1999 Megalobrachium Stimpson, 1858 Minyocerus Stimpson, 1858 Neopetrolisthes Miyake, 1937 Neopisosoma Haig, 1960 Novorostrum Osawa, 1998 Orthochela Glassell, 1936 Pachycheles Stimpson, 1858 Parapetrolisthes Haig, 1962 Petrocheles Miers, 1876 Petrolisthes Stimpson, 1858 Pisidia Leach, 1820 Polyonyx Stimpson, 1858 Porcellana Lamarck, 1801 Porcellanella White, 1852 Pseudoporcellanella Sankarankutty, 1962 Raphidopus Stimpson, 1858 Ulloaia Glassell, 1938
The fossil record of porcelain crabs includes species of Pachycheles, Pisidia, Polyonyx, Porcellana, and a further six genera known only from fossils: 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. However, it was subsequently determined to be a true crab. With the new oldest porcelain crab being Vibrissalana from the same locality.
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