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 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. 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. They feed by combing plankton and other organic particles from the water using long setae (feathery hair- or bristle-like structures) on the mouthparts.
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 <small>Nakasone & Miyake, 1969</small>
- Allopetrolisthes <small>Haig, 1960</small>
- Ancylocheles <small>Haig, 1978</small>
- Capilliporcellana <small>Haig, 1978</small>
- Clastotoechus <small>Haig, 1960</small>
- Enosteoides <small>Johnson, 1970</small>
- Euceramus <small>Stimpson, 1860</small>
- Eulenaios <small>Ng & Nakasone, 1993</small>
- Heteropolyonyx <small>Osawa, 2001</small>
- Heteroporcellana <small>Haig, 1978</small>
- Liopetrolisthes <small>Haig, 1960</small>
- Lissoporcellana <small>Haig, 1978</small>
- Madarateuchus <small>Harvey, 1999</small>
- Megalobrachium <small>Stimpson, 1858</small>
- Minyocerus <small>Stimpson, 1858</small>
- Neopetrolisthes <small>Miyake, 1937</small>
- Neopisosoma <small>Haig, 1960</small>
- Novorostrum <small>Osawa, 1998</small>
- Orthochela <small>Glassell, 1936</small>
- Pachycheles <small>Stimpson, 1858</small>
- Parapetrolisthes <small>Haig, 1962</small>
- Petrocheles <small>Miers, 1876</small>
- Petrolisthes <small>Stimpson, 1858</small>
- Pisidia <small>Leach, 1820</small>
- Polyonyx <small>Stimpson, 1858</small>
- Porcellana <small>Lamarck, 1801</small>
- Porcellanella <small>White, 1852</small>
- Pseudoporcellanella <small>Sankarankutty, 1962</small>
- Raphidopus <small>Stimpson, 1858</small>
- Ulloaia <small>Glassell, 1938</small>
The fossil record of porcelain crabs includes species of Pachycheles, Pisidia, Polyonyx, Porcellana, and a further six genera known only from fossils:
- Annieporcellana <small>Fraaije et al., 2008</small>
- Beripetrolisthes <small>De Angeli & Garassino, 2002</small>
- Eopetrolisthes <small>De Angeli & Garassino, 2002</small>
- Lobipetrolisthes <small>De Angeli & Garassino, 2002</small>
- Longoporcellana <small>Müller & Collins, 1991</small>
The earliest claimed porcelain crab fossil was Jurellana from the Tithonian aged Ernstbrunn Limestone of Austria.