Porcelain crab


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, 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, 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. They feed by combing plankton and other organic particles from the water using long setae 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:
The fossil record of porcelain crabs includes species of Pachycheles, Pisidia, Polyonyx, Porcellana, and a further six genera known only from fossils:
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.