Porcelain crabs are decapod crustaceans in the family Porcellanidae, which superficially resemble true crabs. They are a good example of carcinisation, whereby a non-crab-like animal (in this case a relative of a squat lobster) evolves into an animal that only a specialist would know is not a true crab. They live in all the world's oceans, except the Arctic Ocean and the Antarctic.
Porcelain crabs are small, usually with body widths of 1-2 cm. 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.
Porcelain crabs can be distinguished from the true crabs by the apparent number of walking legs (three instead of four pairs, the fourth pair is actually hidden under the carapace), the apparent lack of a wrist (carpal) segment on the chelipeds, and long antennae originating on the front outside of the eye stalks. The abdomen of the porcelain crab is long and folded underneath it, free to move. In fact, when alarmed, the crab might swim by flapping its abdomen.
Porcelain crabs share the general body plan of a squat lobster (Galatheidae), but their bodies are more compact and flattened, an adaptation for living and hiding under rocks, as well as squeezing into little nooks and crannies. The porcelain crabs are quite fragile animals, and will often shed their limbs, hence their name. This trait is useful for these crustaceans because it helps them escape if a limb is grabbed by a predator or caught between rocks shifting in the current. A broken-off piece of limb may still continue to move for a short while, distracting the would-be predator as the crustacean flees. The lost appendage can grow back over several moultings. Porcelain crabs have large chelae (claws), which are used for territorial struggles, but not for catching food. Feeding is accomplished instead by combing plankton and other organic particles from the water using long setae (feathery hair or bristle-like structures) on the mouthparts, where they will later be scraped off and consumed into the mouth. These animals will also scavenge on the sea floor for detritus and in aquaria, they will consume meaty pieces of fish or shrimp.
Some of the common species of porcellanids in the Caribbean 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.