Like some lizards and spiders, crustaceans are wont to wank off their appendages in the event where autoamputation and a period of cosmetic disfigurement is preferable to remaining whole and dead in the tummies of predatory sea monkeys. The limbs of lobsters and crab-like decapods have fracture planes on the third joint (ischium) close to the base. An alarmed animal can trigger a spasmodic contraction of the muscles in the fracture plane to cause the shedding of the leg. A blood clot forms and a new limb buds off from behind the scar, taking a few moults to reach its full length. This trick might seem useful for ducks prone to find themselves trapped in unseemly receptacles or hoping to swap a tired old bird for a fresh young cock, but unfortunately, ersatz organs do not necessarily attain the dimensions of their predecessors...
The order Decapoda (meaning ten-legged) includes the crustaceans most well known to man, from spice-drowned Scylla and wine-dunked Penaeids to sweet-fleshed flowery swimmers and coconut-stealing robbers on the South Seas. Essentially, decapods comprise the creatures we know as shrimps, prawns, lobsters and crabs. Except for the penaeid shrimps (the best known member being the overrated tiger prawn), decapods also exhibit a more advanced ontogeny, with their newly-hatched larvae already in the zoae stage rather than the bizarre nauplius form assumed by most other crustaceans.
True crabs are grouped in the infraorder Brachyura and they come in a wide array of shapes and sizes from pea-sized hitchhikers to 3 m wide giant spider crabs with claws that will tear apart diving ducks. Closer to home, some crabs tailor spongy outfits for their nekkid backs. Others cart leaves around to hide their bashful bellies. Fiddlers brandish their oversized diddlers at each other on the wet mud, while platoons of ten-legged soldiers troop and thread out cannonballs of sand.
Dangling from a parallel branch are the Anomurans, a group of beasts popularly regarded as crabs, but bearing the distinct difference of having only three pairs of visible walking legs (the claws count as one pair of legs), the last pair being tucked away in the gill chamber (although a few Brachyurans have also lost their final pair of legs secondarily). The hermit crabs and squat lobsters are the most easily distinguishable, thanks to their extended abdomen, while the stone or king crabs (Lithodidae) bear a superficial resemblance to true spider crabs. The superfamily Hippoidea contains clawless beach crawlers that can only be described as crab-like by an unscrupulous imagination.
The porcelain crabs (Porcellanidae) are probably the most Brachyuran-like of the Anomurans, but apart from the leg count, they have a freely-flapping abdomen that can serve to propel them backwards in lobster fashion. They never attain dining plate size though, being shrimpier than most shrimps with bodies barely an inch across. They are filter feeders, straining fine food from the water with their bristly mouthparts; their claws are used mainly for intraspecific battles. With their flattened bodies, porcelain crabs are adapted to life in tight spots such as rock bottoms and coral crevices. Some species, like this Porcellanella picta found at Chek Jawa, are associated with sea pens, hiding inconspicuously amongst the cnidarian's feathery polyps and sharing the harvest of planktonic debris gathered by their sessile host. Often, a cohabitating pair of porcelain crabs will occupy a sea pen. Other species are found in hosts ranging from gorgonians and soft corals to sea anemones and even the occupied shells of large hermit crabs.
Returning to our breaking point, these little crabs get their name from their heightened tendency (even for crustaceans) to stuff a spare leg in a foe's jaws than risk an intact encounter of fatal impact. The common local species is appropriately attired, being off-white and somewhat translucent like the fine china on which we serve peppered portunids. Too minute for even crab cake and oft overlooked thanks to their cryptic coats and colourful homes, these innocuous crustaceans ply their tender trade by our shores, safe for now from scares that may yet some day shatter their busy bliss with no hope of regrowth.
Helmut Debelius, Crustacea Guide of the World, IKAN-Unterwasserarchiv 2001.
Waldo L. Schmitt, Crustaceans, University of Michigan Press 1971
Ria Tan & Alan Yeo, Chek Jawa Guidebook, RMBR/Simply Green 2003. Available online here.