Diagnosis.—Carapace with distinct transverse ridges on gastric region; rostrum trilobate, with small spine at lateral angle of each lateral lobe; 1 supra-ocular spine and 2 epibranchial spines present; branchial margins convex, with 3 median spines and some smaller denticles in front of row of spines. Second article of antennal
peduncle with distinct anterior lobe bearing anteroproximal spine. Chelipeds with distinct transverse or oblique, squamiform ridges on dorsal surfaces of carpi and palms; carpi each with 4 or 5 spine-tipped, broad teeth on dorsoflexor margin, dorso-extensor margin with 5–7 spines along almost entire length; chelae each with distinct longitudinal crest of short ridges on midline, extensor margin with row of spines and long plumose setae, dorsoflexor margin with small subdistal spine. Ambulatory legs relatively slender; meri each with row of spines and plumose setae on extensor margin, lateral flexor margins each with distinct distal spine on second and third pereopods but unarmed on fourth pereopod; carpi each with disto-extensor spine on second pereopod; dactyli each with 3 or 4 corneous spines on flexor margin. Ridges on surfaces with row of short plumose setae on anterior margins.
Petrolisthes scabriculus is a small, marine, porcelain crab (family Porcellanidae).The porcelain crab family includes a diverse set of 230 species from tropical and subtropical waters.Porcelain crabs closely resemble the familiar true crabs (infraorder Brachyura).However, porcelain crabs have independently converged upon crab-like features and lifestyle, and are actually part of the distantly related infaorder Anomura, along with the hermit crabs and squat lobsters.
A quick test to tell porcelain crabs from true crabs is to count the visible legs.Both groups are decapods, meaning they have 10 legs.However, while true crabs have five conspicuous pairs of legs (often the first pair is a set of claws), porcelain crabs have just four visible pairs (again, the first pair is a claw).Their fifth pair of legs is reduced, and they hold it up close to the body, under their carapace.Rather than using their fifth pair for locomotion, as do true crabs, porcelain crabs use them for grooming.These limbs are covered with brush-like setae, and used to clean off debris and sedimentation especially from their gills.Some porcelain crab species have also been observed using these back limbs to brush off parasites.
Petrolisthes scabriculus are widespread. Their range (based on reports of their findings) spans the shores of islands in the East China Sea (Ryukyu Islands) through the South China Sea (including Taiwan), the Gulf of Thailand south to Indian ocean (Christmas Island) the Philippines, Indonesia, New Guinea, western and eastern Australia, New Caledonia and the Loyalty Islands, and French Polynesia.Some researchers in believe they may, in fact, occur across the Pacific. These porcelain crabs inhabit coral reefs and the sea floor in shallow subtidal waters, to about 76 m (228 feet) deep.They are often associated with corals.
Unlike true crabs, which are mostly omnivorous scavengers, porcelain crabs are specialized filter-feeders.Access to spaces with high water flow and appropriate substrate allowing filtration may be the most limiting resource for porcelain crabs and the cause of inter-species competition.
While little is known of their behavior, P. scabriculus carry out courtship rituals in which males transfer a sperm package to females.The sperm fertilize eggs externally brooded by females.Eggs hatch to release swimming larvae.
100 species make up the genus Petrolisthes, this comprises about a third of the known Porcelain crab species. The taxonomy of the genus is has a long, complicated history, as there are several complexes that have not been resolved. The closest relative of P. scabriculus is thought to be the porcelain crab P. miliaris.The two species overlap in distribution and have been described as possibly synonymous, however they have a different pattern of spines on their carapace.Two coloration forms of P. scabriculus and of P. miliaris are also known from different habitats in the South Pacific.It is possible that P. scabriculus is actually more than one species.Clearly, more research is required to determine species boundaries.
(Dana 1852; Fleischer et al. 1992; Hiller et al. 2006; McNeill 1968; Osawa 2014, 2016; Osawa and McLaughlin 2010; Parducho 2015; Werding and Hiller 2015)
