Broad-clawed porcelain crab is a small crustacean reaching about one centimeter in length. It belongs to a family of porcelain crabs (Porcellanidae) which resemble true crabs (Brachyura) due to their dorso-ventrally flattened bodies, beneath the pereon folded abdomens and sideways walking, however they actually belong to Anomura. Dorsally, they are reddish-brown with dense greenish-brown bristles. Ventrally, they are lighter in color and without bristles.

The body consists of a cephalothorax, pereon and pleon (abdomen). The cephalothorax and pereon together are covered by a carapace which extends laterally all the way to the bases of the legs and forms a branchial chamber. The abdomen is short and folded beneath the pereon. Their exoskeleton is highly calcified.

Into the cephalothorax are fused an acron, five head segments and three additional body segments. The head region bears stalked compound eyes and two pairs of filiform antennae (antennae I and II). The antennae I are short and biramous, while the antennae II are considerably longer than the body. In true crabs both pairs of antennae are short. The antennae have chemo- and mechanosensory function. Mouthparts comprise a pair of mandibles, a pair of maxillae I, a pair of maxillae II and three pairs of maxillipeds. Evolutionary, the maxillipeds originate as appendages of the first three thoracic segments.

The pereon consists of five segments. Due to the carapace covering the dorsal side the segments are indistinguishable. The first pair of appendages on the pereon (pereopods) are large chelate chelipeds. Their chelae (claws) are wide and flattened thus the name of the species. The second, third and fourth pair are the walking legs. The fifth pair is reduced in size, partially hidden in the branchial chamber and is used for gill cleaning. In true crabs the fifth pair of pereopods is large and used for walking. The leaflike gills originate at the bases of pereopods.

The abdomen is comprised of six posterior body segments (pleomeres) and a telson. It is symmetrical, short and folded beneath the pereon. Five pairs of pleopods are reduced to various degrees. In females the third, fourth and fifth pair are present, are leaf-like and function in carrying eggs. In males only the second pair is present and is modified as gonopods which function in the transfer of sperm to female. The sixth pleomere has flattened uropods which are absent in true crabs. The telson is large, flattened and consists of seven plates.

The broad-clawed porcelain crab is distributed along the Mediterranean and European Atlantic coasts. It is abundant in the intertidal zone, hiding under rocks and in crevices. It is rarely encountered in deeper areas, where instead the long-clawed porcelain crab (Pisidia longicornis) and Pisidia bluteli are common. Both species have long claws and lack bristles.

Despite its large and strong claws, the broad-clawed porcelain crab is not a predator but feeds as a filter-feeder and detritivore. The long bristles on the third pair of maxillipeds are used to catch suspended food particles, which are then with the second pair of maxillipeds transferred towards the mouth. The chelipeds are rarely used in feeding, instead they are used in interactions with conspecifics and in defense against predators. The chelipeds are larger in males than in females. This is a consequence of sexual selection since the males with larger chelipeds are more successful in competition with other males and thus mate with females more often. Heterochely is present, with one of the chelipeds being larger than the other in one individual. Individuals with larger left or right cheliped are usually the same common.

The broad-clawed porcelain crab has more defense strategies against predators than just its large claws. It usually hides on the underside of a rock where it tightly clings to the rock so that it is almost impossible to grab it. If the predator does grab it, it is capable of the autotomy of one of the limbs, usually the cheliped. The predator gets a small bite; however, the porcelain crab does get away to safety. The porcelain crabs got their name on the basis of their fragile structure and the shedding of their limbs to escape predators. During the subsequent molts the porcelain crab gradually regenerates the lost limb.

From their eggs the larvae termed zoeae are hatched. The zoeae are a few millimeters large, have a distinct carapace with a long rostrum and two long backwards facing spines. These protrusions of carapace serve in defense against predators and increase the buoyancy of the larvae.