Pachygrapsus marmoratus has a shell (carapace) length of up to 3.6 cm. It is violet brown to almost black in colour with a marbled pattern of yellowish brown. The carapace is almost square and slightly broader than long, with distinct diagonal ridges visible. It has a convex shape both longitudinally and transversely. The front edge between the eyes is particularly straight and wide with three pointed teeth on each side. The chelipeds are large and slightly unequal in size. The male chelipeds are more robust than those of the female. The closest segment to the carapace (merus) on the walking legs are prominent, and the last segment (dactyl) has small spines.This species can be distinguished from the two other species of Pachygrapsus occurring in the Mediterranean by the three teeth on the front of the carapace antero-lateral margins. It is very quick moving and exceedingly difficult to catch on account of its singular ability to slip into the narrowest of crevices.
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