P. marmoratus has a square carapace 22–36 millimetres (0.87–1.42 in) long, which is dark violet brown with marbling in yellow.[2] It can be distinguished from related species of Pachygrapsus in the Mediterranean Sea (Pachygrapsus maurus and Pachygrapsus transversus)[3] by the presence of three teeth on each side of the carapace.[2] It is capable of very rapid movements, and it uses this ability to dart into crevices, making it difficult to catch.[2]
P. marmoratus is an omnivore, but not an opportunist; similar proportions of algae and animals are consumed however abundant they are in the habitat. The favoured animals in the diet of P. marmoratus are mussels, limpets and its own species.[4] When attacking the limpet Patella depressa, Pachygrapsus marmoratus uses a consistent method, which is usually unsuccessful.[7] On more sheltered shores, P. grapsus eats fewer mussels, but compensates with a greater consumption of barnacles.[8]
The larvaeofP. marmoratus are planktonic and may survive for up to 31 days. This results in high levels of gene flow between populations, and allows the species to rapidly colonise new areas.[10]
^ abS. Cannicci; M. Gomei; B. Boddi; M. Vannini (2002). "Feeding habits and natural diet of the intertidal crab Pachygrapsus marmoratus: Opportunistic browser or selective feeder?". Estuarine, Coastal and Shelf Science. 54 (6): 983–1001. doi:10.1006/ecss.2001.0869.
^Jean-Claude Dauvin (2009). "New record of the marbled crab Pachygrapsus marmoratus (Crustacea: Brachyura: Grapsoidea) on the coast of northern Cotentin, Normandy, western English Channel". Marine Biodiversity Records. 2: e92. doi:10.1017/S1755267209001109.
^Ana Catarina Ferreira Silva; Sónia Brazão; Steve J. Hawkins; Richard C. Thompson; Diana M. Boaventura (2009). "Abundance, population structure and claw morphology of the semi-terrestrial crab Pachygrapsus marmoratus (Fabricius, 1787) on shores of differing wave exposure". Marine Biology. 156 (12): 2591–2599. doi:10.1007/s00227-009-1283-1.
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