First record of Pachygrapsus transversus (Gibbes, 1850) in Italian waters (Brachyura, Grapsidae).

Pachygrapsus transversus (Gibbes, 1850) is reported for the first time from Italian waters. This crab was collected at Ustica Island and at several other localities on NW Sicily (southern Tyrrhenian Sea) on vermetid reefs, an intertidal habitat frequent in the sampling area. P transversus, whose distributional range covers the warm and warm-temperate areas of the eastern and western Atlantic and the eastern Pacific, was already known from the eastern Mediterranean since the 1920s; prior to this paper it was recorded in the western Mediterranean only in 1980 and 1994 with very few specimens. There are no data to ascertain the origin of the Sicilian population, i.e., natural or ship-mediated diffusion; either is considered possible in theory. Yet, the presence of young specimens (around 5 mm carapace length) and ovigerous females, as well as the numerical abundance at one of the Sicilian sites, suggest that the NW Sicily population is stable and well acclimatized.