Pachygrapsus maurus and Pachygrapsus transversus (Crustacea: Decapoda) in the central Mediterranean: new colonizers or overlooked species?

The warm-Atlantic crabs Pachygrapsus maurus (Lucas, 1846) and P. transversus (Gibbes, 1850) are two congeneric species showing a tendentially disjoined distribution in the Mediterranean Sea, with cores in the Algero-Provencal and Levantine basins, and sporadic records elsewhere. In the north-central Mediterranean, first records of these taxa date back centuries ago, but subsequently they were no longer reported for almost a century. We hereby report their additional spread in central Mediterranean and discuss the recent proliferation of sightings in the area. The recently ascertained occurrence of both species in the Strait of Messina confirms that some less stenotherm taxa of tropical/subtropical origin may colonize some sheltered habitats despite cooling of surface water due to the local upwelling. By contrast, the different settlement success observed so far in the north-central Tyrrhenian Sea showed that P. maurus might be able to further spread and colonize the entire area, whereas P. transversus might be hampered by interspecific competition with the autochthonous P. marmoratus further than climatic constraints.