Since its first sighting in 2008 on the central part of the western coast of northern Cotentin, the marbled crab Pachygrapsus marmoratus has been regularly sampled in the sites where it was first reported. The presence of ovigerous females and juveniles confirmed the presence of a reproductive population in the English Channel. However, the absence of this species in other sites around the Cotentin, prospected for the survey of the progression of two invasive Hemigrapsus decapods is questionable. It is suggested that the species may have good abilities to colonize new areas, considering the larval dispersal with a long planktonic larval phase, but failed in this particular case, probably due to non-favourable conditions.

The sighting of the marbled crab Pachygrapsus marmoratus on the western coast of northern Cotentin confirms the presence of this species in the English Channel. This new sighting may be related to the northern progression of the species probably due to increased sea surface temperatures.

The occurrence in British coastal waters of the marbled crab Pachygrapsus marmoratus is confirmed for the first time. Features are given to enable this crab to be distinguished from native species. Its spread along the French Atlantic coast and introduction into British waters is discussed.