A worldwide community photographing and learning about wildlife
there are several others even some 'imported' from Asia..... don't know, will search around
I also found this, wich contains lots of different color shadings of the species:
http://eol.org/pages/343455/media
Wiki indicates that "P. marmoratus has a square carapace 22–36 millimetres (0.87–1.4 in) long, which is dark violet brown with marbling in yellow"
But there are several other (I think) Pachygrapsus species in the Mediterranean...
Hm I'm not sure is this one, I have seen too many Pachygrapsus marmoratus in my life and none was violet and so dark apart from this one, and this one is quite big too, medium size!