There is zero indications for let you be helped here (name/location of the zoo, thematic of the tank, size of the crab, etc etc) but I think it's reasonable to assume that it can be a species of Inachus, most probably Inachus phalangium or Inachus dorsettensis. MarLIN - The Marine Life Information Network - Leach's spider crab (Inachus phalangium)
@Kakapo Both the crab itself and the tank it was in were fairly small, the crab's body wasn't much larger than a grape, it shared its tank with European prawns, but there was no signage for it. The place in question is called the Lake District Coast Aquarium, in England. Looking at the link you posted, it's likely that that is the species. Thanks very much!
