A gate-crashing giant crab which arrived at the new Bristol Aquarium during a delivery of sea water has become its first unofficial resident.

The massive crab, which measures more than 20cm across the shell and has fist-sized claws, was donated to the new Blue Reef Aquarium in Harbourside by a Dorset-based haulage firm which has been providing sea water for the £4 million attraction.

The aquarium, which is set to open next month, will eventually be home to thousands of other freshwater and marine creatures from around the world including giant octopus and tropical sharks. "The crab is a fantastic specimen and he has settled in well to life in our giant rockpool display," said Blue Reef's Zahra d'Aronville. "He's going to be in splendid isolation for the next week or so until the official fish deliveries start but he seems to be enjoying all the attention he's getting and he will definitely have his place in the aquarium's history.

"The only problem is that the driver of the tanker who gave him to us did it with the proviso that we had to keep the name that he'd come up with for him – so Santa Claws has come early to Bristol this year."

The new aquarium will feature a series of new displays showcasing marine creatures from around the world in a range of themed environments.