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© Todd Warshaw
Shocked staff at the Blue Planet Aquarium in Cheshire Oaks have discovered a mysterious fish which has been living unnoticed in one of their displays.

The 40-centimetre-long sand sole was discovered during routine cleaning work on an open native rock-pool tank.

Stock checks confirmed there were no records of the flatfish having arrived at the aquarium and staff believe it could have remained undiscovered for years - potentially since the aquarium opened 13 years ago.

Acting curator Colin Grist said: "They are, as a flat fish, very cryptic, so that might explain how it has remained unseen for possibly years.

"There is no record of the species being acquired and introduced in recent years so it could possibly be a survivor from the days the aquarium was built.

"Flatfish in general are extremely well camouflaged and sand soles in particular are real masters of disguise which are able to blend in perfectly with their environment.

"Although we haven't been aware of its existence it seems to have been doing extremely well. It's in perfect condition and has obviously been feeding well and probably waiting until the lights go out to go out hunting.

"It's extraordinary that it's managed to have remained hidden for so long but it does go to show just how well evolved it is to its natural environment."

Found in the English Channel and off south-west England and Wales, the sand sole is a bottom-dwelling species usually found on gravel, sand or muddy seabeds in shallow waters from 20 metres down to 50 metres in depth.

The fish has an elongated, oval body up to 40cm in length, with its eyes on the right side of its body.