Stunned aquarium staff have discovered that a shark they did not know they had has been living undetected in their ocean tank for years.
Staff at Blackpool Sea Life Centre were baffled when two shark eggs were discovered in a tank occupied by resident sharks which give birth only to live young.
The centre's marine experts speculate that one of several former curators must have introduced a small tropical carpet shark to the display without recording the fact - and it has been in hiding ever since.
"When we found the first egg during a routine dive in the ocean tank just before Easter, we initially thought it must be artificial, and part of the tank's theming decor," said senior aquarist Martin Sutcliffe. "We were all completely baffled when we took a closer look and realised it was real, and then we found another one about three weeks later."
Sea Life's veterinary consultant and fish specialist Sue Thornton confirmed that the four-inch-long eggs come from a carpet shark - a family so-called because of their habit of lying static on the seabed.
Mr Sutcliffe said: "The ocean tank is a massive half-a-million litre display with numerous dark nooks and crannies amongst the theming, and it is just feasible that a small shark could have stayed hidden.
"A small carpet shark would possibly feel threatened by the larger sharks in the tank, which is the only explanation we can come up with for it keeping out of sight for so long."
Now staff are seeking help from visitors to try to track down and identify their mystery guest.
Mr Sutcliffe said: "Ideally, we would like to move her to a smaller tropical tank and provide her with a mate. The sudden appearance of these eggs suggests she has reached maturity and if we partnered her with a mature male there's every chance we would get some babies."
Along with hundreds of colourful shoaling fish, the Sea Life tank features a bowmouth shark plus sandbar, nurse, black-tipped and white-tipped reef sharks - all of which produce free-swimming offspring.