Emma Padgett, 19, was about to take a mouthful of her favourite lunchtime snack when she noticed something in the Marie Rose sauce.
A baby fish head, along with eyes, teeth and whiskers, was on the end of her fork.
Miss Padgett of Newton Hill said: “I just screamed. I thought it was alive at first and I could not believe it. It looked like a snake’s head rather than anything fishy.
“I buy them all the time and had eaten one for tea the night before. It has put me off prawns for life, which is a shame as I love them.”
Miss Padgett, a junior stylist at Stylehouse hairdressers in The Springs, bought the tub from a Co-op store in Outwood. Her boss Claire Moore, of Alverthorpe, complained to the company, which sent them a packet to return the item for identification.
The Express contacted several marine biologists, who confirmed the creature was the remains of a fish. Some said it could be a dragonette or lepadogaster (clingfish).
Dr Michael Berenbrink, from the University of Liverpool’s Institute of Integrative Biology, said: “This appears to be a juvenile species, perhaps part of the scorpion fish family.
“Further examination would be needed in order to identify it accurately, which could be difficult if it is very young.”
A Co-operative Group spokesman confirmed they had received a complaint and made arrangements for the snack to be returned so it can be sent to their supplier for analysis.