A woman opened a tin of Princes Tuna Chunks to find a tiny creature with prominent black eyes and a tail looking back at her.
Zoe Butler, 28, from Nottingham, was opening the canned fish for her daughter's dinner but screamed in horror when she saw what looked like a "gut sack" with eyes..
"I opened the top of the lid and saw a purply thing, a gut sack or intestine – then I turned it round and pushed it with a fork and saw it looking back at me," she told Nottingham Post.
"It's got like a spiny tail along the bottom – it's quite grim," she added. "I dropped the fork, jumped back, screamed a bit and shouted for my nan to come and have a look."
Dr Hany Elsheikha, associate professor of veterinary parasitology at the University of Nottingham, told the paper it could be some form of "juvenile crab."
Ms Butler, who bought the £1 tin from Asda in the Arnold area of the city, complained to Princes.
The mother-of-two said that she just wanted to find out what the tuna invader is, adding: "I didn't set out to get compensation and I don't a want lifetime's supply of tuna!"
A spokesman for the firm said: "We were contacted by Mrs Butler and immediately responded to apologise."
It arranged for Ms Butler to send them the mystery creature "so that we can look into this matter fully."
She wrote on Twitter this week: