The bug - which has the scientific name cymothoa exigua - was discovered inside the mouth of a red snapper bought from a London fishmonger.

The 3.5cm creature had grabbed onto the fish's tongue and slowly ate away at it until only a stub was left.

It then latched onto the stub and became the fish's "replacement tongue".

Excited

Scientists are very excited by the find.

Dr Jim Brock of the Horniman Museum in south London told Metro newspaper: "I have not seen this species in all my 13 years at the museum so it's a remarkable find."

What the bug looks like

The bugs are usually found off the coast of California, so it's possible the fish was imported to the UK.

Freaked out

But don't be too freaked out - scientists say the creature does not pose any threat to humans and only attaches itself to fish tongues.