Fishermen near the Minquiers - islands under the jurisdiction of Jersey - found the isopod, a type of louse, inside a weaver fish.
Marine researcher Paul Chambers, from the Société Jersiaise, was one of the fishing party and identified the find.
He said he was surprised to find the isopod away from the Mediterranean sea.
Isopods are normally about 2cm (1in) long and live in fish, surviving on the animal's blood, in warm waters.
Mr Chambers told BBC Jersey: "When we emptied the fish bag out there at the bottom was this incredibly ugly looking isopod.
"Really quite large, really quite hideous - if you turn it over its got dozens of these really sharp, nasty claws underneath and I thought 'that's a bit of a nasty beast'.
"I struggled for weeks to find an identification for this thing until, quite by chance I stumbled across something that looked similar in a Victorian journal.
"Apparently there's not too much ill effect to the fish itself except it's lost its tongue."
Experts at the University of Southampton confirmed that the creature was an isopod and that there had been several sightings of them in Cornwall in 1996.
Mr Chambers added: "It doesn't affect humans other than if you do actually come across a live one and try and pick it up - they are quite vicious, they will deliver a good nip."