The pearlfish makes its home in the anus of its victims, emerging from the shadows to feed before ducking back into the darkness.
But if food is scarce, it devours its host from the inside out, chomping its way through the respiratory system and muscles as it goes.
It will even breed inside the bumhole, with as many as 15 pearlfish being found inside one unfortunate sea critter.
“They’ve got a toxic skin. Once they get to full size not much eats them”Professor Jamie Seymour
The bizarre species is typically found in tropical waters of the Atlantic, Indian and Pacific Oceans.
And thankfully, it prefers the rear ends of sea cucumbers and similar creatures to human hosts.
Professor Jamie Seymour from Australia’s James Cook University described the gruesome fish in an online video.
He said” “There have got to be better places to live than up the rear end of a sea cucumber. Why would you wanna live there?
“Well the answer I think is pretty obvious – not much eats sea cucumbers. They’ve got a toxic skin, so once they get to full size there’s not much eats them.
“So if you’re a little fish with no sharp projections or anything, you can hide in there, nothing is going to eat you, you can sleep in there and not have to worry.
“And if things get really tough you can start to feed on the sea cucumber as well. All in all, for the fish, a real bonus.
“Still don’t know quite what’s in it for the sea cucumber though.”
Professor Seymour revealed the secrets of the fish on the YouTube channel, The Nature of Science.
He even probed the inside of a sea cucumber with a micro-camera, describing the digs as “pretty horrific”.
It comes after a fish discovered with legs and a bullet-fast bite was branded an “alien”.
While a new species of shark has been found in the depths of the ocean – and it’s pitch black.