So very often the truth is stranger than fiction.
Back in 2009, the staff at Newquay Blue Reef Aquarium in the UK, noticed that their corals in one of their aquarium were being were were being destroyed, and fish being eaten - and they couldn't figure out why. Remember, this is all happening in an aquarium - so it was unlikely something was coming in and doing the damage. The culprit had to beliving inside the actual enclosure and doing the damage - but no one had any idea what could be doing it! Not certain what to do, they set traps to find out what was going on.
In the morning the traps were destroyed - and nothing caught.
So they did a stake out and discovered Barry.
A 4 ft marine polychaete worm.Quite the fighter, Barry was able to bite through a 20 lb line, and is covered in numbing bristles, and put up quite the defense before being captured.
He was placed in his own aquarium and later became a tourist attraction. I couldn't find whether Barry is still alive at the aquarium, but in my browsing I discovered that it is not at all uncommon for marine aquarium enthusiast to find polychaetes causing damage to their stock - though generally they are inches long rather than feet long. Barry probably came into the aquarium in the coral rocks as a juvenile.
Enjoy your evening :)