Larva/Parasite/Worm? Please help ID the strangest creature in my tank

[yuzusama]

I was checking on my 40G reef tank at midnight as usual, and this fast-swimming worm like creature scared the *** out of me. This is the first time I see a creature like this in my 15 months old tank. Does anyone know what this is? Could it be a larva or parasite? I got it quarantined and would only release it back to the tank if it is beneficial.

I mean it just looks too creepy and bizarre.

 

[yuzusama]

 

[Tired]

That's an epitoke of some sort. Bristleworms and other polychate worms make these; they actually break off the back portion of their body, which goes swimming away into the current. It has reproductive organs in it, and is meant to spawn along with loads of other epitokes, reproducing out in the current while allowing the worm itself to stay hidden and safe. Likely from one of the common bristleworms. Harmless.

 

[OrionN]

It is beneficial fauna in your tank. Great scavengers and produce larvae ad food for the filter feeder. They can bust mid swim to form a cloud of smoke which are larvae that feed other animal or if live form new worms.