Interesting. Those look less bristly than what I have. Maybe it is the camera or container playing tricks. Have you tried cutting them to see if you get two viable worms?
I will try to get some good shots of mine posted for comparison.
I agree. Looks like they can get very Big. I have been wanting to catch and rid of them for a while now. I just had a Snowflake Clown disappear and believe that worm was the culprit. It is a Eunicid Tomoko. I looked them up and these look just like them. These hitchhikers must have came on some live Fiji rock I got from Chris. I read where they are from Fiji. Ugly looking worms.
Here's three large bristle worms (at least I think so) living in close quarters with a damsel. I don't think they can move fast enough to catch a fish. I also have a 3.5π inch worm in another tank with 3 clown gobies who tend to sit very still. A cleanup crew can make quick work of a dead fish.
And a few others for grins and to prove the glass isn't always filthy. I've seen the biggest one stretch from the bottom of the tank to the top.
All of these are examples of bristle worms. I have some monsters in my tank as well. I also had some Eunice worms and side by side it is easy to see a difference. I have never seen a bristle worm take interest in a live fish before.