Well that makes for an interesting read at 2am. I think I'll never reach into my tank again unless using tweezers, holy crap these things are nasty animals, quite a thrilling read, thank you
Did you ever catch your worm? I caught mine last night....dumb luck. Was rescaping picked up a rock and he was mostly out, coiled under it...started to dive into that rock, quickly freshwater dipped and he backed out.
Was not near as big as the one you have burrowing... was probably 8-10 inches long, but maybe 1/2 the size of a pencil thick.
I had one 15 years ago in my first reef tank and you can see them. During the day they hang out near the sand bed and you can see their five or so tentacles that come off of their face. At night they come out but are very skittish. They don't look brown like the photo the one I had red and off white legs in alternate pairs and the segmented body was iridescent. They are scary and beautiful at the same time. Best way to find out if you have one is by looking at the tank at night after the light have been off for a while. They are super fast though. I never caught mine until a power outage/surge nuked the tank. I moved out here and left the tank in my parents care and they didn't know what to do so it sat for a while. This thing lived for 3-4 weeks while the tank sat. I'd say it was about 6 feet and still very active when I removed it from the tank.
That is creepy!! I hate brittle worms i can only imagine this but if I had this is my tank I'd be so scared I wouldn't even dip my hand into the water...