They're great detritus eaters. Alot of people used to(and some still do) consider them dangerous, but I have TONS of coral and no damage to any from the bristle worms in my tank. When I pulled all the LR out of my tank a couple months ago to put my fish in QT, I found one that was yellow and red that was about 6" long when scrunched up, but stretched to at least 12". It just burrows through the sand and harms nothing. It scared the !@#* out of my wife, but I think it is cool.