ID'd to Amphinomidae by Dot Norris. From there the only tricky part of the Light's key is the "caruncle" which on this worm is the little wavy thing extending dorsally from the head back two segments. Branchiae present on all segments.

I found this worm pumping for ghost shrimp in sand, so it's possible it was living comensally in a ghost shrimp burrow.