I don't know if I agree with the bristle worm scenario. Bristle worms sting usually whether they want to or not. Larger clams are less susceptible to the stings and don't really notice them that much unless they are completely unprotected at the byssal opening. If you have clams in your tank that are attached to a rock or even in the sand (especially in the sand) you can usually lift an established clam up (DON'T OR YOU COULD KILL IT) and find several of bristle worms on the underside.
The reason they are most of the time found near the byssal opening and not near the ligament is PROBABLY due to the decaying matter coming out of that area. Granted, as I said above though, the smaller ones are going to be more prone to noticing the stings and even become rather stressed out due to them, then a larger one would. That is just in high enough numbers however.
Just a thought. But byssal threads SMELL. I have not gotten around to it that much, but I think they are constantly decaying due to lack of blood flow and new ones are constantly being created. Nothing is more enticing to a bristle worm then the smell of matter dying off.
