Steve,
Bristleworms, large and small, are oppportunistic scavenger feeders, as far as I have always understood. I agree that I may have only caught the animal that was "cleaning up" after the kill, and it sort of bothers me that I may have caught and killed an animal that might have be beneficial to my tank overall
I wonder, though, why the killer would have killed the snail then left it only half eaten and out in the open in the cave. I would have imagined a mantis would have dragged the snail back to its liar. But the worm wouldn't have to, it could just partially come out of its hole to kill then feed.
Looking at the snails that have been found dead, yes, most of them have been margaritas. However, I have margaritas in my three tanks, all bought at the same time from the same lfs, and tank temps for all three are at 78-79F. The other two tanks have not experienced any margarita deaths, so I have to assume something particular to the 120g death tank is doing in the snails and the bivalve.
After reading your post, I took a look at the activity levels of the snails in the 120g, both margaritas and astreas. The margaritas are chowing down like there's no tomorrow on the algae on the glass. They are definitely more active than the astreas in that tank, which has always been the case when I've had both astreas and margaritas in the past. As well, I have never had a successive die off of margaritas in the years I've kept them in my tanks as I am having now.
So, really, I'm hoping the worm we caught last might was the killer. If not, I don't know how long it will before we discover what animal, if any, is doing in our snails
I suppose only time and vigilance will tell if the b-worm was the culprit after all, or if something else is killing them, if we ever find out what the cause of their deaths is