Well, Hanna, I can't imagine that anyone would recommend C. brunneus as a good addition to a reef tank. Although these predatory muricid gastropods tend to specialize on other gastropods in the wild, there is always a risk that it could attack your clam.
It may not find that the effort is worth the payoff if you keep the animal well-fed on open clams, mussels, and periwinkles from the local fish market, but there is no guarantees on that. Most likely, even if it were hungry or bored with it’s diet, it would prefer to eat the common snails that are added to a reef tank as part of the clean-up crew (such as Astrea, Trochus, and Nassarius.
However, there is always a great deal of individual variability in the prey preferences and diet breadth of any predator, so there is no way to say for sure whether or not your particular snail is likely to attack clams in your aquarium. Having said that, the snail is perfectly capable of killing a clam, and there is certainly the possibility that it could happen...
Rob
