I would agree; the only thing I can think of would be a predatory whelk or snail... If you are bored there is a good article on what they are and how they accomplish their goals here.
http://www.dailykos.com/story/2007/1/19/185713/309 Yeah, it is only really about one type, but it gives you a good idea of what you may be dealing with and how they kill.
From that article... "Oyster drills feed mainly on bivalve mollusks, but are equally adept at penetrating the defenses of barnacles, periwinkles, and when times get tough, even others of its own species." So I guess you could always buy a nice, expensive maxima as bait.
