Kim's posting brings up another aspect - that baiting can be used without
actual trapping.  Many small carnivores, like Nassariidae, spend much of their
time burrowing, or simply resting buried in the sediment; but the smell of
food can bring them out in a hurry.  There are a couple of ways to approach
this.  I have had the experience of tossing a broken mussel (Mytilus edulis)
onto a shallow muddy bottom, and a half hour later returning to find the
mussel absolutely buried in hungry mud nassas (Nassarius obsoletus).  There
have also been some papers written (years ago) on methods of concentrating
gastropods in a small area by baiting.  Burrowing snails can be collected by
screening methods - shoveling or fanning the sediment onto a screen, and
sifting it through.  But it's a lot of work if you have to sift many square
meters of sediment to find a few snails.  This method of collecting is much
more productive if you can get the snails to congregate in an area of one or
two square meters before you start screening.  This can be accomplished by
baiting.  A dead fish or other suitable bait can be secured to the bottom
either by driving a stake through it and into the bottom, or by laying a heavy
flat rock on top of it.  It is left there anywhere from overnight to a few
days.  At that time, some snails may be found on the bait, but in the sand or
mud surrounding the bait, the concentration of snails is likely to be much
higher than in a comparable unbaited area, and they can be easily collected by
screening.  This method has also been used to collect fresh water snails like
Campeloma.  I recall one paper which described concentrating Campeloma by
staking down a small burlap bag filled with chicken dung.
Paul M.