Going to the known location of the nest of the last great auk or giant
condor and catching or killing the beast is one matter.  The collecting of
specimens of a "rare" marine mollusk is not a comparable situation.  First,
as time and experience have repeatedly shown, the designation "rare", as
applied to marine mollusks cannot be taken to mean "rare in nature", but
only "rare in collections". Such a designation most often means that we
simply have not yet discovered the area where the main population resides.
Cypraea fultoni, for example, was one of the greatest "rarities" for
centuries, until a few years back, when a commercial fishing vessel trawled
through their previously unknown habitat and brought up over a thousand of
them. Tucker Abbott, in his book "Kingdom of the Seashell", stated (not an
exact quote because I don't have the book in front of me)  ... "for any
species living today there must be a breeding population of at least several
thousand individuals".  Of course "several thousand individuals", if that
constituted the entire population of the species, might still be grounds for
labeling a species "rare". But more likely there are tens of thousands or
hundreds of thousands of individuals unobtrusively living their daily lives
somewhere in the 96% of ocean habitats we have not yet explored.

So, I think that collecting a species of marine mollusc that is currently
considered rare is not really an issue. "Go out and collect all you can
find" might sound wanton. But in fact, if the species is truly rare, you
probably won't find a single specimen except by an incredible stroke of
luck; that's what "rare" means; and if you do manage to find one, collecting
it is not going to significantly alter the population dynamics of the
species.  On the other hand, if you find and collect a couple of dozen
specimens, that simply demonstrates that the species is not actually rare.

Note that my comments refer to marine species.  A fresh water species known
from only one stream or a land snail restricted to one small island may very
well be hunted to extinction, which doesn't mean that every last specimen is
collected, but only that the population density is reduced to the point
where the species can no longer reproduce itself effectively.  But it is
probably impossible for human beings to bring any marine mollusc to the
brink of extinction, except by wholesale destruction of habitat, and even
then only for species which have limited geographic range.

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