All right, Jenny, I admit that was a bit flippant, and the other answers
you received have not all been straightforward either. The ultimate reason
is that English, or any other language, does not represent reality very
well. We can easily pose questions that do not correspond to any kind of
reality, such as the classic, "Have you stopped beating your wife?" or the
even more absurd, "Why have you stopped beating your wife?" Your questions
are not outrageous, but they do not admit of clearcut answers either, so
I'm afraid we had a little fun at your expense. OK, as an apology, here's a
straightforward reply to two non-straightforward questions:
 
To get to specifics, it is a historical accident that, in English, we have
two words for those who study shells. In older usage, they were synonyms
(our lexicographer, Gary Rosenberg, can correct me if I'm wrong on this).
In today's usage, a conchologist is someone who emphasizes the shell, and a
malacologist is someone who studies the whole animal, with, as you said, a
continuum between them. But it is just an accident that we have two words
instead of one; there's no deep meaning to be found here. When I said that
the best malacologist is the one who has studied the most shells, of course
I was speaking poetically, not with scientific precision. An old habit from
someone who wrote poetry and studied linguistics before majoring in
geology.
 
As to the relative value of a gem shell and well-preserved soft tissue, my
answer is another question: For what purpose? The shell is of no use at
all, valueless, for studies of the food value of scallops, for instance.
And although you might get some DNA out of a shell, it's not the best way
to do it, and you wouldn't have a gem shell afterwards. To answer the
question, "How long does a scallop live?", I'd want to cut and count the
rings of a shell, and I'd want neither a gem nor an alcoholic specimen for
that. For comparison with fossils, which lack soft tissues, I'll take the
gem, or at least a representative shell specimen. The preservatives that
are used for soft tissues are harsh on shells, so the collector has to be
decisive.
 
To give a practical answer for someone who is planning to document the
species of a poorly known area: Try to get some of each. If you collect a
gem-quality specimen and another of the same species that is less than gem,
and have alcohol at hand, I'd recommend that the gem be cleaned and the
second be put in alcohol. If you have only one specimen of a species and it
is gem, then you have to evaluate whether this species is better identified
from an alcoholic specimen or a gem shell, and act accordingly, since your
chief purpose is to identify species. Within the limits posed by good
conservation practice, I'd also recommend that you collect several
specimens of each species to show the limits of species variation.
 
Incidentally, maintaining a collection of alcoholic specimens is a big job.
Alcohol evaporates from the jars and must be replaced periodically. The
collection is a very real fire hazard and must be treated accordingly.
Fumes must be vented properly, no smoking allowed, and so on. Me, I'd
arrange to send alcoholic specimens on to an institution that already
handles such specimens. For instructions (which you already have, Jenny,
but I'm including this for newcomers), the Conch-L Archive has a thread on
alcoholic specimens and how to prepare, store, and transport them.
 
Apology accepted?
 
Andrew K. Rindsberg
Geological Survey of Alabama