> You do come up with creative insights.  Regarding your below comments, I'm
> not sure I understand how leaving shells where they die results in CO2
> emissions, but placing them in collections does not.

If the shells would have been broken down and chemically eroded,
releasing CO2, if left outdoors, but are protected and kept from
breaking down when you collect them, then a small amount of CO2 is
being removed from circulation for a long time.  In fact, carbonate
skeletons (or inorganically precipitated carbonates) are probably
rather better than plant material as a way to sequester carbon dioxide
for the very long term-limestone is rather more widespread in the
goelogic record than coal.  In the short term, however, even a
Tridacna ranch doesn't use as much CO2 as a tree farm.

Of course, as Paul pointed out, if you transport the shells with a
fossil fuel-burning vehicle, you probably aren't producing a net
carbon dioxide decrease.  However, it's equally true that collecting
shells is unlikely to cause serious problems, except in the case of
rare species (mostly non-marine), commercial-scale collecting, or
destructive techniques.

--
Dr. David Campbell
425 Scientific Collections
University of Alabama
"I think of my happy condition, surrounded by acres of clams"

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