> As my passion for shells has touched numerous other people, they
> have asked me, "How old is this, or that shell."  I have no idea.
> How old do shells grow to?

The current record is about 200 years for Margaritifera margaritifera
(pearly mussel) form Scandinavia, though deep sea species might pass
that, and Arctica islandica well over 100 years is known.  On the
other hand, many species probably live no more than a year-Donax
variabilis, the coquina, averages a few months.  Many shells have some
sort of growth lines, like tree rings.  The problem is being sure
whether these are annual, seasonal, tidal, reflecting occasional
growth stoppage for unpleasant conditions, reflecting growth stoppage
to put energy into reproduction instead, etc.  This can be tested by
growing a shell under observation, by looking for size classes (if
they breed once a year, you ought to see distinct sizes and can count
how many, though this doesn't help once they reach near-maximum size,
if a maximum exists), or by doing careful chemical analysis of tiny
bits of shell along the direction of growth, looking for seasonal
variations.  The average large shell probably lives a few years to a
few decades.

--
Dr. David Campbell
425 Scientific Collections Building
Department of Biological Sciences
Biodiversity and Systematics
University of Alabama, Box 870345
Tuscaloosa AL 35487-0345  USA

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