There are a few complicating factors in the size versus age for
mollusks.  Influences include temperature, food supply, and other
environmental factors as well as breeding condition.  Once a mollusk
matures, it may not stop growing (some do, some don't).  However,
growth slows considerably because much of the nutrition and energy
that once went into growth is now put into reproduction.  If
conditions are unsuitable for reproduction (too cold, too hot,
parasites destroyed the gonads, etc.), then an animal may grow more
instead of reproducing.  Thus, extra large individuals may result from
multiple causes.  Similarly, growth lines may result from low
temperature, high temperature, low tide, etc. and cannot be assumed to
be annual.  If there's regular seasonal variation at a locality, then
there will be stable isotope variations that can be measured to
confirm the time between growth lines.

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

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