One obvious question comes to mind in reading all this - how do we know
that a particular growth ring is an ANNUAL growth ring?  Growth rings,
as the name indicates, mark periods of growth, separated by periods of
non-growth, or at least greatly reduced growth.  But growth spurts can
be caused by changes in temperature, temporary blooms of algae and other
food species, and various other factors.  I can easily imagine a clam
with 150 growth increments marked on its shell.  But 150 ANNUAL growth
increments is a very different story.  Also, I had thought that annual
growth lines were pretty much restricted to shallow water species of
temperate climates, indicative of major changes in ambient temperature,
as well as seasonal reproductive cycles of food species.  Tropical
species, I thought, do not usually show annual lines, due to the lack of
extreme environmental temperature fluctuations, and, I thought deep
water species also lacked annual lines for the same reason??  Any
information on this?

Paul M.