> Is there, indeed, any evidence that longevity in these clams is related to
> depth or shell size?  Has there been any radiometric verification of the
> ages of these "Killer" and other large/old species?  When including
> bomb-radiocarbon- and lead-dating, there are sometimes great discrepancies
> among aging techniques of fish and molluscs; esp. compared with
> ring-counting; and esp.  in deep-sea species.  As any tropical forest
> ecologist will tell you, growth regions (in trees as well as calcareous
> mollusks) don't always equal years when there are no annual growth seasons
> to which the organism responds.

Giant clams (Tridacna) grow fast for moderately long time periods, so
their age is in the low decades.  Maximum size for them is about 4
feet.  I don't know if anyone has aged the giant fossil rudists or
inoceramids of comparable to slightly larger size.

Much of the age-dating in mollusks uses carbon and oxygen stable
isotopes as a check to confirm that growth rings are annual.  Due to
various seasonal influences, the ratio of 13C to 12C and 18O to 16O
typically varies through the year.  Since shells are CaCO3, there's
plenty of both to measure.  In extratropical settings, annual rings
are the norm, though whether summer or winter is a time of little
growth deends on the species and location.  Also, little to no growth
may occur in a given year once adulthood is reached, making ring
counting difficult.

Deep-sea and high-latitude species seem to be most likely to live for
very long times, reflecting large portions of their life spent waiting
for better conditions.  Thus, some freshwater mussels live for decades
in many parts of the world (if their habitat doesn't get wiped out),
but Scandanavian Margaritifera are known to pass 2 centuries.  Ages
for deep-sea taxa are less well documented.

On the other hand, a few months is the normal lifespan for Donax variabilis.

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

----------------------------------------------------------------------
[log in to unmask] - a forum for informal discussions on molluscs
To leave this list, click on the following web link:
http://listserv.uga.edu/cgi-bin/wa?SUBED1=conch-l&A=1
Type your email address and name in the appropriate box and
click leave the list.
----------------------------------------------------------------------