Paul,

No thoughts of my own, but more from Doug Jones. Here is the section
relevant to your question from Dr. Doug Jones' 1989 Am. Conch. article.

"...The regular spacing of external growth rings on shells and their
progressive crowding as the animals grow older continue to prompt people
to interpret them as age markers, analogous to annual rings in trees.

"Actually, for many decades the growth patterns in molluscan shells have
been the subject of serious biological and paleontological inquiry.
Early workers concentrated on growth structures visible on the external
surfaces of shells, such as ribs, frills, and concentric rings on the
bivalve shells. These structures often were considered (usually without
much evidence) to form annually, so that by counting the number of
rings, a clam's age could be determined. Despite extensive study,
however, the use of external shell growth patterns in most species has
been found to be rather limited. The inability to distinguish true
periodic structures on shell surfaces from random disturbance marks
(e.g., those induced by severe storms), and the difficulty in obtaining
accurate ring counts because of extreme crowding toward the edge of old
shells, have contributed to skepticism concerning the reliability of
external shell rings as age indicators.

"Within the last few decades, studies of molluscan growth structures
have focussed upon periodic patterns within shells. These studies have
dramatically changed our ideas about the lifespans of typical bivalve
species. For instance, longevity estimates of some common coastal
bivalves, once thought to be fairly ephemeral creatures, have been
extended to 50 years or more, while certain offshore species are known
to live for over 100 years.

"Internal shell growth patterns are best viewed in cross-sections taken
from the umbo to the growing shell margin. This technique is readily
applied to bivalve shells, where a straight cut by a rock-cutting saw
equipped with a diamond blade reveals the entire growth history of the
animal in the sectioned shell. The majority of shells that have been
examined in cross sections possess annual patterns of growth increments,
typically recognizable as large white bands alternating with thinner
dark bands or rings. The combination of one dark and one white increment
represents an annual cycle of shell growth. The dark rings correspond to
the rings on the shell's exterior (where there may also be spurious
disturbance marks) and can form in response to reduced shell
calcification rates resulting from a variety of factors, including cold
winter or warin summer temperatures, annual spawning cycles, etc.

"Because of this potential variability in timing and cause of ring
formation, the periodicity of supposed yearly increments in a given
bivalve species should be adequately documented before age assessments
are made. This is frequently accomplished by mark-and-recovery
experiments where live bivalves are marked, released for a known
interval of time such as a year or two, recaptured and sectioned. The
number of rings that formed in the elapsed time, and hence the
periodicity of growth ring formation can then be determined. Other
documentation techniques include monthly collection and analysis of
specimens from local populations to assess seasonality of growth
increment formation as well as a variety of sophisticated
chemical/isotopic approaches.

"Studies of internal shell growth increments confirm that annual rings
are generally the most ubiquitous and useful periodic growth feature in
bivalves, directly analogous to annual tree rings. However, they are not
the only periodic shell structures. About 25 years ago, R.M. Barker
suggetsed that a whole hierarchy of periodicities are preserved as
alternating light and dark increments in the bivalve shell. These range
in size from a few microns to a few centimeters and are thoughts to
refelect periodicities such as subdaily tidal cycles, daily light-dark
cycles, fortnightly tidal cycles, and annual (seasonal) temperature
cycles. Most of these smaller scale increments can be studied only with
the aid of a microscope...."

Lynn Scheu
Louisville, KY

Paul Monfils wrote:
>
> 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.