I am working on that question regarding some species of freshwater mussels. So far, I have counted the "rings" on the outer surface of the shell and I have counted the "rest marks" that are laid down in the ligament that holds the two shells together as well as apart. Both of those features are produced by the animal during the spawning period and during the winter. In my species, those events occur at about the same time each year; consequently, those marks are called "annuli". So long as there are no other events during the year that would stimulate a resting response from the animal, there sould be one mark per year caused by a drop in the growth rate. A toxic spill can produce such a response so there might be a closer spacing of the annuli in the ligament. On the other hand, if the animal doesn't spawn and the winter has been mild, there might not be a rest mark produced that year. The animal I am studying, Margaritifera falcata, the Western Pearlmussel is purported to live 80 to more than a hundred years. Animals that old have shells about 6" long but the growth rate is directly linked to temperature - the warmer the climate, the faster they grow but the maximum decreases. The old ones are more often found in the northern lattitudes.

I have noticed irregulatities in the shells of some marine snails that look like they might be rest marks, too. They are usually interruptions in the spacing of the ornamentation. I've never looked for shells showing those marks so I could compare the shells to see whether the marks apear at the same locations on each shell and whether the numbers of the marks vary with the shell lengths. I think I'll start looking for marks on the Fringed Dogwinkle, Nucella lamellosa and the Northern Moonsnail, Polinices lewisii here in the Puget Sound where they are very common. and see if each has some obvious feature that might be linked to an annual cycle.

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? I have not seen a "small/baby cone". Yet I do not believe they are born 50 mm long. Where are all the babies? For example, I never saw a small Tiger Cowrie. Where are they hiding? Can anybody give us some insight?