Likely to be cyclic expressions of growth like the annular rings on trees.
And therefore probably tied to environmental factors as you observed.
-----Original Message-----
From: Peter Froehlich <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Sunday, May 10, 1998 10:00 AM
Subject: Growth scars
 
 
>Can anyone tell me what would cause growth scars in gastropods?
>At a beach where I often go shelling there is a population of Ilyanassa
>obseleta.  All of these shells seem to have a growth scar in the same
>location on the shell, about 5-6mm back from the lip.  Would environmental
>changes create these kind of scars?  I have also observed the same type of
>scars on Nucella lapillus which live nearby but they don't seem to effect
>as large a proportion of that population.  But I haven't observed the same
>type of scars on the Lunatia heros (moon snails) also found at this beach.
>The scaring only seems to occur on species which inhabit the various
>intertidal areas - mudflats and intertidal rocks.  I apologise for not
>checking the Littorina littorea (common european periwinkles)- I regard
>them as the intertidal equivalent of dandilions.
>
>Since N lapillus (dog winkles) and I. nassa (mud nassa's) occupy very
>different microenvironment's I am thinking that the cause must have been
>environmental such as a sudden change in food supply or other environmental
>conditions (water salinity? temperature? storm damage? ).  I don't think
>that it would have been a predator causing physical damage to the shells as
>I don't think the scars would have been that even (scars form a straight
>line and are located in the same place on the shells).
>
>If it is likely to have been an environmental change is there anyway to
>determine how long ago it took place from the amount of shell whorl laid
>down after the scar?
>
>Any thoughts or theories on this would be welcome.