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.