I remember hearing years ago that it had something to do with the competition
for the nutrients that they need to grow their shell. There was, or so I was
told, more competition as you get closer to the equator. Having no group like
this to verify I have more or less passed this on to others over the years.
If it's way out in left field, just be thankful that all those I passed it on
to were not collectors.

Frank (getting colder, I have a beaut also, but looks like it won't be a
white Christmas) in Massachusetts