This scenario seems highly unlikey.  The edible snails that you are refering to
are Helicidae, either Helix aspersa or Otala lactea.  The local snails are
Helminthoglytidae.  My understanding is that the differences in reproductive
organs is rather distinct from species to species and I would suspect that,
from family to family, the differences would be rather insurmountable.  This
sounds like a wonderful malacological  "urban myth".

I wonder, has anyone ever collected these types of stories?  It could make for
enjoyable reading.

All the best,

Phil Liff-Grieff
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Andrew Rindsberg wrote:

> Paul Monfils wrote, "Imported snails interbreeding with local snails?  Very
> interesting! Presumably they were closely related forms?  Same genus?  At
> least same family?  Any additional information on this would be welcomed.
> Another consideration - are there some species that can reproduce
> hermaphroditically?"
>
> I don't know the specific names, but the classic example is that of an
> edible landsnail that was imported from France to California many years ago.
> When I lived in California in the 70's, I was told that they had interbred
> with the native snails, reducing their size. They were certainly present in
> overwhelming numbers, and did enormous damage to gardens. If anyone could
> affirm or deny this little story with details, I'd appreciate it.
>
> Getting back to James Cheshire's innocent question: Another good reason not
> to transport living snails away from their habitats is that some freshwater
> snails carry parasites (viruses, bacteria, worms, etc.), some of which cause
> serious diseases in people. Even if the parasites do not affect humans, they
> might well be able to infest native snails, birds, domestic or wild animals,
> etc.
>
> Well, folks, I have to step out of this discussion awhile and get back to
> other work... have fun! And listen to what Kurt has to say. He knows!
>
> Andrew K. Rindsberg
> Geological Survey of Alabama