Dear Beth,
 
Good question. I suspect that to be the case with our Florida Crown Conch -
Melongena corona (Gmelin, 1791). Oysters and cannibalism seem connected
with large size with regard to that species.
With Neverita (or Polinices), where forms overlap, at inlet areas,
interbreeding does not seem to take place. The distinctive shells remain
distinctive. I have collected both forms in ethanol in hope that a
researcher will want to examine the beasts. I think your idea to collect
the two forms alive and keep them in an aquarium for comparison is an
excellent one. I'm set up to photograph them, but will have to wait for the
next season - November or December. Shame on me for not thinking to do this
when I had the chance.
 
Phil P.