I have collected both forms of Neverita duplicata off Louisiana. The
low-spired form was collected off Cameron Parish in 2 fathoms and the
high-spired form off Vermilion Parish, also in two fathoms. This makes them
basically sympatric, since the two parishes are side by side. Obviously,
neither depth nor habitat have to do with their differences, although my
high-spired form has rust stains and the flat form is perfectly clean.
Which reminds me of many other species that are collected sympatrically but
a specific species may have stains and the others are clean.  I assume this
happens because they are embedded in different strata? Could this also be a
<specific> characteristic
 
Emilio F. Garcia
115 Oak Crest Dr.
Lafayette, LA 70503
USA