It seems that Nucella lapillus may follow the pattern described for
certain other gastropods, of greater color variation toward the
extremes of the geographic range.  Ross described colorful
populations way up there in Nova Scotia.  I have seen similar
populations in central Maine, with shells showing various shades of
orange, yellow, grey, black, tan and brown, and often banded with a
contrasting color, or with white.  There are virtually no all-white
shells in these populations.  Only a hundred or so miles south, in
central Massachusetts, north of Cape Cod, the species looks radically
different.  Around Plymouth are stretches of rocky shoreline with tens
of thousands of Nucella, virtually every one of them pure white - in
some areas, maybe a few brown ones mixed in, less than 1%.  No bright
colors, no banding at all.  The shore structure, vegetation, and water
temperature in these areas are very similar to those in Maine.
However, south of Cape Cod, in southern Massachusetts, and moving
down into Rhode Island, colorful and banded populations are again
encountered, though not showing as much variation as the northernmost
populations.
The reference cited by Aydin is very interesting, as it suggests that
the shell structure of these animals is environmentally, not
genetically controlled.  It is easy to see how populations in
different habitats might evolve different phenotypes, but if
artificially transplanting a population to a new habitat brings about
such changes in next generation offspring, it is obvious the change is
produced by environmental factors, not heredity.  What might the
actual mechanism(s) be?  Any ideas?  A stronger, thicker shell might
provide protection from predators (though even "thin-shelled" Nucella
lapillus still have a very strong shell), but certainly the presence
of predators is not the stimulus that actually produces the change.
Has anyone tried transplanting individuals from the central
geographic range into habitats near the extremes of the range (or
vice versa), to see if the observed color variations are
environmentally induced?  (Probably not, given that the environmental
conditions at the two extremes of a species' range are often very
different, while the conditions in the central range typically fall
between those at the two extremes).  Not likely that both cold and
warm water could stimulate production of colored shells, while
intermediate temperatures would cause white shells.
 
Paul M.