Nucella lapillus is one mollusc that is extensively discussed in the book I
recommended yesterday (Biology of Rocky Shores). One reference cited is:
Gibbs, P.E. (1993). Phenotypic changes in the progeny of Nucella lapillus
transplanted from an exposed shore to sheltered inlets. J. Molluscan Studies,
59:187-194. In a shore exposed to waves shells had large apertures and thin
shells, but those from a sheltered shore had thick shells and small
apertures. The explanation is that at exposed shores predators (crabs) are
rare so the animals can afford having a thin shell while having a large
aperture means that they have larger feet to better hold on to the rocks. In
sheltered shores it is the opposite: there are crabs, but no waves.
 
So, it is important for a collector to note the detailed habitat of a shell,
not just the microhabitat, but also the general surroundings, wave exposure,
and so on. In a given location, say an island, one bay may be exposed, while
the one next to it may be more sheltered, giving rise to differences in shell
shapes, etc.
 
A.