Dog Whelk/ Nucella lapillus

Is the shell thickness of N.lapillus genetic or is it as a response to the environment or both? e.g. shells are thinner on more sheltered shores but is this as a result of there being more predators found there therefore natural selection has meant that those with thicker shells do not survive and this is passed on to offspring or is it that there is less exposure therefore the muscular foot is not as large?