The dogwhelk, Nucella lapillus, is an intertidal predator that displays classic ecotypic variation. Dogwhelks from exposed shores typically have small shells with large apertural openings while dogwhelks on protected shores have large, robust shells with thick apertural lips. The morphology of each ecotype is adaptive in its respective environment as the exposed shore morph minimizes the risk of dislodgement in heavy surf and the protected shore morph is more resistant to the shell-crushing predators common on protected shores This morphological variation has been attributed to wave exposure, site-specific and chromosomal factors and phenotypic plasticity. Through morphological analyses we have documented extensive site-specific variation in five morphological traits. Specifically, we found that site-specific factors rather than exposure explained a greater proportion of the variance across the five shell traits we examined. We have also documented the presence of a chromosomal polymorphism in Western Atlantic populations of the dogwhelk which were previously believed to be monomorphically of the 2n = 27 karyotype. We have found that chromosome number ranges from 2n = 26 to 2n = 32 in dogwhelk populations in Maine. Furthermore, we suggest that chromosome number is correlated with morphology and may explain the site-specific variation we observed in our morphological survey. Lastly, we transplanted snails of different karyotypes from exposed and protected shores to four different protected shores in order to examine the plastic response in shell lip thickness and aperture width across sites, karyotype, and exposure. The results of this experiment suggest that chromosome number plays a role in the phenotypic response of shell traits to environmental stimuli and that this response is variable depending upon which trait is being examined. We assert that the drastic morphological variation observed in N. lapillus is not simply due to wave exposure, but is instead a plastic response mediated by chromosomal factors.