Shell morphological variation in the dogwhelk ( Nucella lapillus ) along the Bay of Fundy, New Brunswick, Canada

Many marine gastropods exhibit remarkable intraspecific diversity in shell morphology. While shell form often represents adaptations to abiotic and biotic stressors, the function and drivers of other morphological features such as colour remain understudied. We quantified spatial variation in both shell form (size, shape and thickness) and colour in dogwhelks (Nucella lapillus), a common predatory gastropod in the North Atlantic rocky intertidal zone. To test whether morphological trends exhibit clinal or patchy patterns along a strong tidal gradient, we sampled 808 dogwhelks along the New Brunswick coast of the Bay of Fundy in Canada, which experiences the largest tides in the world. Shell form metrics were strongly correlated: As shells got longer, they also got wider, thicker and had larger apertures. Sites differed significantly in average shell form, but there was no clear spatial pattern associated with tidal range. Instead, Bay of Fundy dogwhelks are predominantly the elongate ‘protected’ ecotype, which is thought to prevent desiccation stress and protect dogwhelks from predators on low-wave-energy shores. Interestingly, shell colour exhibited a spatially patchy pattern, whereby a subset of sites exhibited elevated colour polymorphism. One site in particular, Beaver Harbour, had substantially higher colour diversity and bandedness than all other sites. The highly localized and patchy spatial pattern of colourfulness may be influenced by the species’ limited dispersal capacity. Future research can build on this natural history work to elucidate the genomic basis of colour in dogwhelks.