RAD sequencing of common whelk, Buccinum undatum, reveals fine-scale population structuring in Europe and cryptic speciation within the North Atlantic.

Buccinum undatum is a subtidal gastropod that exhibits clear spatial variation in several phenotypic shell traits (color, shape, and thickness) across its North Atlantic distribution. Studies of spatial phenotypic variation exist for the species; however, population genetic studies have thus far relied on a limited set of mitochondrial and microsatellite markers. Here, we greatly expand on previous work by characterizing population genetic structure in B. undatum across the North Atlantic from SNP variation obtained by RAD sequencing. There was a high degree of genetic differentiation between Canadian and European populations (Iceland, Faroe Islands, and England) consistent with the divergence of populations in allopatry (F ST > 0.57 for all pairwise comparisons). In addition, B. undatum populations within Iceland, the Faroe Islands, and England are typified by weak but significant genetic structuring following an isolation-by-distance model. Finally, we established a significant correlation between genetic structuring in Iceland and two phenotypic traits: shell shape and color frequency. The works detailed here enhance our understanding of genetic structuring in B. undatum and establish the species as an intriguing model for future genome-wide association studies.

Shell morphology and color of the subtidal whelk Buccinum undatum exhibit fine-scaled spatial patterns.

Geographical patterns in morphology can be the result of divergence among populations due to neutral or selective changes and/or phenotypic plasticity in response to different environments. Marine gastropods are ideal subjects on which to explore these patterns, by virtue of the remarkable intraspecific variation in life-history traits and morphology often observed across relatively small spatial scales. The ubiquitous N-Atlantic common whelk (Buccinum undatum) is well known for spatial variation in life-history traits and morphology. Previous studies on genetic population structure have revealed that it exhibits significant differentiation across geographic distances. Within Breiðafjörður Bay, a large and shallow bay in W-Iceland, genetic differentiation was demonstrated between whelks from sites separated by just 20 km. Here, we extended our previous studies on the common whelk in Breiðafjörður Bay by quantifying phenotypic variation in shell morphology and color throughout the Bay. We sought to test whether trait differentiation is dependent on geographic distance and/or environmental variability. Whelk in Breiðafjörður Bay displayed fine-scale patterns of spatial variation in shape, thickness, and color diversity. Differentiation increased with increasing distance between populations, indicating that population connectivity is limited. Both shape and color varied along a gradient from the inner part of the bay in the east to the outer part in the west. Whelk shells in the innermost part of Breiðafjörður Bay were thick with an elongate shell, round aperture, and low color diversity, whereas in the outer part of the bay the shells were thinner, rounder, with a more elongate aperture and richer color diversity. Significant site-specific difference in shell traits of the common whelk in correlation with environmental variables indicates the presence of local ecotypes and limited demographic connectivity.