Gastropod shell differentiation following colonization of an invasive intertidal macrophyte in Atlantic Canada

In the 19th century, a lower intertidal macrophyte, Fucus serratus (Linnaeus), from western Europe, was introduced to Nova Scotia, Canada where it successfully established, co-existing with native macrophytes F. vesiculosus (Linnaeus) and Ascophyllum nodosum (Linnaeus). We first examined whether a common gastropod in Nova Scotia, Littorina obtusata (Linnaeus, 1758), which grazes on the native macrophytes, has exploited the invasive and on finding this, we secondly examined whether there has been any phenotypic differentiation on the invasive. Among 98 sites surveyed around Nova Scotia and Newfoundland in May and June, 1994, eleven had the invasive macrophyte, all of which showed colonization by L. obtusata including egg masses, juveniles and adults. Among 2135 shells photographed for digital image analyses, those on the invasive differed from those on the native macrophytes with respect to (1) RGB (red/green/blue) channels, (2) HSV (hue, saturation, brightness) phenotypes, (3) protoconch pattern and (4) adult shell size. Nitrogen and carbon stable isotope signatures on muscle tissues from a subset of L. obtusata indicate foraging fidelity on the invasive rather than temporary occupation. We suggest that these cumulative phenotypic responses to the invasive macrophyte that vary in extent and direction within and among localities reflect localized adaptation and offer a unique opportunity for quantifying early stages of phenotypic and genomic differentiation in a novel ecological niche.