Influence of genotype and geography on shell shape and morphometric trait variation among North Atlantic blue mussel (Mytilus spp.) populations JONATHAN P. A. GARDNER 1,2 * and RAYMOND J. THOMPSON 1 1 Ocean Sciences Centre, Memorial University of Newfoundland, St John’s, Newfoundland, A1C 5S7, Canada 2 Centre for Marine Environmental & Economic Research, School of Biological Sciences, Victoria University of Wellington, PO Box 600, Wellington, New Zealand Received 19 December 2007; accepted for publication 12 August 2008 The influence of geography and genotype on shell shape (outline) and trait (morphometric) variation among North Atlantic blue mussels and their hybrids has been examined. Shape differences among reference taxa (Mytilus trossulus, Mytilus edulis and Mytilus galloprovincialis) were consistent with an association between taxon-specific genes and shape genes. Newfoundland M. edulis ¥ M. trossulus populations and northern Quebec M. trossulus populations exhibited an uncoupling of taxon-specific genes from shape genes, whereas Nova Scotia M. trossulus populations and SW England M. edulis ¥ M. galloprovincialis populations exhibited an association between taxon-specific genes and shape genes. We found no evidence of a geographic effect (NE versus NW Atlantic) for shape variation, indicating that the genotype effect is stronger than any geographic effect at macrogeographic scales. Pronounced differences were observed in trait variability consistent with an association between taxon- specific genes and trait genes in European populations, and trait divergence of New York M. edulis from all European mussels. Trait variability in mussels from Newfoundland, Nova Scotia and northern Quebec indicated an uncoupling of taxon genes from trait genes, whereas trait variability in SW England M. edulis ¥ M. gallopro- vincialis populations was consistent with background genotype, indicating a strong association between taxon genes and trait genes. A pronounced macrogeographic split (NE versus NW Atlantic) regardless of taxonomy was observed, indicating that geography exerts a greater influence than genotype on trait variation at the macrogeo- graphic scale. This is consistent with pronounced within-taxon genetic divergence, indicative of different selection regimes or more likely of different evolutionary histories of mussels on either side of the North Atlantic. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 96, 875–897. ADDITIONAL KEYWORDS: blue mussels – Elliptical Fourier Analysis – genotype-phenotype interactions – genotypic variation – hybridization – introgression – phenotypic variation – Principal Component Analysis. INTRODUCTION The taxonomic status of smooth-shell blue mussels (genus Mytilus) has been controversial and unclear for decades, with more than 30 taxonomic designa- tions being recognized by various authorities at different times, largely as a consequence of the con- siderable phenotypic plasticity characteristic of the genus (Soot-Ryen, 1955; Koehn, 1991; McDonald, Seed & Koehn, 1991; Gosling, 1992; Seed, 1992). The characterization of Mytilus biochemical genetic variation from nearly all parts of its anti-tropical distribution has resulted in the identification of three smooth-shell species, M. edulis, M. galloprovincialis and M. trossulus (Koehn et al., 1984; McDonald & Koehn, 1988; Varvio, Koehn & Väinölä, 1988; Koehn, 1991). More recent application of molecular markers has supported these taxonomic designations (Gardner *Corresponding author. Current address: Centre for Marine Environmental & Economic Research, School of Biological Sciences, Victoria University of Wellington, P O Box 600, Wellington, New Zealand. E-mail: jonathan.gardner@vuw.ac.nz Biological Journal of the Linnean Society, 2009, 96, 875–897. With 9 figures © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 96, 875–897 875