In the northwest Atlantic Ocean, sea scallop (Placopecten magellanicus) has been characterized by a latitudinal genetic cline with a breakpoint between northern and southern genetic clusters occurring at ~45°N along eastern Nova Scotia, Canada. Using 96 diagnostic single-nucleotide polymorphisms (SNPs) capable of discriminating between northern and southern clusters, we examined fine-scale genetic structure of scallops among 27 sample locations, spanning the largest geographic range evaluated in this species to date (~37–51°N). Here, we confirmed previous observations of northern and southern groups, but we show that the boundary between northern and southern clusters is not a discrete latitudinal break. Instead, at latitudes near the previously described boundary, we found unexpected patterns of fine-scale genetic structure occurring between inshore and offshore sites. Scallops from offshore sites, including St. Pierre Bank and the eastern Scotian Shelf, clustered with southern stocks, whereas inshore sites at similar latitudes clustered with northern stocks. Our analyses revealed significant genetic divergence across small spatial scales (i.e., 129–221 km distances), and that spatial structure over large and fine scales was strongly associated with temperature during seasonal periods of thermal minima. Clear temperature differences between inshore and offshore locations may explain the fine-scale structuring observed, such as why southern lineages of scallop occur at higher latitudes in deeper, warmer offshore waters. Our study supports growing evidence that fine-scale population structure in marine species is common, often environmentally associated, and that consideration of environmental and genomic data can significantly enhance the identification of marine diversity and management units.

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We thank the people and organizations who contributed scallop samples, including NOAA (Devora Hart, Jakub Kircun), Maine Department of Marine Resources (Kevin Kelly), Fisheries and Oceans Canada (Amy Glass, Alan Reeves, Kyle Matheson, Leslie-Anne Davidson, Shawn Robinson, Leslie Nasmith, Andrew Cooper, Elizabeth Coughlan, Phil Sargent and Carole Turbide) and Sam Truesdale. We also thank the staff of the Aquatic Biotechnology Lab at the Bedford Institute of Oceanography for assistance with DNA extractions, RAD library preparation and SNP genotyping, and Praveen Nadukkalam Ravindran for bioinformatic support. This work was supported by a National Sciences and Engineering Research Council of Canada (NSERC) visiting fellowship awarded to S.J.L.

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  1. Fisheries and Oceans Canada, Northwest Atlantic Fisheries Centre, St. John’s, NL, A1C 5X1, Canada

    • Sarah J. Lehnert
    • , Emma V. A. Sylvester
    •  & Ian R. Bradbury
  2. Fisheries and Oceans Canada, Bedford Institute of Oceanography, Dartmouth, NS, B2Y 4A2, Canada

    • Claudio DiBacco
    • , Nicholas W. Jeffery
    • , J. Ben Lowen
    • , Brendan F. Wringe
    •  & Ryan R. E. Stanley
  3. Department of Biology, Memorial University of Newfoundland, St. John’s, NL, A1C 5S7, Canada

    • Mallory Van Wyngaarden
  4. Aquatic Biotechnology Laboratory, Fisheries and Oceans Canada, Bedford Institute of Oceanography, Dartmouth, NS, B2Y 4A2, Canada

    • Lorraine C. Hamilton


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