Basin-scale population genetic structure of the planktonic copepod Calanus finmarchicus in the North Atlantic Ocean

Pelagic marine invertebrates have extensive potential for gene flow, although barriers to gene flow and entrainment in ocean currents may lead to reproductive isolation or drift, and thus to genetic differentiation of populations. The planktonic calanoid copepod Calanus finmarchicus shows significant geographic variation in life history traits across subarctic zones of the N. Atlantic Ocean. Population genetic analysis of C. finmarchicus examined allelic variation at 24 single nucleotide polymorphic (SNP) sites in three nuclear protein-coding genes: citrate synthase, heat-shock protein-70, and AMP-activated protein kinase. Samples were collected during 2005 from 10 areas representing the Northwest (NW), North Central (NC), and Northeast (NE) Atlantic gyres. Hypotheses of two or more distinct populations of C. finmarchicus were examined based on SNP variation within the three genes analyzed both separately and together using AMOVA (Arlequin Ver. 3.11), CLUMPP (Ver. 1.1), GENALEX (Ver. 6.2), Genepop (Ver. 4.0.10), and Structure (Ver. 2.3). All analyses revealed evidence of small but significant differentiation among areas within gyres (e.g., FSC = 0.0306, p < 0.0001 for two populations; FSC = 0.0344, p < 0.0001 for three populations; pairwise FST values for all 10 areas ranged from 0.0000 to 0.2400), which may reflect ecologically-important, short-term (on the order of months) variation driven by geographic variation in life history traits. Support for underlying large-scale differentiation, which may reflect persistent barriers to gene flow associated with entrainment in ocean gyres, was provided by various analyses, with numbers of distinct C. finmarchicus populations ranging from two to four. Analysis of molecular variation supported two populations, while clustering and population assignment supported two, three, or four populations. The Barents Sea sample was especially distinctive: one test using AMOVA was non-significant among gyres without this sample and differentiation among area populations within gyres was reduced. Analysis of additional genes, higher resolution sampling, and comparisons across different years are needed to resolve the spatial limits and number of distinct C. finmarchicus populations across the N. Atlantic Ocean basin.