DNA sequence variation for a 350 base-pair portion of mitochondrial 16S rRNA gene was determined for the copepod, Calanus finmarchicus, collected from eight sites in the western N. Atlantic Ocean and seven sites in the Norwegian Sea. Among 104 individuals for which sequence data were obtained there were 30 mitochondrial haplotypes. One haplotype occurred in 59 individuals and was found in all but two of the samples, while 21 haplotypes were unique. Both nucleotide diversity (pi = 0.0061) and haplotype diversity (h = 0.67) were moderate. There were significant differences in the frequencies of the haplotypes among the samples (P < 0.001 by chi-square test by Monte Carlo simulation). One sample collected from the Norwegian Sea and one sample from the northern Gulf of St. Lawrence were distinct from all other samples, perhaps as a result of advection from different oceanographic regions. Chi-square tests resolved four groups: one consisting of the three Georges Bank, two Gulf of Maine, and two (of three) Gulf of St. Lawrence samples, another consisting of most of the Norwegian Sea samples, and two others consisting of the distinct samples listed above. These groups differed significantly in haplotype frequencies (P < 0.001). A population genetic statistic used to describe population genetic structure, F-ST = 0.188, indicated some differentiation of these groups. The pooled samples on either side of the ocean (all western N. Atlantic vs, all Norwegian Sea samples) differed in haplotype frequency (P < 0.033). Based on sequence variation for this mitochondrial gene, C. finmarchicus is genetically structured on meso- and large scales; gene flow across the species range is high. Future studies will examine the relationship between population genetic structure and physical structure at meso- to large scales in the ocean, as defined by ocean circulation patterns.
