A complex pattern of population structure in the North Pacific giant octopus Enteroctopus dofleini (Wülker, 1910)

We investigated the population structure of the North Pacific giant octopus, Enteroctopus dofleini (Wulker, 1910) in Alaskan waters. Octopuses were collected from five locations (Dutch Harbor (DH; n ¼ 45), Kachemak Bay (KB; n ¼ 45), Prince William Sound (PWS; n ¼ 18), Glacier Bay (GB; n ¼ 33), and Stephen's Passage (SP; n ¼ 39)). All samples were sequenced at the cytochrome oxidase I (COI) locus of the mitochondrial genome. We identified two major mtDNA haplogroups. Sequence divergence ranged from 0.2 to 2.9%. Haplotypes were not distributed evenly among the sampled populations, producing an enigmatic pattern of population structure. We observed no genetic differen- tiation between DH, KB and GB, or between PWS and SP. FST was extremely high for all other pair- wise comparisons, ranging from 0.871-0.948. We did not observe an isolation-by-distance pattern or a strong clinal gradient in haplotype frequencies, as typically detected in other marine species. Strong genetic drift, serial bottlenecks or sweepstakes events may contribute to the pattern observed. The high level of sequence divergence observed at the COI locus could also suggest cryptic species within the E. dofleini complex, with limited geographical overlap of populations and gene flow. Additional samples were contributed by researchers from British Columbia (n ¼ 1), Seaside, Oregon (n ¼ 4), Neah Bay, Washington (n ¼ 2), Puget Sound, Washington (n ¼ 1), and Kodiak Island, Alaska (n ¼ 2)) While sample sizes were low for these locations, prompting their exclusion from population based analyses, all individuals were of the predominate haplotype found in Alaska.