Abstract
We investigated population genetic structure within and between ocean basins in the common mesopelagic copepod, Haloptilus longicornis, using the mitochondrial marker cytochrome oxidase subunit II (mtCOII, 43 locations, n = 1059). We found highly significant genetic divergence among ocean basins (global F ST = 0.20; p F CT = 0.21, Pacific: F CT = 0.15, AMOVA, p F CT measures genetic divergence among groups). In the Atlantic, a region of low abundance for H. longicornis in equatorial waters coincided with the location of the observed genetic break (∼ 0–12°N), suggesting the presence of a physical or bio‐physical barrier that effectively limits migration among subtropical gyre systems for this species. Using oceanographic data from a basin‐scale transect, we provide the first environmental portrait of an open‐ocean dispersal barrier for the marine plankton. Within all four Atlantic and Pacific subtropical gyres, we found a general lack of genetic subdivision among sites, as has been observed in a few other globally distributed plankton species.