Authors: Pereira, Ricardo; Bein, Bernhard;Lima, Françoise;Leite, Tatiana;Lima, Sergio;

Temporal variation of population size and genetic connectivity determine current patterns of genetic diversity within species, and hence their capacity to respond to natural selection. Yet, these demographic processes remain largely unknown in oceanic systems, where many species are still being described and are exploited. Here, we present one of the first population genomic studies in a cephalopod, Octopus insularis. Using genomic data, we identify the South Equatorial current as the main barrier to gene flow between southern and northern parts of the species range, followed by discontinuities in the habitat associated with depth. We find that genetic diversity of insular populations significantly decreases after colonization from the continental shelf, also reflecting low habitat availability. We find a stronger population expansion for coastal relative to insular populations, consistent with increases in habitat availability since the Last Glacial Maximum. The direction of gene flow is coincident with currents and eddies between isolated populations. Together, our results show that oceanic currents and habitat breaks are determinant in the diversification of marine species, shaping standing genetic variability within populations. Moreover, our results show that insular populations are particularly vulnerable to current human exploitation and selective pressures, calling the revision of their protection status.