Three-dimensional post-glacial expansion and diversification of an exploited oceanic fish

Vertical divergence in marine organisms is being increasingly documented, yet much remainsto be done to understand the role of depth in the context of phylogeographic reconstructionand the identification of management units. An ideal study system to address this issue is thebeaked redfish, Sebastes mentella – one of four species of “redfish” occurring in the NorthAtlantic – which is known for a widely-distributed “shallow-pelagic” oceanic type inhabitingwaters between 250 and 550m, and a more localised “deep-pelagic” population dwellingbetween 550 and 800m, in the oceanic habitat of the Irminger Sea. Here, we investigate theextent of population structure in relation to both, depth and geographic spread of oceanicbeaked redfish throughout most of its distribution range. By sequencing the mitochondrialcontrol region of 261 redfish collected over a decadal interval, and combining 160 rhodopsincoding nuclear sequences and previously genotyped microsatellite data, we map the existenceof two strongly divergent evolutionary lineages with significantly different distributionpatterns and historical demography, and whose genetic variance is mostly explained by depth.Combined genetic data, analysed via independent approaches, are consistent with a latePleistocene lineage split, where segregation by depth likely resulted from the interplay ofclimatic and oceanographic processes with life-history and behavioural traits. The on-goingprocess of diversification in North Atlantic S. mentella may serve as an “hourglass” tounderstand speciation and adaptive radiation in Sebastes and in other marine taxa distributed across a depth gradient