Genetics in the Ocean's Twilight Zone: Population Structure of the Glacier Lanternfish Across Its Distribution Range

The mesopelagic zone represents one of the few habitats that remains relatively untouched from anthropogenic activities. Among the many species inhabiting the north Atlantic mesopelagic zone, glacier lanternfish (<i>Benthosema glaciale</i>) is the most abundant and widely distributed<i>.</i> This species has been regarded as a potential target for a dedicated fishery despite the scarce knowledge of its population genetic structure. Here, we investigated its genetic structure across the North Atlantic and into the Mediterranean Sea using 121 SNPs, which revealed strong differentiation among three main groups: the Mediterranean Sea, oceanic samples, and Norwegian fjords. The Mediterranean samples displayed less than half the genetic variation of the remaining ones. Very weak or nearly absent genetic structure was detected among geographically distinct oceanic samples across the North Atlantic, which contrasts with the low motility of the species. In contrast, a longitudinal gradient of differentiation was observed in the Mediterranean Sea, where genetic connectivity is known to be strongly shaped by oceanographic processes such as current patterns and oceanographic discontinuities. In addition, 12 of the SNPs, in linkage disequilibrium, drove a three clusters' pattern detectable through Principal Component Analysis biplot matching the genetic signatures generally associated with large chromosomal rearrangements, such as inversions. The arrangement of this putative inversion showed frequency differences between open-ocean and more confined water bodies such as the fjords and the Mediterranean, as it was fixed in the latter for the second most common arrangement of the fjord's samples. However, whether genetic differentiation was driven by local adaptation, secondary contact, or a combination of both factors remains undetermined. The major finding of this study is that <i>B. glaciale</i> in the North Atlantic-Mediterranean is divided into three major genetic units, information that should be combined with demographic properties to outline the management of this species prior to any eventual fishery attempt.