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

The large estimates of mesopelagic fish biomass have long fuelled harvesting interests in the relatively untouched twilight zone of the ocean. The silvery lightfish-one of the most abundant species inhabiting the North Atlantic mesopelagic layer-is a candidate for such a fishery despite its enormous ecological importance and the insufficient knowledge about its population genetic structure. To address this knowledge gap, 863 individuals sampled across the North Atlantic Ocean and into the Mediterranean Sea were genotyped using a panel of 170 genome-wide SNP loci. Analyses revealed habitat-driven differentiation into three main units: Mediterranean Sea, oceanic samples, and Norwegian fjords. These groups were not completely isolated from each other as introgression from the Mediterranean Sea was detected in the Eastern Atlantic façade extending from Moroccan waters northward to 47° N, within an otherwise genetically homogeneous oceanic cluster. The complex topography of the Greek Seas seemed to shape the genetic structure in the Mediterranean Sea whereas along the Norwegian coastline, sills did not appear to hinder genetic exchange among fjords ranging 200 km apart, likely reflecting a combination of the position of the species in the water column and its swimming ability. This genetic information should be integrated with ecological and demographic properties to outline the management boundaries of this species prior to any eventual fishery attempt.