Mesopelagic fishes experience an extreme body transformation from larvae to adults. DNA barcoding has been useful in establishing phylogenetic relationships among mesopelagic and other deep-sea fishes, however no molecular evidence to confirm the identity of their larvae was available so far. Molecular analyses were conducted to identify the larvae and adults of 18 species of the most abundant and frequent mesopelagic fishes in the western Mediterranean, across 14 genera. For myctophids, 7 of 9 species of the subfamily Lampanyctinae and all species from Myctophinae (6) were analysed. At least 565 nucleotides of the mtDNA CO1 region were sequenced. The 12S rRNA region was selected for C. braueri and C. pygmaea (372 nucleotides). Pairwise nucleotide distances were notably higher inter- than intraspecifically allowing to discern between congeners such as Cyclothone braueri and C. pygmaea, Hygophum benoiti and H. hygomii, Lampanyctus crocodilus and L. pusillus, and Notoscopelus bolini and N. elongatus. The good fit between larvae and adult sequences for each species proved the accuracy of earlier larval descriptions based on morphology and pigmentation characters. A list of the genetic markers used to identify each species with their accession numbers in Genbank is provided here along with their developmental stage, sampling location and depth.

Juvenile: Juvenile fish that finished transformation from larva to juvenile and that are not yet mature.Transforming: In the process of metamorphosis from larva to juvenile.