New findings on the early life stages of five codlet species (Teleostei, Gadiformes, Bregmacerotidae) from the northwestern Pacific, including a putative undescribed Bregmaceros lineage.

Codlets (Bregmacerotidae) play a major role in marine ecology, acting as trophic links between primary consumers and higher predators in mesopelagic communities; however, little information has been accumulated on their early life stages. In this study, 53 larval and juvenile specimens of Bregmacerotidae, collected from the northwestern Pacific Ocean between May 2017 and July 2024, were examined using 390-bp of mitochondrial DNA (mtDNA) 16S rRNA sequence analysis for molecular species identification. Five codlet species were identified: Bregmaceros anchovia (n = 1), B. japonicus (2), B. nectabanus (4), B. neonectabanus (6), and Bregmaceros sp. (20), a candidate undescribed species closely related to B. atlanticus. It was also discovered that previously recorded codlet larvae and juveniles appear to have been misidentified due to extreme similarity among external traits. Diagnostic characters enabling species identification at different developmental stages included the presence of melanophores on the lower right side of the preopercle, the pattern of lateral body melanophores, the presence of melanophores on the caudal peduncle, and the degree of first dorsal fin development. Here, we provide morphological descriptions and diagnostic keys for larvae and juveniles of these five taxa and hypothesize the existence of a putative undescribed Bregmaceros lineage in the region.