New records of three rare mesopelagic fishes, Nessorhamphus danae, Photostylus pycnopterus and Kali falx in South China Sea

The first recorded specimens of Derichthyidae, Alepocephalidae, and Chiasmodontidae in the South China Sea were collected during a fishery survey. Morphological identification confirmed these as newly recorded genera and species of mesopelagic fishes in the South China Sea. In August 2024, a specimen of the family Derichthyidae was collected and identified as Nessorhamphus danae based on its morphological characteristics: its key diagnostic features included a snout length approximately 3−4 times the eye diameter and a dark stripe along the ventral midline. In May 2024, a specimen of the family Alepocephalidae was collected and identified as Phtostylus pycnopterus, which has a key diagnostic feature: small, stalked photophores. In August 2023, a specimen of the family Chiasmodontidae was collected and identified as Kali falx, whose key diagnostic features were that the internal row of premaxillary and dentary tooth were both three, recurved, flanged. In this work, we identified three newly recorded mesopelagic fish species: Nessorhamphus danae, Phtostylus pycnopterus and Kali falx. Additionally, the genus Phtostylus is newly recorded in the South China Sea. This study expands the known distribution ranges of these genera and species, enhances the fish diversity, and provides foundational data for further research of mesopelagic fishes in the South China Sea.