Taxonomy and morphological comparison of larva and juvenile stages in the genera of Lobianchia and Idiolychnus

Globally, the fish genera Lobianchia Gatti, 1904 and Idiolychnus Nafpaktitis et Paxton, 1978 belong to the Myctophidae family, within the Myctophiformes order. The genus Lobianchia includes two species: Lobianchia dofleini (Zugmayer, 1911) and Lobianchia gemellarii (Cocco, 1838) , while the genus Idiolychnus comprises only one species Idiolychnus urolampus (Gilbert et Cramer, 1897) . Currently, there are limited reports on Lobianchia and Idiolychnus in Chinese waters, with adults, larvae and juveniles of Lobianchia gemellarii documented in the waters off eastern Taiwan, and larvae and juveniles identified as Lobianchia gemellari and Lobianchia urolampus collected from the Kuroshio Current region off the East China Sea. To further elucidate the taxonomy and early developmental morphology of these species, an identification key for the these two genera Lobianchia Gatti, 1904 and Idiolychnus Nafpaktitis et Paxton, 1978 was compiled. Following a review of the data and samples from the aforementioned surveys conducted from June to July 1984 (summer) and December 1984 to January 1985 (winter), the larvae and juveniles previously identified as L. gemellari and L. urolampus, were corrected to L. gemellarii (Cocco, 1838) and I. urolampus (Gilbert et Cramer, 1897), respectively. Additionally, the early developmental morphology of L. gemellarii post-larva of 5.10 mm in BL and the I. urolampus juvenile of 6.80 mm in BL were described respectively. The developmental morphological characteristics of larvae and juveniles of L. dofleini, L. gemellarii, and I. urolampus juvenile were discussed. Upon comparison, the aliform pectoral fins of Lobianchia and Idiolychnus larvae and juveniles represent a distinctive morphological feature. A comparative analysis of melanophore shape, quantity, and distribution on the body surface of larvae and juveniles from these two genera was performed. As analyzed, melanophores shape and quantity, which conceivably vary with habitats, exhibit species-specificity. Thus, melanophores shape and quantity can be used as morphological characteristics for species identification. The developmental sequence and distribution of the branchiostegal organ (Br) in Lobianchia and Idiolychnus larvae and juveniles were elucidated. Analysis of the distribution data of Lobianchia and Idiolychnus larvae and juveniles across various oceans suggests that Lobianchia dofleini larvae and juveniles are rarely found in the Northwest Pacific. Spawning stocks of Lobianchia gemellarii in the Kuroshio Current region off the East China Sea reproduce during winter, spring and summer, but the specifics of their spawning habits remain unclear. This study contributes to understanding of the morphology and ecology of lanternfishes early life stages and offers scientific evidence for the utilization and management of these small-sized marine fish resources.