Diversity of lanternfish (Myctophidae) larvae along the Ninety East Ridge, Indian Ocean

Since the 19th century, the impact of seamounts on the distribution of plankton has been a topic of considerable interest. The influence of seamounts on the biogeographic patterns of marine organisms is complex, with some aspects still under debate. It is generally accepted that seamounts can drive the upwelling of nutrient-rich deep waters. Tidal amplification, flow acceleration, and internal waves can further enhance vertical mixing, leading to increased primary productivity near seamounts. Seamounts may also act as barriers to the migration of marine organisms, affecting gene flow. Research on Pacific seamounts suggests these features might serve as "stepping stones" for the dispersal of marine species across the ocean. However, investigations of seamounts in the eastern Indian Ocean remain limited. Focusing on the Ninety East Ridge region in the eastern Indian Ocean, this study collected zooplankton samples using horizontal (surface) and vertical (0-200 m) plankton nets and measured temperature and salinity profiles with a conductivity, temperature, and depth (CTD) sensor. A total of 544 fish larvae were identified, including 260 lanternfish larvae, representing 38 species across 12 genera, determined through COI DNA barcoding. Phylogenetic trees and haplotype networks were constructed to analyze genetic distances and population structures of lanternfish species. Among the samples, intra-specific genetic distances ranged from 0% to 2.99%, while inter-specific distances ranged from 1.88% to 25.71%. Except for <i>Notolychnus valdiviae</i> (Brauer, 1904), the maximum intra-specific distances were lower than the minimum inter-specific distances for all species. Haplotype analysis of nine species revealed significant variations in haplotype number, structure, and spatial distribution. Specifically, <i>Ceratoscopelus warmingii</i> (Lütken, 1892) and <i>N. valdiviae</i> exhibited a notable north-south divergence pattern, consistent with the temperature and salinity distribution of the region's water masses. This conclusion was supported by analysis of molecular variance analysis, suggesting that larval stages of certain lanternfish species may struggle to cross boundaries between water masses. However, the remaining species showed no significant north-south distribution differences, possibly due to their adaptive capabilities, vertical migration patterns, or the duration of their planktonic larval stages. These findings suggest that seamounts and water mass distribution have varying implications for lanternfish species, potentially influencing gene flow and horizontal distribution patterns, which could contribute to speciation. Global climate change-induced alterations in ocean currents may profoundly impact the genetic diversity of fish species. This study provides new insights into the diversity of lanternfish in the Ninety East Ridge region and offers valuable data for understanding the biogeography of seamounts.