In recent years, poleward shifts in marine species distributions have been reported worldwide under ocean warming. Especially, western‐boundary currents including Kuroshio have great potential to carry marine fish larvae from lower to higher latitudes and accelerate range expansions. A key biological trait influencing species‑specific dispersal ability is the duration of the pelagic larval stage. Species with long pelagic larval durations (PLDs) are more likely to be transported over long distances, resulting in more homogeneous population genetic structure from subtropical to temperate regions. In contrast, species with shorter PLDs tend to disperse over shorter distances, and their genetic connectivity from subtropical to temperate regions is expected to proceed gradually through a stepping‑stone pattern across multiple generations.
To clarify the relationship between dispersal ability and population genetic structure in tropical reef fishes expanding into temperate zones, we conducted population genomic analyses using genome-wide sequencing (GRAS-Di) for two coral reef fishes expanding into Japanese temperate coasts with different PLDs: Dascyllus trimaculatus (20–30 days) and Chaetodon lunulatus (33–40 days). Genetic samples were collected from the Ryukyu Islands (subtropical sites) and Kochi prefecture (temperate sites) along the Kuroshio current. GRAS‑Di generated 48,209 SNPs for D. trimaculatus and 70,123 SNPs for C. lunulatus. Principal component analysis and admixture analysis based on these SNPs revealed no clear genetic differentiation between subtropical and temperate populations in either species. The relationships between genetic differentiation and geographic distance showed no isolation‑by‑distance patterns for both species. Together, these findings highlight that tropical reef fishes with varying dispersal abilities can expand into temperate ecosystems via the Kuroshio Current, without experiencing detectable genetic bottlenecks during their poleward expansion.