Large-scale marine eDNA database reveals seasonal latitudinal diversity gradients in Japan driven by vagrant fish

The spatial and temporal patterns of marine diversity are essential for the sustainable use and management of marine ecosystems. Here, we investigated latitudinal and seasonal diversity gradients of fish genera in Japanese waters using environmental DNA (eDNA) data from 1,765 surveys conducted between 2017 to 2022. We found a negative relationship between genus richness and latitude, consistent with latitudinal diversity gradient (LDG). Importantly, the strength of the gradient varied seasonally, with steeper slopes observed during warm seasons compared to winter. This seasonal restructuring of LDGs may reflect the increased occurrence of tropical vagrant fish, rather than the commonly assumed pattern of seasonal migration. These transient taxa may temporarily elevate observed richness at higher latitudes but do not persist through winter conditions. As a result, short-term increases in biodiversity under warming conditions may not necessarily indicate stable ecological change. Our findings highlight that eDNA-based richness metrics can capture both persistent community structure and transient ecological processes, underscoring the need for careful interpretation of biodiversity patterns in a changing climate.