Year-round dynamics of Noctiluca scintillans in Yatsushiro Bay, Japan suggest possible foreign genetic contributions and long-distance connectivity pathways

Abstract

Noctiluca scintillans is a globally distributed harmful algal bloom (HAB) species known for potentially causing fish mortality and economic losses to fisheries. N. scintillans tends to accumulate near the sea surface, making it particularly susceptible to transport by ocean currents, however, direct evidence of long-distance dispersal has remained limited. Year-round monitoring in Kumamoto revealed that the Indonesian (Jakarta-type, K2) genotype occurs predominantly during the autumn high-abundance period, coinciding with smaller cell sizes that match Jakarta population. To evaluate the plausibility of long-distance transport, we conducted Lagrangian particle-tracking simulation using OSCAR surface currents. The results showed a plausible physical ocean connectivity between Indonesia and Japan within 600 days, with consistent patterns across different particle-release numbers indicating that arrival probabilities remained low but spatially robust. Recognizing that OSCAR provides a 0.25 degrees satellite-derived representation of basin-scale surface circulation that does not explicitly resolve mesoscale eddies, we interpret these trajectories as possible connectivity pathways rather than literal particle tracks. Together, our genetic, morphological, and particle-tracking simulation results indicate that N. scintillans populations in Yatsushiro Bay likely consist of both regional and foreign genetic contributors, highlighting the potential for long-range connectivity under contemporary circulation patterns.