Quantitative assessment of sea slugs (Heterobranchia) assemblages along the western coast of Kyushu, Japan: a baseline for long-term biodiversity monitoring

Quantitative information on the seasonal dynamics of heterobranch sea slug assemblages remains limited in warm-temperate coastal regions, despite their ecological importance as benthic consumers and indicators of environmental change. Here, we conducted a standardized, multi-seasonal SCUBA-based survey of sea slug assemblages at two rocky reef sites (Tatsunokuchi and Nomozaki-Akase) along the northwestern coast of Kyushu, Japan, from February 2024 to November 2025. Across the study period, a total of 81 species comprising 892 individuals were recorded. Species richness and total abundance exhibited pronounced seasonal variation at both sites, with higher values in winter-spring and marked declines during summer. Assemblage composition shifted seasonally from relatively even communities in winter-spring to dominance by a few taxa in summer, a pattern reflected by concurrent changes in diversity indices. Water temperature displayed clear seasonal cycles and was negatively correlated with both species richness and total abundance, indicating a close association between thermal conditions and seasonal changes in sea slug assemblages. While causal mechanisms were not explicitly tested, these consistent patterns highlight the importance of temporal environmental variability in structuring heterobranch communities in this region. This study provides one of the few quantitative, multi-seasonal baselines of heterobranch sea slug assemblages in warm-temperate coastal Japan, offering a reference framework for future ecological monitoring and assessments of environmental change.