A different world: temporal changes in nudibranch community structure over a half-century

Understanding long-term changes in marine biodiversity is essential for evaluating effects of climate change on coastal ecosystems. In this study, we compared nudibranch assemblages in northwestern Kyushu, Japan, during three time periods (1960–1980, 2001–2003, and 2023–2024), based on underwater surveys and historical records. In all, 47 nudibranch species were recorded during 27 diving surveys conducted in 2023 and 2024. Species diversity indices (Shannon–Wiener H and Simpson D) showed higher values during this survey than in 2001–2003. Comparative analysis of species composition revealed significant shifts, with 15 species exhibiting statistically significant differences in relative abundance from the past to the present. Notably, several species common in the past, such as Aplysia kurodai , were rarely observed in the recent survey, while many tropical-subtropical species appeared for the first time. The proportion of tropical-subtropical species increased markedly, whereas subarctic species were no longer detected. Similarity indices (Jaccard’s coefficient and Sorensen–Dice index) indicated that the current community differs markedly from those in earlier periods. These findings suggest a major community reorganization, potentially driven by rising sea water temperatures and other environmental changes. This study highlights the importance of long-term monitoring using multiple indicators to detect and interpret climate-driven biodiversity shifts in coastal marine ecosystems.