A different world: temporal changes in the community structure of sea slugs (Heterobranchia) in northwest Japan spanning more than 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 heterobranch sea slug (hereafter referred to simply as 'sea slugs') assemblages in northwestern Kyushu, Japan, across three survey periods: (1) historical records from 1960-1980, for which the exact survey effort and duration were not documented; (2) surveys conducted from April 2001 to September 2003; and (3) recent surveys conducted from June 2023 to January 2024. A total of 47 sea slug species were recorded from our underwater surveys conducted in 2023-2024. Species diversity indices, including the Shannon-Wiener diversity index (<i>H</i>) and Simpson's diversity index (<i>D</i>), showed higher values during this survey than those of 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. Jaccard's coefficient indicated that the current community differs markedly from those in earlier periods. These findings suggest a major community reorganization, potentially driven by rising seawater 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.