DataSheet_6_The effect of environmental factors on spatial-temporal variation of heterobranch sea slug community in northern Taiwan.xlsx

Sea slug (Subclass Heterobranchia) biodiversity is an important element of marine coral reef communities. However, due to their small size and cryptic behavior, few studies have examined their spatiotemporal variation. Taiwan is in the middle of East Asia Island Arcs, where it is one of the hotspots of sea slug diversity. Meanwhile, the northeast coast of this island lies within the subtropical monsoon region, where the climate shifts from a warm southwest monsoon (SW: May to Sep) to a cold northeast monsoon (NE: Oct to Apr). Between the monsoons, distinct temperatures and currents result in different community structures. To understand how those environmental factors (i.e., monsoon season, diel period and water depth) influence sea slug species richness and assemblages, we surveyed these invertebrates monthly at both night and day by diving in a marine protection area (CJ) and at a nearby non-protected site (MR) from May 2020 to Apr 2021. We showed that the species composition in our sampling area was highly consistent with what was found in Okinawa and Pescadores island, but less similar to those in southern Taiwan or Hong Kong. On the northeast coast of Taiwan, sea slug number and abundance were both higher in CJ than MR, but biodiversity (H’) and the species evenness index J’ was lower in CJ than MR. Additionally, cluster analysis indicated that the sea slug assemblages differed between the two sites. Monsoon season significantly influenced both species richness and species assemblages, with species richness during SW being significantly higher than in the NE monsoon and species assemblages also being distinct between these two seasons. Moreover, species richness increased with water depth (within the surveyed range of 0-25 m). Diel period did not exert a strong impact on sea slug biodiversity, except for species richness at CJ, where it was higher during the day than at night. Our study suggested that changing currents between the monsoon seasons could transport individuals originating from either the north or south of these island arcs. Furthermore, temperature and food sources may differ between seasons and depths, thereby influencing sea slug assemblages in this region.

海蛞蝓（异鳃亚纲，Subclass Heterobranchia）的生物多样性是海洋珊瑚礁群落的重要组成部分。然而由于其体型微小且行为隐蔽，目前针对其时空分布变化的研究尚少。台湾地处东亚岛弧带中段，是全球海蛞蝓多样性热点区域之一。该岛东北部海岸地处亚热带季风区，气候随季风更替呈现明显变化：暖湿西南季风（SW：5月至9月）与冷干东北季风（NE：10月至次年4月）交替主导。季风转换期的水温与海流差异，亦会塑造不同的生物群落结构。为探明季风季节、昼夜周期与水深等环境因子对海蛞蝓物种丰富度及群落组成的影响，本研究于2020年5月至2021年4月期间，采用潜水调查法，每月分别在日间与夜间对两处采样点（海洋保护区CJ及邻近非保护区域MR）的该类无脊椎动物进行了采样。研究结果显示，本采样区域的物种组成与冲绳群岛及澎湖列岛的记录高度吻合，但与中国台湾南部或香港地区的物种组成相似性较低。在台湾东北部海岸，CJ采样点的海蛞蝓个体数量与种群丰度均高于MR，但该区域的生物多样性指数（H’）与物种均匀度指数J’则低于MR。此外，聚类分析结果表明，两处采样点的海蛞蝓群落组成存在显著差异。季风季节对海蛞蝓的物种丰富度与群落组成均具有显著影响：西南季风期的物种丰富度显著高于东北季风期，且两个季风期的群落组成亦存在明显分化。此外，在本次调查的0-25米水深范围内，物种丰富度随水深增加而升高。昼夜周期对海蛞蝓生物多样性整体未产生显著影响，仅在CJ采样点，日间的物种丰富度显著高于夜间。本研究推测，季风季节交替过程中海流的变化，可能会携带源自该岛弧带南北两侧的海蛞蝓个体。此外，不同季节与水深的水温及食物资源存在差异，这可能会调控该区域海蛞蝓的群落组成。