Comparing ichthyofaunal composition between Zostera japonica beds (Plantae, Alismatales, Zosteraceae) and adjacent bare ground within a small temperate estuary in southern Japan

Zostera japonica Ascherson et Graebner, 1907 is a dominant seagrass species that forms coastal habitats in the northwestern Pacific. Bare areas adjacent to these seagrass beds are also recognized as fish habitat. However, few studies have comparatively evaluated the ecological importance of Z. japonica beds and adjacent bare ground within estuarine environments. We conducted seine net sampling to compare ichthyofaunal composition between Z. japonica beds and adjacent bare ground in a small temperate estuary in Kyushu, southern Japan, during three summer seasons (2015, 2016, and 2019), when seagrass growth is at its peak. Fish species richness and the abundance of the predominant species, Redigobius bikolanus (Herre, 1927), were significantly higher in the seagrass beds than over bare ground. Additionally, the size distribution of R. bikolanus was broader in seagrass beds. PERMANOVA analysis revealed a significant difference in abundance-based species composition between the two habitats, and SIMPER analysis identified three species that contributed most to this distinction: R. bikolanus and Gerres japonicus Bleeker, 1854 (both more abundant in seagrass beds), and Gymnogobius breunigii (Steindachner, 1880) (more abundant over bare ground). These findings imply that Z. japonica beds enhance fish diversity and serve as key habitats for dominant fish species, while adjacent bare ground also supports specific fish communities. The results underscore the importance of conservation efforts within estuaries that account for the ecological roles of both seagrass and bare substrates.