River dam prevents the invasion of non-native species of Neocaridina Kubo, 1938 (Decapoda: Caridea: Atyidae) into native habitats: A case study in the Yumesaki River system, Japan

The range expansion of invasive species that threaten biodiversity is caused by repeated cycles of their artificial introduction and subsequent dispersal. While dispersal can occur without human intervention, its pattern is mainly determined by environmental factors and species characteristics. Knowledge regarding the dispersal processes of invasive species and their subsequent interactions with native species is limited, which makes controlling invasive species and conserving native species difficult. We focused on the freshwater shrimps genus Neocaridina Kubo, 1938 in the Yumesaki River system, Japan, which is the habitat of the native species N. denticulata (De Haan, 1844) and has experienced the introduction of closely related invasive species. We examined the genetic population structures and morphological characteristics of the introduced species to elucidate their dispersal process. The results showed that populations comprising only the native species remained upstream of the Sugo Dam, whereas the native species was replaced by the introduced species N. davidi (Bouvier, 1904) at the other sites. Our results demonstrate that the distribution of invasive species can expand throughout the entire river system in rivers without structures that restrict the movement of aquatic organisms. Our findings highlight a rare example of a riverine structure that is generally detrimental to native species, acting as a bulwark for native species against invasive species.