ABSTRACT:The freshwater amphipod Crangonyx floridanus (Amphipoda: Crangonyctidae) has considered to be recently introduced from North America to Japan, and now the recorded sites of collection cover nearly all over Japan excluding the northern part. In this study, we surveyed further areas outside the known distribution ranges and also examined the population genetic structure and the phylogenetic relationships between Japanese and North American populations based on nuclear (18S rRNA) and mitochondrial (COI) DNA sequences. We found that this amphipod already reached to Hokkaido, the northernmost Japan, and suggested the rapid expansion in a pattern of concentric circles from the central part of Japan. The genetic analysis showed the genetic homogeneity of Japanese population in contrast to the genetic diversification in North American Crangonyx populations. The process of introduction, establishment, and expansion of this amphipod in Japan may be explained as follows. A limited number of individuals from a North American native population were probably inadvertently introduced and established somewhere within the Kanto region. After that the local population size increased and the range of its distribution was rapidly expanded over Japan.

The crangonyctid amphipod, Crangonyx floridanus Bousfield, originating from North America (Florida), has invaded the freshwater habitats of Japan. This amphipod was first found in Japan in 1989, in the Furutone-numa oxbow lake of the Tone-gawa River which flows along the Ibaraki-Chiba prefectural boundary. Its strong capacity to adapt to broad and variable environmental conditions, and their superior reproductive efficiency have contributed to its rapid dispersion throughout Japan. These alien amphipods have a significantly broader range of habitats than their native counterparts, and can inhabit a wide range of environments, even those with a relatively high water temperature. They also readily inhabit the oligosaprobic waterways originally used by native species. In the present study, we conducted a comparative ecological and population dynamics research between the alien amphipod C. floridanus and the native amphipod Jesogammarus jesoensis, at the Tate-gawa River study site (Azumino, Nagano Prefecture). Although both the alien and native amphipods inhabit the same area, habitat segregation at the microhabitat level was observed (i.e., they tend to utilize different water plant species as their habitats). It was also suggested that some interaction may be occurring in terms of the timing of their reproductive season in order to facilitate their successful cohabitation. No evidence of a particularly strong competitive relationship was observed between the alien and native amphipod species. This is thought to be due to their niche segregation as a result of the significant differences in their body size, and also to the abundant availability of habitats and food (i.e., rich water plants), along with the high water quality of the environment at the Tate-gawa River study site.