Abstract
The native freshwater shrimp Paratya improvisa was designated as an endangered species by all prefectures in the Kanto region, Japan, except for Kanagawa. The non-native freshwater shrimp Neocaridina spp. was introduced to the region from Western Japan and/or China as a fishing bait and for ornamental reasons, and it has been recorded in the Sagami River Basin, Kanagawa. I investigated the longitudinal distributions for P. improvisa and Neocaridina spp. in downstream of the Shiroyama Dam, Sagami River Basin, Japan. The altitude of longitudinal distribution of P. improvisa (mean ± S.D. =42.8 ± 21.3 m) was significantly higher than that for Neocaridina spp. (mean ± S.D. =24.0±19.7 m). The percentage of P. improvisa individuals in each sampling site where the two species were captured tended to be high in upper sampling sites. These suggest that there is differential environmental preference between the two species in the river basin. The distributions of the two species widely overlapped, suggesting that the two species can inhabit and partition resources in overlapped distribution areas. However percentage of Neocaridina spp. individuals in each sampling site containing both species, generally, was significantly higher than the percentage of P. improvisa individuals. This suggests that Neocaridina spp. dominates P. improvisa in environments where the two species have to compete.
