2002 Volume 57 Issue 2 Pages 91-101
Abstract: The distribution and spatio-temporal variation in population structure of the fluvial neritid gastropod Clithon retropictus(v. Martens, 1879)were studied over one year in the Saigo River, Fukuoka Prefecture, Japan. This species inhabits only the downstream reaches of the river, in the area that extends from freshwater to brackish water, and shows a marked spatio-temporal variation in population structure. The abundance of snails was greatest at the upstream limit of brackish water, where juveniles smaller than 8mm in shell width occurred throughout the year. Very few juveniles were found in the middle reaches of the brackish water and none was found in the freshwater area. Mean shell width of both males and females was smallest at the upstream limit of brackish water and tended to increase both upstream and downstream. Females were everywhere larger than males, and the sex ratio was always nearly one at the upstream limit of brackish water; however, there were many more females than males in the middle reaches of the brackish water area and throughout the freshwater reaches. These results suggest site-specific settlement of larvae at the upstream limit of brackish water, and growth-related movements of small and middle-sized snails both upstream and downstream. The mechanisms of downstream and upstream movement may be different, with the former being caused by passive displacement by floods or by density-dependent dispersal, whereas the latter probably involves active upstream migration.