Abstract
By marking-and-recapture method, upstream and downstream migrations of Clithon retropictus were examined in the upper area of brackishwater zone in the Naka river of Izu Peninsula, because there is an explanation that C. retropictus has intrinsic quality to ascend the river. In this area, there are many cross-shaped concrete blocks arranged parallel to the side wall of this river, forming a continuum of square-shaped pools. At a fixed square-shaped pool (release site), 100 individuals marked with color paint and 241 individuals marked and numbered with color paint and adhesive were released in seven different days from 13 April to 15 November 1989. Their recaptures and releases were repeated almost monthly from June 1989 to September 1990. At this time, migration distances of recaptured individuals were recorded by counting number of square-shaped pool from release site, and their shell sizes were measured. Only individuals recaptured from September to December 1990 were sexed afterward ; 33 males and 35 females. Number of recaptured individuals was compared between upstream and downstream pools 1 to 15 months after release. The results showed no obvious difference between upstream and downstream migration. No difference was observed between males and females, and also among different shell size groups. Therefore, we concluded that C. retropictus has no intrinsic quality to ascend the river, i.e., so-called positive rheotaxis, but shows two-directional upstream and downstream migration. Inhabitation of larger individuals in the upper fresh-water area of the river may be explained by the above-mentioned two-directional migrations and quality to avoid pure sea water.
