イシマキガイの移動活動の季節的変化

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Seasonal changes in the moving activity of Clithon retropictus were studied from the distance moved by this snail during one month as an indicator. In the upstream portion of blackish-water zone in the Naka river of Izu Peninsula, a total of 241 marked individuals were released from May 1989 to November 1989. Recapture was undertaken once a month from July 1989 to September 1990 in order to survey the distance moved by marked individuals. The distance moved from release site was recorded by counting the number of square-shaped pools which are formed between large cross-shaped concrete blocks arranged parallel to the side wall of this river. Out of recapture records, only data of the individuals recaptured one month after release were extracted to study monthly changes in mean and maximum distances. Both mean and maximum distances were large from April to August with slight decrease in June, but decreased from September to March showing the smallest in December and January. Sexual difference of monthly changes in mean and maximum distances was analysed using 33 males and 35 females recaptured. Monthly mean distances moved by males were large in February and May to August, especially larger than those by females in February, May and June. This active movement suggests that reproductive activity of males searching for females to transfer spermatophores in copulation has already begun actively before the active egg-laying period from April to June. It also suggests that active movements after the active egg-laying period are related to the growth after the end of reproductive activity. Monthly changes in the maximum distances moved by males showed a similar pattern as in the mean. On the other hand, the mean distances moved by females showed gradual increase from February to August, showing slight decrease at two peaks of egg-laying in April and June. This slight decrease in the mean distance during active egg-laying period suggests that this period is devoted largely to egg-laying behavior. The mean and maximum distances moved by females in July and August after the peak of egg-laying were larger than by males, and this suggests that females show large growth in this period. Studies on monthly changes in the relation between the distance and the shell size during active moving season from February to November showed that the distance does not increase with the size of individuals but the maximum distance was gained by middle-sized individuals especially during egg-laying season from February to August. Therefore, the egg-laying season was divided into the first period (February to June) from the beginning until the second peak of egg-laying when large energy consumption for egg-laying is expected and the latter period (July to August) when large growth after the end of egg-laying activity is expected. The relation of the distance to the shell size was compared between males and females in these two periods. The maximum distance moved by males was observed in the middle-sized individuals having the shell length of 14.35 mm during the first egglaying period, but during the latter period the maximum one was observed in larger individuals having the shell length of 16.40 mm. The distances moved by small-sized and large-sized individuals were larger in the latter period than in the first one. Increase of the distances moved by small-sized and large-sized individuals in the latter period is considered as a result of lowered reproductive activity. The transition of the shell size showing the maximum distance from the small-size to the large-size suggests a heavy burden on reproductive activity in the small-sized and large-sized individuals during the first egg-laying period. On the other hand, nearly similar tendency was observed in females as in males. The distance moved by large-sized females were larger than by males in the latter period. This suggests that females grow larger in shell size than males.
日本貝類学会の論文
1992-07-31

イシマキガイの移動活動の季節的変化

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