同一河川に生息するカワニナとイシマキガイのニッチ分け
2000 年 59 巻 2 号 p. 135-147
詳細
2000 年 59 巻 2 号 p. 135-147
Life history of coexisting two freshwater snail species, Semisulcospira libertina (Gould, 1859) (Prosobranchia ; Plearocerdiae) and Clithon retropictus (Martens, 1879) (Prosobranchia ; Neritidae) was studied. S. libertina, which is an ovoviviparous snail, is distributed in freshwater streams and lakes in Japan. C. retropictus, which is an oviparous snail, is distributed in brackish water and freshwater of south-east Japan. These two species show coexisting distribution at fresh water area in the middle reaches of Goino River, Kagoshima City, Kyushu Island, Japan. We tried to examine why two snail species was for the same place to be able to coexist. We also investigated what kind of niche segregation two snail species showed. Four permanent plots (5 m×5 m) were established from the upper reaches to the lower part of the Goino River. At the time of survey, about thirty quadrats (50 cm×50 cm) were set in each plot randomly, and the number of snails appeared in the quadrats was counted to estimate the population density of two species. Then the snails appeared were measured for the shell size and returned to the origin plots. In the S. libertina population, frequency distributions of shell size from July 1998 to December 1998 showed that newly hatched juveniles appeared during August to October. In the C. retropictus population, frequency distribution of shell size from July 1998 to December 1998 showed that newly juveniles appeared in October at the river-mouth site and moved from the river-mouth to the upper stream. The difference in microhabitat between two species was studied. It showed that S. libertina tended to prefer a pool to a rapid and C. retropictus had the opposite trend. The stomach contents were observed by the fluorescent microscope with rhodamine filter to show the food preference between two species. It was clarified that C. retropictus was herbivorous and S. libertina was polyphagous. From the facts above mentioned, it is considered that two species segregate their niche by microhabitat and food in the coexisting area.
