| Summary: | 碩士 === 國立臺灣大學 === 漁業科學研究所 === 101 === Ovoviviparous freshwater snail Semisulcospira libertina (Prosobranchia: Pleuroceridae) were wildly distribution in Shihmen Reservoir watershed, and it was seem as indicator species of un-polluted habitat. In this study, we estimated age of S. libertina through stable oxygen isotope ratios (δ18O) of shell, and determined the reproductive biology by dividing embryo stage from female brood pouch.
The study periods were divided to two stages, we surveyed two streams sampling stations (Hsiayun and Sanmin) from July to October 2010 at first, and then surveyed four streams sampling stations (Kuchi, Hsiayun, Shihbichiao and Sanmin) from April 2011 to May 2012. The freshwater snails sampling and environment monitoring were conducted at each station monthly. The age determination of S. libertina were determined by two shell sample from Hsiayun station. The reproductive biology were determined from S. libertina population from Hsiayun and Sanmin station.
The result revealed that the series of δ18O from two shell samples show the fluctuated pattern both. The shell high of two samples were 20.53mm and 24.95mm, δ18O were determined from -6.17 to -8.12 ‰ and from -6.83to -8.55 ‰, age were estimated about 14.5 months and 20 months respectively. The community composition of freshwater snails changed and population density of S. libertina decline. The sex ratio was severely deviated from 1:1 in all streams, and the female much more than the male. The minimal mature shell high (MH50) and shell width (MW50) of Hsiayun were 13.5 and 6.55mm respectively. The minimal mature shell high (MH50) and shell width (MW50) of Sanmin were 17.84 and 7.58mm respectively. The potential shell size to release juveniles of female S. libertina in Hsiayun stream was smaller than Sanmin stream. We suggested that the different reproductive strategy of S. libertina depended on the different habitat and interspecific competition.
The results suggested that the age of freshwater mollusk living in natural habitat would estimate objectively through determining the δ18O of shell. This method would benefit to the studies among the growth, population dynamic, resources assessing, and medical model species.
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