This study derives the paleotemperature equation based on the oxygen isotope records of freshwater-bivalve shells. We examined the oxygen isotope ratios (δ18O) of aragonite shells of the cultured freshwater bivalve Corbicula sandai (Cyrenidae), a species endemic to Lake Biwa, Japan, along with the temperature and δ18O values of water at the culture site. The δ18O values of cultured specimen shells showed clear periodicity corresponding to the culture period; they decreased with increasing water temperature and increased with decreasing water temperature. The observed water temperatures were strongly correlated with the δ18O values for shells minus the δ18O values for water; the following equation for the temperature was calculated by the least-squares method. \(T \left(^\circ \text{C}\right)=18.19 \left(\pm 0.37\right)-3.93 (\pm 0.33)\times ({{\updelta }^{18}\text{O}}_{\text{aragonite}/\text{VPDB}}-{\updelta }^{18}{\text{O}}_{\text{water}/\text{VSMOW}})\) . Using this equation, water temperatures from May 2, 2019, back to 2009 were reconstructed based on the δ18O values of two specimens collected in the lake, then compared with the observed water temperatures at lake depths of 0.5–20 m. In the northern basin of the lake, the water temperature in the shallow layer below the top 15 m, which is the habitat depth of C. sandai, was 26–28°C during the summer and 6–8°C during the winter; the reconstructed water-temperature range of 12–28°C agreed well with the observed higher values. The growth rates of cultured specimens significantly slowed at temperatures below 12–13°C, and the reconstructed minimum-water temperature of approximately 13°C indicates that the equation exhibits reliable accuracy. The growth records of the cultured specimens suggest that the reconstructed water-temperature fluctuations reflect the water temperatures from early spring (April–May) to late autumn (December).
