Analysis of oxygen isotopes in two giant clam (Tridacna) specimens in profile, one H. hippopus and one T. derasa, reveals possible species differentiation regarding isotopic uptake from surrounding seawater into their carbonate shells. Both collected from Malakal Harbor in Palau’s main island of Koror on August 17, 1955, the two shells recorded similar environmental conditions though the T. derasa δ18Oaragonite values were much more negative for the duration of its life. Growth rate, preferred reef location, freshwater influence, and vital effects can dramatically influence the uptake of stable isotopes in carbonate organisms and perhaps explain the observed difference in oxygen isotope values. Furthermore, several sea surface temperature (SST) reconstruction equations were tested (e.g., Aubert et al., 2009; Romanek and Grossman, 1989; Aharon and Chappell, 1983; Watanabe and Oba, 1999; Duprey et al., 2015; Grossman and Ku, 1986) using δ18Oaragonite values measured from modern sub-fossil Tridacna shells. The calculated SSTs were compared to several SST records from Palau, where it was found that the equation developed by Grossman and Ku in 1986 defined for aragonitic species of mollusks [T (ºC) = 21.8 – 4.69 (δ18Oaragonite - δ18Oseawater] was the most accurate.