The micro-bivalve Carditella iejimensis is less than 3.5 mm in shell height and lives in the surface sediment within a submarine cave at Ie Island, Okinawa Islands, Japan. A previous study interpreted that δ ¹⁸O-derived temperature obtained from C. iejimensis shells indicates the water temperature between May and July of each year. This interpretation is based mainly on a statistically significant correlation between shell size and δ ¹⁸O values obtained from empty shells. To assess the validity of this interpretation, we performed high-resolution isotopic analyses on 19 empty shells with the aim of examining ontogenetic variation in δ ¹⁸O values. The results reveal no systematic variation in δ ¹⁸O values, inconsistent with the previous interpretation. Our data indicate little or no influence of growth duration (shell size) on the δ ¹⁸O values of shells greater than 1 mm in height.