Turritella duplicata (Linnaeus, 1758) is a marine suspension-feeding gastropod found in shallow-marine and tidal flat environments throughout Southeast Asia and the Indo-West Pacific. It was studied on a tidal flat in the national park of Nopparat Thara in the district of Krabi, southern Thailand. Turritella duplicata was found primarily in the lower intertidal zone, and 90 specimens were gathered and measured. Their geographical positions on the flat were mapped, revealing zonations in population distribution, both parallel and perpendicular to the shore-line. The size distribution of the shells indicates that the population consists exclusively of adults. δ¹⁸O sclerochronology performed on three shells shows variations of up to 1.5‰ associated with seasonal changes in water parameters, suggesting longevity of about three years and seasonally controlled growth rates. The absence of juveniles and observed population distribution coupled with the sclerochronological data suggest that reproduction occurs in early spring, and that the population represents three cohorts approximately one year in age and older. Migration to deeper water during other seasons cannot be ruled out. Smaller individuals were preferentially found on the inner tidal flat, and individuals older than two years preferentially colonised the outer tidal flat. The carbon isotopic composition of the analysed shells partially tracks these life stages, suggesting that metabolic “vital effects” are influencing the δ¹³C of the shells.