Abstract. Ramah S, Kaullysing D, Soondur M, Taleb-Hossenkhan N, Bhagooli R. 2023. Differential photo-physiological responses of two giant clam species to elevated temperature stress from Rodrigues Island, Western Indian Ocean. Indo Pac J Ocean Life 7: 64-70. Bleaching events leading to mass mortality of coral reef and its associated symbiotic organisms have become an alarming issue worldwide. However, as compared to corals, little has been documented regarding giant clams’ (Tridacnines) thermal photo-physiological susceptibility. Triplicate specimens of the small giant clam Tridacna maxima and the fluted giant clam T. squamosa collected from the lagoon of Rodrigues Island, Western Indian Ocean were exposed at two temperatures, 29°C and 32°C, under a constant low light intensity of approximately 200 µmol quanta m-2 s-1 over a 12-hour duration. The photo-physiological parameters namely, effective quantum yield of photosystem II (?PSII), relative maximum Electron Transport Rate (rETRmax) and maximum Non-Photochemical Quenching (NPQmax) were determined using a Diving Pulse-Amplitude-Modulated (D-PAM) fluorometer prior to and after 3 and 12 hours of exposures. At 29°C the photo-physiological parameters did not vary significantly for both species. At 32°C, T. squamosa and T. maxima exhibited significant declines in ?PSII as at 3 and 12 hours, respectively. The rETRmax of T. squamosa showed a significant decrease at 3 hours while both species showed a significant reduction in their NPQmax functioning as from 3 hours. The experiment also recorded the disintegration of the mantle tissue in T. squamosa after 12 hours. These findings indicate that T. squamosa is thermally more susceptible than T. maxima. Further in-depth investigations on symbionts genetic types and antioxidant responses of both the Tridacna host and symbionts are required to thoroughly understand giant clams’ variable heat stress responses in the era of ocean warming.