Bleaching and recovery in the giant clam Tridacna crocea from the sub-tropical waters of Okinawa

Heat stress-induced bleaching has been shown to lower the photosynthetic and physiological performances and even causes mortality in giant clams. However, there is a lack of information on the responses of sub-tropical giant clams to bleaching stress as well as their recovery from bleaching. Here we exposed the giant clams Tridacna crocea from sub-tropical Okinawa to different temperature conditions for 21 days and then examined their recovery at control temperatures (28–29 °C) within 42 days. T. crocea in control and in 30–31 °C retained their colors, whereas bleaching was observed in 32–33 °C and in the increasing temperatures (29–33 °C, 1 °C change every 5 days). The F_v/F_m decreased at 32–33 °C and in the increasing temperatures (29–33 °C), even before the onset of bleaching, but the F_v/F_m returned to baseline values at day 9 of recovery period. Zooxanthellae density and chlorophyll a + c concentrations were decreased, whereas the zooxanthellae cell size was increased in all elevated temperatures at day 21 of exposure period. Respiration (R) in all temperature conditions was not affected during exposure period, whereas gross photosynthesis (P_g) and P_g/R were reduced at 32–33 °C. At day 42 of recovery period, most bleached individuals regained their colors, phototrophic potential, and zooxanthellae population. No giant clams died within 21 days of exposure period, but mortalities were observed at 32–33 °C and at increasing temperatures (29–33 °C) during recovery period. This study shows that the sub-tropical giant clams are vulnerable to heat stress, but their ability to recover may suggest their persistence in a warming ocean.