In situ giant clam growth rate behavior in relation to temperature: A one-year coupled study of high-frequency noninvasive valvometry and sclerochronology

The life history of 15 giant clams, Hippopus hippopus, was studied in situ in the southern lagoon of New Caledonia; growth rate and animal behavior were studied both by sclerochronology and high-frequency noninvasive (HFNI) valvometry. Electrodes glued on each valve of each specimen recorded the shell-gaping behavior at 0.6-Hz frequency. A nonparametric regression model was used to model clam behavior. The daily increment thickness in the inner layer of five representative clams was measured. H. hippopus has its valves open during the day and partly closed during the night all year round, and shell growth is continuous. The cumulative growth using both techniques was similar, as was the mean daily thickness increment. The occurrence of one increment per day in H. hippopus shell was measured by valvometry. The five sclerochronological profiles were highly similar. Shell growth was significantly correlated to rising sea surface temperature (SST), up to 27 degrees C. At the solar maximum, gaping behavior and increment thickness became erratic. SST- and solar irradiance-related stress could be related to physiological oxidative stress triggered by zooxanthellae symbionts. In the present context of globally increasing SST, our data indicate that the giant clams H. hippopus could live beyond their thermal comfort limits in summer in New Caledonia.