Changing seawater temperature effects on giant clams bleaching, Mannai Island, Rayong province, Thailand

Abstract

In this study, we recorded the effect of changing sea water temperatures on two species of giant clams at Mannai Island, Rayong province. Surveys were carried out between November 2009 and September 2010. The study site was located at a distance of 100m from the shore had a total area of 200m^2. We used a data logger to measure the water temperature at 1m and 5m depths (reef flat and reef slope, respectively). During this study, 12 individuals of T. squamosa and 117 of T. crocea were observed. The clams' shell lengths varied from 20.0 to 35.0cm and from 0.6 to 13.5cm, respectively. The changes in seawater temperature fluctuated between 28.8 – 31.1 ºC during the study. The first bleaching event was recorded in April with 60% of individuals showing faded coloration, 30% partly bleached and 10% bleached completely. In May, bleaching became more severe with 90% completely bleached, 8% partly bleached and just 2% remaining with faded coloring. In June, the temperature began decreasing (below 30ºC on average). Finally, we found that 40 % of the giant clams had died and started becoming re-colored in August. The overview of this study, since exposure to temperatures over 30ºC for longer than two weeks could result in the expulsion of the symbiotic living zooxanthallae. Such a process was considered to have a direct impact on the giant clam's survival. Moreover, the giant clam located in shallow waters has shown the ability to re-color.