Temperature seasonality, the difference between summer and winter temperature, has significant influences on global terrestrial and marine ecosystems. However, most of proxy-based climate records are of limited temporal resolution and thus insufficient to quantify the past temperature seasonality. In this study, high-resolution Sr/Ca ratios of modern (live-caught) and fossil (dead-collected) Tridacna gigas shells from the South China Sea (SCS) were used to reconstruct the seawater temperature seasonality during the late Holocene. The averaged seawater temperature seasonality around 2165 ± 75 BC (4.46 ± 1.41°C, derived from the data of 18 yr) were similar to the seasonality of recent decade (4.41 ± 0.82°C during AD 1994–2005), but the temperature seasonality around AD 50 ± 40 (3.69 ± 1.37°C, derived from the data of 48 yr) and AD 990 ± 40 (3.64 ± 0.87°C, derived from the data of 11 yr) was significantly lower than that during AD 1994–2005. The reduced seasonality around AD 990 ± 40 was attributable to the unusually warm winter during the medieval times, probably caused by the weakening of East Asian Winter Monsoon. Our study highlighted the potential of T. gigas shells in providing high-resolution seasonality climate information during the late Holocene.

The impetus for this study was the mass bleaching event of giant clams in 1997–1998 at several reefs in the Great Barrier Reef (GBR, Australia). From September until December 1999, the study investigated the effects of high light intensities and increased temperature accompanied by a nutrient limitation on the metabolism of Tridacna gigas, to test, if these environmental changes can induce bleaching. In a 50-day trial similar conditions of the mass bleaching event were imitated stressing clams, collected from Orpheus Island and Nelly Bay, by a two- to threefold light intensity and an increased temperature of 4–6 °C. The objectives of the experiments were to determine whether high light intensities can induce changes in chlorophyll content or alter zooxanthella cell sizes and populations in the tissue. After 50 days of exposure to high light intensity the mantle tissue of stressed clams exhibited a decreased number of zooxanthellae per unit area from 19.8 ± 0.8 (× 107·cm–2) to 0.2 ± 0.2 (× 107·cm–2) (mean ± CL). Additionally, the average cell size of zooxanthellae were downsized from 7.4 ± 0.1 μm to 5.3 ± 0.1 μm (mean ± CL). Subsequently, the chlorophyll content of both, chl a and chl c1, declined as well, chl a from 192 ± 4 to 0.1 ± 0.1 μg·ml–1 and chl c1 from 145 ± 6 to 0 μg·ml–1 (mean ± CL). This study shows that increased light intensity and temperature are the main causes for bleaching in giant clams. Thus, the study confirmed the four major aspects involved in bleaching: (1) loss of symbiotic algae, (2) decrease of chl a/c1 in the remaining symbiotic algae, (3) retention of small zooxanthellae in the tissue and (4) release of ammonium (NH4+) into the water column while nutrient uptake of ammonium was largely blocked.