Anomalous δ 18 O signal in a giant clam shell ( Tridacna maxima ) from the Lakshadweep Archipelago, India: signature of thermal stress during a coral bleaching event

A giant clam shell ( Tridacna maxima ) collected from the lagoon of Minicoy Island in the southern Lakshadweep Archipelago, India, was used for a high-resolution stable isotope (δ 18 O, δ 13 C) analysis. The results reveal a cyclic pattern in δ 18 O shell values, interpreted as combined signatures of seasonal temperature and δ 18 O sw fluctuations over the period from 2004 to 2014. These δ 18 O shell cycles are characterized by a slight background scatter governed by rainfall events leading to short-term and limited freshening of the water in the partly restricted lagoon. The most striking features of the isotope data are exceptionally negative outliers in δ 18 O shell values beginning in mid-2010. This first anomalous isotope excursion is followed by a phase of lowered growth rates lasting until the beginning of 2011. It is observed that this sudden change in oxygen isotope composition and shell precipitation was caused by anomalous sea surface warming, which was previously documented for the region in 2010 and caused widespread coral bleaching throughout the Lakshadweep Archipelago. Even though several other negative excursions in δ 18 O shell values follow, the cyclicity in the isotope signal and the growth rates become again more regular in the distal part of the shell, indicating a gradual recovery of the bivalve after the initial thermal stress event. The results reveal that even short high-temperature events can significantly perturb the biology of giant clams and require long recovery phases. This information is particularly significant for conservation efforts for this endangered bivalve group in a world with ongoing global warming.