Microstructure and growth rate variability in a giant clam (Tridacna maxima) from the Lakshadweep Archipelago, India: implications for their use as biological monitors to trace extreme weather events

The giant clam shells are considered excellent marine bioarchives and have been used in studying current and past climate variabilities. Here, we present the microstructure and growth rate variability of a giant clam shell (T. maxima) collected from the Minicoy Island Lakshadweep, India. We employed FE-SEM, EDX, Raman spectroscopy and petrographic analysis to identify coherent records of the past marine environmental settings and factors controlling growth rates in T. maxima. A consistent growth rate was observed throughout the shell ontology, except for the year 2010, when a relatively slow growth rate was observed due to exceptionally warm water temperatures. Slower microstructural growth is confirmed by an anomalously negative excursion in δ¹⁸O values caused by an anomalous sea surface warming event. Such kind of warming was previously documented for the region in 2010 that caused widespread coral bleaching throughout the Lakshadweep Archipelago. Interestingly, our results do not reveal any major growth hiatus in the T. maxima even after experiencing this anomalous thermal stress event, demonstrating the ability of these bivalve shells to reliably reconstruct past environments. These findings advance the potential of combining data from shell microstructure, petrographic, and isotope (δ¹⁸O_shell) analyses, to reconstruct the past extreme events preserved in giant clam shells from the Lakshadweep Archipelago.