Development of pH Visualization Methods to Study Shell Formation in Juvenile Pearl Oyster Pinctada fucata

Biomineralization in pearl oysters (Pinctada fucata) is well-studied due to their economic value and research accessibility. Their shells comprise an outer calcite prismatic layer and an inner aragonite nacreous layer, presenting the classic calcite-aragonite polymorphism problem in biomineralization research. While molecular and genetic aspects of shell formation are increasingly understood, direct observation of formation mechanisms remains limited.

This study applied microscopic pH imaging techniques, previously successful in foraminifera research, to observe pH dynamics during shell development. Post-settlement individuals (shell length ~0.5 mm) from Mie Prefecture Fisheries Research Institute were examined using HPTS (pyranine) fluorescence microscopy. Observations revealed specific pH distributions, with elevated levels (~8.1 compared to ambient seawater ~7.7) parallel to growth lines near shell thickening areas, while soft tissue regions showed lower pH (<6.0), likely corresponding to digestive areas. These findings indicate active pH regulation during shell formation in bivalves.

Further research should investigate pH pattern responses to varying environmental conditions, particularly regarding climate change parameters. Studies of specific mechanisms creating these pH gradients and comparisons across developmental stages would enhance our understanding of biomineralization processes, benefiting both fundamental research and pearl cultivation practices.