The daily growth band patterns of Tridacna derasa – Evidence from culture experiments

Biogeochemical archives, such as tree rings and corals, with yearly-to-monthly resolution, have been used to reconstruct past climate change. However, they can not reconstruct weather events that occurred on daily-hourly time scales. Tridacna, the largest bivalve in the world, inhabiting the tropical and subtropical Pacific-Indian Ocean regions, exhibits daily growth bands measuring ∼5 to ∼60 μm and has the facility for reconstructing past extreme weather events. However, the precise daily growth band patterns across a day remain unclear, hindering any comprehensive understanding of the daily growth band patterns of Tridacna and their climatic implications. In this study, a 30-day controlled culture experiment on Tridacna derasa was carried out to address this and explore the main factors influencing the hourly Sr/Ca profiles. Results demonstrated that wide dim increments formed during daytime while narrow bright lines formed at nighttime. The hourly Sr/Ca profiles exhibited clear daily cycles synchronized with light cycles, with higher Sr/Ca at night and lower Sr/Ca values during daytime. The Sr/Ca ratios were not constant even under constant water temperature and light intensity, suggesting a limited influence of external environment factors and highlighting the role of internal biological regulation in Sr/Ca variation. A hypothesis was proposed here: During daylight, light-driven enzymatic activity coupled with H⁺ removal mechanisms regulates and lowers the Sr/Ca ratio, but the enzyme species and activities may be different, thus leading the variability of Sr/Ca at the identical light intensity; During nights, the organic matrix regulation mechanisms in extrapallial fluid (EPF) predominates, leading to higher Sr/Ca values, but the composition and concentration of organic matrix are not constant, leading to the variability of nightly Sr/Ca. Our findings provide a research foundation for paleoweather/paleoenvironment studies based on Tridacna.