The daily growth bands pattern of Tridacna - evidence from culture experiments.

Biogeochemical archive such as tree rings, corals, and stalagmites have proven effective in reconstructing past paleoclimate patterns at year-month resolutions. However, they are not suitable for investigating weather changes on a day-hour scale. Tridacna, the largest bivalve in the world, which inhabits the Tropical-subtropical Pacific-Indian Ocean area, exhibits unique daily growth bands ranging from 5 to 60μm. This characteristic grants Tridacna the potential to reconstruct extreme weather events in the past, thus paving the way for paleoweather studies. Tridacna engages in symbiosis with zooxanthellae and relies on photosynthesis for energy. The growth rate and pattern of its daily growth bands correlate with diurnal alternations. Currently, the utilization of Tridacna daily growth bands and high-resolution geochemical element ratios holds potential for establishing a daily resolution biogeochemical proxy. However, the precise mechanism of the circadian rhythm during Tridacna's growth period in natural conditions remains unclear. Consequently, our comprehension of the pattern exhibited by Tridacna's daily growth bands is limited, impeding progress in the development of hourly scale paleo-weather analysis. In this study, a 30-day artificial experiment was conducted on Tridacna derasa to investigate the significance of shell daily growth bands and hourly element ratios in reflecting environmental conditions. The results of calcein labeling revealed that the wide dim areas in the daily growth bands corresponded to daytime and exhibited valley values in Sr/Ca ratio, while the narrow bright areas corresponded to nighttime and showed peak values in Sr/Ca ratio. At the daily-hourly scale, when the water temperature remained constant, the Sr/Ca daily variation showed potential for recording the light daily cycle. However, it is important to note that the influence of the light daily cycle on the Sr/Ca variation may be indirect. As a result, a hypothesis was proposed: the Sr/Ca variation is directly controlled by the internal regulation of Tridacna. During the daytime, the Sr/Ca ratio was primarily regulated by physiological activities. Due to light stimulation, the activity of certain enzymes in the inner mantle was enhanced, leading to an increase in Ca²⁺ concentration and a decrease in Sr/Ca ratio. During the nighttime, the Sr/Ca ratio was mainly regulated by organic matrices. In the extrapallial fluid (EPF), the presence of high Sr²⁺-selective organic matrices promoted the incorporation of Sr²⁺ into the shell, resulting in a higher Sr/Ca ratio.