Culturing experiments reveal mechanisms of daily trace element incorporation into Tridacna shells

Giant clams such as Tridacna are exceptionally well suited for studying past environmental changes on daily to multidecadal timescales. The visible growth bands in their shells, which can be yearly, seasonal or even daily, are accompanied by changes in the elemental composition of the shell and provide insights into their growth and environmental history. The daily elemental cycles, particularly in Mg/Ca and Sr/Ca, can be used to determine age and growth rates. However, the mechanisms creating the visible day and night banding and the associated elemental cycles, remain unclear. To better understand the mechanisms of El/Ca incorporation into the shells of Tridacna during day and night growth, we performed controlled growth experiments using ¹³⁵Ba-labelled seawater. The isotope spike was alternatingly applied in 12 h intervals in order to individually and unequivocally mark day and night growth segments in Tridacna. These experiments show that Tridacna calcification rates are nearly five times higher during the day than at night. In addition, based on the observed changes in shell composition we deduce that the bivalve’s extrapallial fluid (EPF) reacts to changes in seawater chemistry within tens of minutes, both during day and night. A full compositional replenishment is achieved after approximately 1 d, assuming a similar residence time for all elements. During daytime, El/Ca (for El = B, Mg, Sr, Ba) decrease, while Na/Ca increases. The opposite behaviour occurs at night. The night peak in El/Ca occurs in the earliest morning, shortly before the change between spiked and non-spiked water at 07:30 UTC+2. Daily El/Ca cycles are likely dominantly driven by variations in active Ca²⁺ and HCO⁻₃ transport into the EPF, influenced by light availability, circadian rhythms and/or energy availability (from both photosymbionts and filter feeding), rather than a closed-system Rayleigh fractionation process driven by contrasting El-distribution coefficients alone. We propose that active Ca²⁺ and HCO⁻₃ pumping into the EPF might also drive diurnal changes of growth rate, shell structure and possibly organic content.