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

Abstract. Giant clams such as Tridacna sp. are exceptionally well suited for studying past environmental changes on various timescales, from daily to multidecadal. 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 Tridacna shell during day and night growth, we performed controlled growth experiments using ¹³⁵Ba-labelled seawater. It was alternatingly applied in 12-hour 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. The bivalve’s extrapallial fluid (EPF) reacts to changes in seawater chemistry within tens of minutes, both during day and night, with full compositional replenishment achieved after approximately one day. 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 7:30. Daily El/Ca cycles are likely caused by variations in active Ca²⁺ and HCO₃^- 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. We propose that active Ca²⁺ and HCO₃^- pumping into the EPF might also drive diurnal changes of growth rate, shell structure and possibly organic content.