the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 25 Jul 2025
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.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2025-3479', Anonymous Referee #1, 02 Sep 2025
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AC1: 'Reply on RC1', Iris Arndt, 11 Sep 2025
Dear Anonymous Referee #1,
We really appreciate the very constructive, helpful and overall encouraging comments on our manuscript and agree that addressing the issues raised in the specific comments section would improve the manuscript. We aim to improve the clarity of Fig. 4, distinguish between true replicates grown in the same water and replicates that just grew under the same experimental conditions, add detail to the feeding procedure and add an error estimate to the alkalinity measurements. To add a comparison of the presented El/Ca data from cultured specimens to those from naturally-grown Tridacna is a good idea, which we will aim to include in the discussion. We are also grateful for the technical corrections and will try to shorten sentences that seem too long and complicated. A decision on final edits, however, can only be made once all reviews have been received.
Best regards,
Iris Arndt on behalf of all co-authors
Citation: https://doi.org/10.5194/egusphere-2025-3479-AC1
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AC1: 'Reply on RC1', Iris Arndt, 11 Sep 2025
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RC2: 'Comment on egusphere-2025-3479', Daniel Killam, 14 Sep 2025
The work of Arndt et al represents a valuable expansion of the previous investigation of Warter et al., 2018, with many novel interpretations that make it a valuable contribution to the advancement of giant clam biology. The effort to quantify the daily calcification of these animals in particular is quite important to other efforts to understand how they will fare during a time of environmental change. The methodology of LA-ICPMS sampling at 2 micron resolution, and mention of the variation of different under-studied elemental ratios such as Na/Ca and B/Ca is also an important advancement. This approach shows how shell records are valuable to investigate the modern-day physiology of these living animals, and I endorse its publication once the following line-by-line comments are addressed.
Daniel Killam
22: I'd advocate for the authors noting here that they are estimating the EPF composition based on the composition of external seawater in a culture setup. This is totally a valid approach, but the abstract made it sound like direct sampling of the EPF was conducted, which does not seem to be the case.
86: It is crucial to know the species of these clams, for reproducibility purposes. I would be happy to personally identify the species if the authors have pictures of the clams. Feel free to email pictures.
109: Do you know the brand/composition of coral food? The diet could be important for other workers to reproduce the results. I think it was useful that the clams were fed (that is, of course different than knowing if they actually ate the food), since other studies often do not feed the clams, which leads to changes in behavior, particularly for juvenile clams of this size, which are generally more reliant on filter-feeding. Did you observe them feeding during the experiment? Production of feces/pseudofeces, valve clapping, etc.
140: The measurement of alkalinity differences at night and day is a great approach and one of those things that makes me slap my forehead, and wonder why someone hadn't thought to do that before, especially since in the aquarium trade, giant clams are known as infamous alkalinity sinks. Were any other environmental parameters measured, even intermittently? I'd be particularly interested in nutrient measurements like nitrate or ammonium (more on that below)
204: What direct influence would the calcification have on DO? I would have thought O2 is more related to photosynthesis. In daylight hours, the clams can be net sources of O2 (Fisher et al., 1985), while at night, they conduct respiration, analogous to similar processes seen in plants. Were there any algae in the tanks with them adding to these processes, or were they the only significant biomass in the observation tanks?
336: One source of potential error in estimations of day-night differences in EPF residence time: did the clams partially close at night? They tend to close partially at night (see Killam et al., 2023) in a defensive posture (this is not always the case, such as some aquarium settings where predation is not an issue, so yours might not have). But if they close, their overall internal volume would be smaller, and as such their extrapallial space would be smaller in volume.
359: What nutrients are higher in daytime? Generally, N-bearing nutrients would be expected to be lower in the day on average due to assimilation and higher at night due to remineralization/respiration being dominant.
Section 4.4.1: I suspect that in your tanks, this was not a major factor, but the reason I asked about nutrient measurements is that in other experimental setups where other biota might be present in the aquarium, I believe nitrification would represent a significant source of error trying to replicate this approach. In closed-system aquarium setups, ammonium waste from the inhabitants is converted to nitrate, a process which can destroy alkalinity. In your constrained single-species tanks, I bet that the clams were quickly re-absorbing all ammonium produced to feed their symbionts, making it not a factor of concern in your calculations. However, I'd advise adding a mention here of that process to aid in reproducibility, since this hidden sink of alkalinity could make replication difficult in some research aquaria. Also for that reason, if you have any observations of nutrients like NH4/NO3/NO2/etc during the experiment, please include them.
376: It is better to refer to them as Tridacninae or tridacnines, since they are in the family Cardiidae
Section 4.4.2: I think it's great you included this section discussing the role of organic matter, which I believe is greatly under-studied.Citation: https://doi.org/10.5194/egusphere-2025-3479-RC2
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General Comments
The manuscript “Culturing experiments reveal mechanisms of daily trace element incorporation into Tridacna shells” presents novel and well-designed culturing experiments that address important knowledge gaps in our understanding of biomineralization processes in Tridacna and their application as high-resolution paleoclimate archives. The use of 135Ba as a tracer to unequivocally differentiate between day- and night-time shell growth represents a novel methodological advancement, providing robust constraints on the mechanisms of trace element (El/Ca) incorporation. The link made between EPF dynamics, light availability, and elemental ratios is compelling and will be of interest to both geochemists and marine biologists. The work is highly relevant to Biogeosciences, as it improves the mechanistic understanding of high resolution geochemical proxy development in marine bivalves, with implications for interpreting sub-daily climate signals.
The manuscript is clearly structured, generally well-written (although I would recommend making many sentences shorter where possible), and supported by a thorough literature review. The experimental setup is carefully designed and technologically sharp and the analytical work by the novel LA-ICPMS configuration is impressive - clearly the author's expertise. The conclusions are both significant and well supported by the data.
Overall, the study provides important insights for the Tridacna sclerochronology community (myself included) and for paleoclimatology. With some very minor clarifications, improvements in figure presentation, and minor language editing, the paper will be an excellent contribution.
Specific Comments
- Figures 3 and 4 are central to the paper. In Figure 3, I would suggest expanding it width-wise. I know Fig. 4 provides an expanded version but the legibility in Fig. 3 is not great and should be improved. This would make the timing of the spiking vs the periodicity in the El/Ca values (which is key) a bit clearer. In Fig. 4, explain the green bars (as you do in Fig. 3) in the figure caption or in the figure.
- In terms of experimental, given that two clams share each jar, and therefore the same water, can they be considered as true replicates? Isn’t the carbonate chemistry in the water an important part of the experimental unit?
- Line 109: A few more details about the feeding regime would be good. Quantity? Timing? DOM vs particulate matter? This is important for understanding food vs light contributions.
- Consider uncertainties in using 12h ΔALK to estimate calcification.
- How does the data from this study compare with daily-resolved trace element/Ca data from clams that grew in natural reef settings (e.g. in some of your other publications)? E.g. amplitude, relative phasings. If similar, this would strengthen any arguments you had that your observations apply to Tridacna more generally, not just those with very specific culture conditions. (Juvenile, small individuals in 28 °C, 37 psu, constant light regime (12:12 h) under lab flow are quite different from heterogeneous reef conditions. Whether the 5× daytime calcification and the precise phasing of maxima/minima are similar across sizes, species, and natural diel PAR/temperature cycles remains to be tested – worth acknowledging perhaps?)
- The discussion could more explicitly link findings to potential paleoenvironmental applications (ENSO, storm reconstructions, etc.), strengthening the broader impact.
Technical Corrections
- Language generally fluent, but some sentences are long and could be simplified for clarity.
- Don’t capitalise ‘tridacnid’
- Minor typos: Hyphen needed in ‘non spiked’ on p121. Centre data in table.