Ultradian rhythms in shell composition of photosymbiotic and non-photosymbiotic mollusks

The chemical composition of mollusk shells is a useful tool in (paleo)climatology since it captures inter- and intra-annual variability in environmental conditions. Trace element and stable isotope analysis with improved sampling resolution now allows in situ determination of the composition of mollusk shell volumes precipitated at daily to sub-daily time intervals. Here, we discuss hourly resolved Mg inline-formula $M1inlinescrollmathml/$ 8pt14ptsvg-formulamathimg1b4178c77ca0d4bfee6c9ddd864f3a43 bg-20-3027-2023-ie00001.svg8pt14ptbg-20-3027-2023-ie00001.png  Ca, Mn inline-formula $M2inlinescrollmathml/$ 8pt14ptsvg-formulamathimg527256ea34e0af356380afd605ccefc0 bg-20-3027-2023-ie00002.svg8pt14ptbg-20-3027-2023-ie00002.png  Ca, Sr inline-formula $M3inlinescrollmathml/$ 8pt14ptsvg-formulamathimge653eaf840568ee76bb20ba3bf368ae0 bg-20-3027-2023-ie00003.svg8pt14ptbg-20-3027-2023-ie00003.png  Ca, and Ba inline-formula $M4inlinescrollmathml/$ 8pt14ptsvg-formulamathimg57ee8123d9c9aefcf23d9c7f6463c158 bg-20-3027-2023-ie00004.svg8pt14ptbg-20-3027-2023-ie00004.png  Ca profiles measured by laser ablation inductively coupled plasma – mass spectrometry (ICP-MS) through shells of the photosymbiotic giant clams (Tridacna maxima, T. squamosa, and T. squamosina) and the non-photosymbiotic scallop Pecten maximus. Precise sclerochronological age models and spectral analysis allowed us to extract daily and tidal rhythms in the trace element composition of these shells. We find weak but statistically significant expressions of these periods and conclude that this cyclicity explains less than 10 % of the sub-annual variance in trace element profiles. Tidal and diurnal rhythms explain variability of, at most, 0.2 mmol molinline-formula⁻¹ (inline-formula∼ 10 % of mean value) in Mg inline-formula $M7inlinescrollmathml/$ 8pt14ptsvg-formulamathimg36bd7baae116a5efc17e692d563c2b51 bg-20-3027-2023-ie00005.svg8pt14ptbg-20-3027-2023-ie00005.png  Ca and Sr inline-formula $M8inlinescrollmathml/$ 8pt14ptsvg-formulamathimgfb147fccdcf98a9911cf3d26a8f6dc33 bg-20-3027-2023-ie00006.svg8pt14ptbg-20-3027-2023-ie00006.png  Ca, while ultradian Mn inline-formula $M9inlinescrollmathml/$ 8pt14ptsvg-formulamathimg880d1b22cfae9b4167ff115d05c6894c bg-20-3027-2023-ie00007.svg8pt14ptbg-20-3027-2023-ie00007.png  Ca and Ba inline-formula $M10inlinescrollmathml/$ 8pt14ptsvg-formulamathimg3af55808dad7e355d8e0b0b2a0272ce7 bg-20-3027-2023-ie00008.svg8pt14ptbg-20-3027-2023-ie00008.png  Ca cyclicity has a median amplitude of less than 2 inline-formulaµmol molinline-formula⁻¹ mol molinline-formula⁻¹ (inline-formula∼ 40 % and 80 % of the mean of Mn inline-formula $M15inlinescrollmathml/$ 8pt14ptsvg-formulamathimg7572a9d7afeaa92ba0e8bb6f686362bd bg-20-3027-2023-ie00009.svg8pt14ptbg-20-3027-2023-ie00009.png  Ca and Ba inline-formula $M16inlinescrollmathml/$ 8pt14ptsvg-formulamathimgc6f00d13d95b9183e3e2526db4298e27 bg-20-3027-2023-ie00010.svg8pt14ptbg-20-3027-2023-ie00010.png  Ca, respectively). Daily periodicity in Sr inline-formula $M17inlinescrollmathml/$ 8pt14ptsvg-formulamathimga3a809672b156f3719eee3cbaf593ee5 bg-20-3027-2023-ie00011.svg8pt14ptbg-20-3027-2023-ie00011.png  Ca and Ba inline-formula $M18inlinescrollmathml/$ 8pt14ptsvg-formulamathimg62d2c8208bbdf49afb8db19c9f7b6b50 bg-20-3027-2023-ie00012.svg8pt14ptbg-20-3027-2023-ie00012.png  Ca is stronger in Tridacna than in Pecten, with Pecten showing stronger tidal periodicity. One T. squamosa specimen which grew under a sunshade exhibits among the strongest diurnal cyclicity. Daily cycles in the trace element composition of giant clams are therefore unlikely to be driven by variations in direct insolation but rather reflect an inherent biological rhythmic process affecting element incorporation. Finally, the large amount of short-term trace element variability unexplained by tidal and daily rhythms highlights the dominance of aperiodic processes in mollusk physiology and/or environmental conditions over shell composition at the sub-daily scale. Future studies should aim to investigate whether this remaining variability in shell chemistry reliably records weather patterns or circulation changes in the animals' environment.