SCLEROCHRONOLOGY OF GIANT CLAMS CULTURED IN A 2.6 MILLION LITER MESOCOSM: SEARCHING FOR PROXIES OF PHOTOSYMBIOSIS, ACID-BASE CHEMISTRY AND FOOD SUPPLY (Invited Presentation)

The geochemical composition of the shells of giant clams (Tridacninae) are useful climate proxies on reefs, due to their longevity and rapid growth. These unusual bivalves achieve their large size due to a partnership with symbiotic algae of the same varieties found in reef-building corals. Extensive research has investigated the influence of photosymbionts on coral carbonate chemistry, but less work has been done to find proxies of photosymbiosis in giant clam shells. Previous attempts to find geochemical signals of photosymbiotic development in giant clams have been challenged by overprinting by external environmental variability. There is thus a need for studies culturing giant clams in a controlled environment simulating the tropical ocean, but allowing for fine control of light, temperature and detailed monitoring of production and acid-base chemistry.

Here we present results of a 1.5 year long culturing study of Tridacna derasa grown in the 2.6 million L ocean mesocosm at Biosphere 2, Arizona. The clams grew from 30 to 70 mm, remaining at a stable 25 °C temperature and a pH of 7.9-8.2. Solution-based and laser ablation (LA) ICPMS techniques were used to determine Sr/Ca, Mg/Ca, Ba/Ca, Li/Ca, Mn/Ca, P/Ca, and B/Ca. Additionally, a subset of shells were analyzed via LA-MC-ICPMS, allowing measurement of boron isotope (δ¹¹B) values at 20 µm resolution. Finally, weight percent shell organic matter (SOM) was calculated via loss on ignition analyses.

Results indicate a decrease in B/Ca and δ¹¹B during the experiment, and increases in Ba/Ca, Li/Ca and Sr/Ca. Given growth at relatively constant pH (with diurnal variability), the decreases in B/Ca and δ¹¹B are likely related to decreased pH at the calcification site as the clams accelerated their growth with time in the B2 ocean tank. However, the shells show increases and decreases in both variables with a daily periodicity. This has been observed for other elements investigated in giant clam shells (e.g. Sr, Mg, Ba), but here we show that δ¹¹B and B/Ca also respond to light-influenced circadian shell growth. LA and solution approaches largely corroborate each other (despite the potential for confounding by SOM around 2%, 2-10x higher than is common for adults), and results are similar between the dozen individuals studied, suggesting that giant clams are reproducible archives of microenvironmental variability on reefs.