Elevated temperature and carbon dioxide levels alter growth rates and shell composition in the fluted giant clam, Tridacna squamosa

Abstract

Giant clams produce massive calcified shells with important biological (e.g., defensive) and ecological (e.g., habitat-forming) properties. Whereas elevated seawater temperature is known to alter giant clam shell structure, no study has examined the effects of a simultaneous increase in seawater temperature and partial pressure of carbon dioxide (pCO2) on shell mineralogical composition in these species. We investigated the effects of 60-days exposure to end-of-the-century projections for seawater temperature (+ 3 °C) and pCO2 (+ 500 µatm) on growth, mineralogy, and organic content of shells and scutes in juvenile Tridacna squamosa giant clams. Elevated temperature had no effect on growth rates or organic content, but did increase shell [24Mg]/[40Ca] as well as [40Ca] in newly-formed scutes. Elevated pCO2 increased shell growth and whole animal mass gain. In addition, we report the first evidence of an effect of elevated pCO2 on element/Ca ratios in giant clam shells, with significantly increased [137Ba]/[40Ca] in newly-formed shells. Simultaneous exposure to both drivers greatly increased inter-individual variation in mineral concentrations and resulted in reduced shell N-content which may signal the onset of physiological stress. Overall, our results indicate a greater influence of pCO2 on shell mineralogy in giant clams than previously recognized.

Giant clams modify crystallographic and geochemical pathways of shell formation in response to turbidity

“…Our findings on the geochemistry and architecture of shells in this study have important implications for their biomechanical properties (e.g., hardness and elasticity), giving indication of the defensive capabilities and resilience of biogenic skeletons (9,32). Overall, our results show higher crystallographic orderliness and significantly lower El/Ca ratios in the high turbid site, compared to the low turbid site.…”

“…The geochemical fingerprint of shells is thought to underpin biomineral design, associated crystallographic properties and their relationship to the environment because geochemical variations play a key role in the dictation of mineral formation (32). Shell geochemistry shows a strong physiological component in many marine calcifiers because element-to-calcium (El/Ca) ratios deviate from surrounding ambient seawater (i.e., non-equilibrium fractionation) (e.g., 33).…”

Universality of ecological memory for local and global net ecosystem exchange, atmospheric CO2, and sea surface temperature

“…The inevitable effects of these disrupted ecosystems, when uncontrolled, may push these ecosystems to their tipping points and be irreversibly damaged. These rising levels observed in many global environmental systems such as sea surface temperature and anthropogenic CO 2 emissions have been disrupting the natural cycles of atmospheric events 1,2,3 . Among the most impacted natural cycles is the global carbon balance whereby observed fluctuations are indicative of changing ecosystem behavior, such as microbial regulations of greenhouses fluxes 4 , altered peatland vegetation phenology 24 , shifting of forest ecosystems from sink to source, that may result to the tipping of carbon balance 5 .…”