Comparison of Mediterranean Pteropod Shell Biometrics and Ultrastructure from Historical (1910 and 1921) and Present Day (2012) Samples Provides Baseline for Monitoring Effects of Global Change

Anthropogenic carbon perturbation has caused decreases in seawater pH and increases in global temperatures since the start of the 20th century. The subsequent lowering of the saturation state of CaCO3 may make the secretion of skeletons more problematic for marine calcifiers. As organisms that precipitate thin aragonite shells, thecosome pteropods have been identified as being particularly vulnerable to climate change effects. Coupled with their global distribution, this makes them ideal for use as sentinel organisms. Recent studies have highlighted shell dissolution as a potential indicator of ocean acidification; however, this metric is not applicable for monitoring pH changes in supersaturated basins. In this study, the novel approach of high resolution computed tomography (CT) scanning was used to produce quantitative 3-dimensional renderings pteropod shells to assess the potential of using this method to monitor small changes in shell biometrics that may be driven by climate change drivers. An ontogenetic analysis of the shells of Cavolinia inflexa and Styliola subula collected from the Mediterranean was used to identify suitable monitoring metrics. Modern samples were then compared to historical samples of the same species, collected during the Mediterranean leg of the Thor (1910) and Dana (1921) cruises to assess whether any empirical differences could be detected. Shell densities were calculated and scanning electron microscopy was used to compare the aragonite crystal morphology. pH for the collection years was hind-cast using temperature and salinity time series with atmospheric CO2 concentrations from ice core data. Historical samples of S. subula were thicker than S. subula shells of the same size from 2012 and C. inflexa shells collected in 1910 were significantly denser than those from 2012. These results provide a baseline for future work to develop monitoring techniques for climate change in the oceans using the novel approach of high-resolution CT scanning.

自20世纪初以来，人为碳扰动已导致海水pH值下降、全球气温升高。碳酸钙（CaCO₃）饱和态的持续降低，可能会使海洋钙化生物的骨骼分泌过程面临更大挑战。作为一类形成薄文石壳的生物，被壳翼足类（thecosome pteropods）被证实对气候变化影响尤为敏感。加之其全球分布特性，这类生物成为理想的指示生物（sentinel organisms）。
近期研究将壳体溶解视为海洋酸化的潜在指示指标，但该指标并不适用于监测过饱和海盆中的pH变化。本研究采用高分辨率计算机断层扫描（computed tomography，CT）这一创新方法，对翼足类壳体进行定量三维重建，以评估该方法用于监测气候变化驱动因子所引发的壳体生物测量参数细微变化的潜力。
研究对采集自地中海的弯形海若螺（Cavolinia inflexa）和细柱翼足螺（Styliola subula）的壳体开展个体发育分析，以确定适宜的监测指标。随后将现代样本与1910年Thor号科考航次地中海航段、1921年Dana号科考航次地中海航段采集的同种历史样本进行对比，以检测是否存在可观测的经验性差异。
研究人员计算了壳体密度，并利用扫描电子显微镜（scanning electron microscopy，SEM）对比了文石晶体的形貌特征。基于冰芯数据获取的大气CO₂浓度，结合温度与盐度时间序列，对样本采集年份的海水pH值进行了回溯推算。
结果显示，2012年采集的细柱翼足螺（S. subula）壳体厚度低于同尺寸的历史样本；1910年采集的弯形海若螺（C. inflexa）壳体密度显著高于2012年的对应样本。本研究结果为未来利用高分辨率CT扫描这一创新技术开发海洋气候变化监测手段提供了基准参照。