Does seawater acidification affect survival, growth and shell integrity in bivalve juveniles?@en

Anthropogenic emissions of carbon dioxide are leading to decreases in pH and changes in the carbonate chemistry of seawater. Ocean acidification may negatively affect the ability of marine organisms to produce calcareous structures while also influencing their physiological responses and growth. The aim of this study was to evaluate the effects of reduced pH on the survival, growth and shell integrity of juveniles of two marine bivalves from the Northern Adriatic sea: the Mediterranean mussel Mytilus galloprovincialis and the striped venus clam Chamelea gallina. An outdoor flow-through plant was set up and two pH levels (natural seawater pH as a control, pH 7.4 as the treatment) were tested in long-term experiments. Mortality was low throughout the first experiment for both mussels and clams, but a significant increase, which was sensibly higher in clams, was observed at the end of the experiment (6 months). Significant decreases in the live weight (-26%) and, surprisingly, in the shell length (-5%) were observed in treated clams, but not in mussels. In the controls of both species, no shell damage was ever recorded; in the treated mussels and clams, damage proceeded via different modes and to different extents. The severity of shell injuries was maximal in the mussels after just 3 months of exposure to a reduced pH, whereas it progressively increased in clams until the end of the experiment. In shells of both species, the damaged area increased throughout the experiment, peaking at 35% in mussels and 11% in clams. The shell thickness of the treated and control animals significantly decreased after 3 months in clams and after 6 months in mussels. In the second experiment (3 months), only juvenile mussels were exposed to a reduced pH. After 3 months, the mussels at a natural pH level or pH 7.4 did not differ in their survival, shell length or live weight. Conversely, shell damage was clearly visible in the treated mussels from the 1st month onward. Monitoring the chemistry of seawater carbonates always showed aragonite undersaturation at 7.4 pH, whereas calcite undersaturation occurred in only 37% of the measurements. The present study highlighted the contrasting effects of acidification in two bivalve species living in the same region, although not exactly in the same habitat.人为二氧化碳排放正导致海水pH值下降与碳酸盐化学组成改变。海洋酸化可能会对海洋生物形成钙质结构的能力产生负面影响，同时干扰其生理响应与生长过程。本研究旨在探究pH值降低对北亚得里亚海两种海洋双壳类幼体的存活、生长以及壳体完整性的影响，这两种双壳类分别为地中海贻贝（Mytilus galloprovincialis）与条纹文蛤（Chamelea gallina）。本研究搭建了户外流水培养系统，在长期实验中设置两组pH梯度：以自然海水pH为对照组，pH 7.4为酸化处理组。首轮为期6个月的实验中，两种双壳类的初始死亡率均较低，但实验结束时死亡率显著上升，且条纹文蛤的死亡率增幅显著高于地中海贻贝。处理组条纹文蛤的活体重量下降26%、壳长下降5%，该差异具有统计学显著性；而地中海贻贝未出现此类变化。两组对照组的双壳类均未出现壳体损伤；而处理组的两种双壳类则出现了不同模式与不同程度的壳体损伤。地中海贻贝的壳体损伤程度在暴露于低pH环境3个月后达到峰值，而条纹文蛤的损伤程度则随实验推进持续加重，直至实验结束。实验期间，两种双壳类的壳体损伤面积均持续扩大，地中海贻贝的损伤面积峰值达35%，条纹文蛤则达11%。处理组与对照组条纹文蛤的壳体厚度在实验3个月后出现显著下降，而地中海贻贝的壳体厚度下降则出现在实验6个月后。第二轮实验为期3个月，仅设置地中海贻贝幼体的低pH暴露组。实验结束后，自然pH组与pH 7.4处理组的贻贝在存活率、壳长及活体重量方面均无显著差异。但反之，处理组贻贝自实验第1个月起即可观察到明显的壳体损伤。海水碳酸盐化学监测结果显示，pH 7.4环境下始终处于文石不饱和状态，而方解石不饱和状态仅在37%的监测样本中出现。本研究揭示了同区域（但生境略有差异）的两种双壳类对海洋酸化的响应存在显著差异。