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

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在导致海水pH值下降以及碳酸盐化学体系改变。海洋酸化可能会损害海洋生物构建钙质结构的能力，同时还会影响其生理响应与生长。本研究旨在评估pH值降低对北亚得里亚海两种海洋双壳类幼体的存活、生长及壳体完整性的影响：分别为地中海贻贝（Mytilus galloprovincialis）与条纹帘蛤（Chamelea gallina）。研究搭建了室外流水养殖系统，在长期实验中设置两组pH梯度：以自然海水pH为对照组，以pH 7.4为酸化处理组。首次实验全程（6个月）中，贻贝与条纹帘蛤的初始死亡率均处于较低水平，但实验结束时死亡率显著升高，且条纹帘蛤的死亡率增幅更为显著。处理组条纹帘蛤的活体重显著下降26%，壳长也意外降低5%，而贻贝未出现此类变化。两种双壳类的对照组均未出现壳体损伤；而处理组的贻贝与条纹帘蛤则以不同方式、在不同程度上出现了壳体损伤。酸化处理仅3个月后，贻贝的壳体损伤严重程度便达到峰值，而条纹帘蛤的损伤程度则随实验推进逐渐加重，直至实验结束。两种双壳类的壳体受损面积均随实验推进不断扩大，贻贝受损面积峰值达35%，条纹帘蛤则为11%。处理组与对照组条纹帘蛤的壳体厚度在实验进行3个月后出现显著下降，而贻贝的壳体厚度则在实验6个月后才出现显著降低。第二次实验时长为3个月，仅将幼龄贻贝暴露于低pH环境中。实验进行3个月后，自然pH组与pH 7.4组贻贝的存活率、壳长及活体重均无显著差异。但与之相反，处理组贻贝自实验第1个月起便出现了明显的壳体损伤。海水碳酸盐化学监测结果显示，pH 7.4组始终处于文石不饱和状态，而方解石不饱和状态仅在37%的检测样本中出现。本研究揭示了栖息于同一区域（但并非完全相同生境）的两种双壳类对海洋酸化的响应存在显著差异。