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导致海水pH值下降与碳酸盐化学组成改变。海洋酸化不仅会对海洋生物形成钙质结构的能力产生负面影响，同时还会干扰其生理响应与生长发育。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：这两种生物分别为地中海贻贝（Mytilus galloprovincialis）与波纹巴非蛤（Chamelea gallina）。 本研究搭建了户外流水养殖系统，设置两组pH梯度（以天然海水pH为对照组，pH 7.4为酸化处理组）开展长期暴露实验。首次为期6个月的长期实验中，贻贝与蛤蜊的初始死亡率均较低，但实验结束时死亡率显著升高，且蛤蜊的死亡率增幅显著高于贻贝。酸化处理组的波纹巴非蛤出现活体重量显著下降（降幅达26%），且壳体长度亦出现意外的显著缩短（降幅5%），而地中海贻贝未出现此类变化。两组对照组的双壳类均未记录到壳体损伤；而酸化处理组的贻贝与蛤蜊则出现了不同模式与不同程度的壳体损伤。暴露于低pH环境仅3个月后，地中海贻贝的壳体损伤程度即达到峰值，而波纹巴非蛤的损伤程度则随实验进程持续升高，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，地中海贻贝的受损面积峰值达35%，波纹巴非蛤则为11%。酸化处理组与对照组的波纹巴非蛤壳体厚度在实验3个月后即出现显著下降，而地中海贻贝的壳体厚度则在实验6个月后才出现显著降低。 第二次实验为期3个月，仅针对地中海贻贝幼体开展低pH暴露处理。实验开展3个月后，天然pH组与pH 7.4处理组的贻贝在存活率、壳体长度及活体重量上均无显著差异。但反之，酸化处理组的贻贝自实验第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处理组始终处于文石不饱和状态，而方解石不饱和状态仅在37%的监测样本中出现。本研究揭示了栖息于同一片海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。