Sea hare Aplysia punctata (mollusca: Gastropoda) can maintain shell calcification under extreme ocean acidification

Ocean acidification is expected to cause energetic constraints upon marine calcifying organisms such as molluscs and echinoderms, because of the increased costs of building or maintaining shell material in lower pH. We examined metabolic rate, shell morphometry, and calcification in the sea hare Aplysia punctata under short-term exposure (19 days) to an extreme ocean acidification scenario (pH 7.3, 2800 µatm pCO2), along with a group held in control conditions (pH 8.1, 344 µatm pCO2). This gastropod and its congeners are broadly distributed and locally abundant grazers, and have an internal shell that protects the internal organs. Specimens were examined for metabolic rate via closed-chamber respirometry, followed by removal and examination of the shell under confocal microscopy. Staining using calcein determined the amount of new calcification that occurred over 6 days at the end of the acclimation period. The width of new, pre-calcified shell on the distal shell margin was also quantified as a proxy for overall shell growth. Aplysia punctata showed a 30% reduction in metabolic rate under low pH, but calcification was not affected. This species is apparently able to maintain calcification rate even under extreme low pH, and even when under the energetic constraints of lower metabolism. This finding adds to the evidence that calcification is a largely autonomous process of crystallization that occurs as long as suitable haeomocoel conditions are preserved. There was, however, evidence that the accretion of new, noncalcified shell material may have been reduced, which would lead to overall reduced shell growth under longer-term exposures to low pH independent of calcification. Our findings highlight that the chief impact of ocean acidification upon the ability of marine invertebrates to maintain their shell under low pH may be energetic constraints that hinder growth of supporting structure, rather than maintenance of calcification.

海洋酸化预计会对软体动物、棘皮动物等海洋钙化生物造成能量限制，因为在较低pH环境下构建或维持贝壳外壳的成本会升高。本研究针对海兔（Aplysia punctata）设置了两组实验：一组置于极端海洋酸化场景（pH 7.3，2800 µatm pCO₂）中进行19天短期暴露，另一组作为对照组维持常规环境条件（pH 8.1，344 µatm pCO₂），以此检测其代谢速率、贝壳形态计量特征与钙化作用。该腹足类及其近缘类群分布广泛、局部丰度较高，拥有可保护内脏器官的内壳。研究人员通过封闭舱呼吸测量法检测实验个体的代谢速率，随后取出贝壳并利用共聚焦显微镜进行观察。在驯化周期末期，通过钙黄绿素染色量化6天内的新增钙化量；同时对贝壳远端边缘的新生预钙化壳宽度进行定量，以此作为整体贝壳生长的替代指标。结果显示，低pH环境下海兔的代谢速率降低30%，但钙化作用未受显著影响。该物种即便在极端低pH环境下，乃至面临代谢速率降低带来的能量限制时，仍能维持钙化速率。本发现进一步佐证，钙化作用本质上是一种自主的结晶过程，只要血腔环境适宜即可发生。不过，研究发现新生非钙化贝壳物质的沉积过程可能受到抑制，这意味着在长期低pH暴露下，即便不考虑钙化作用，整体贝壳生长也会出现下降。本研究结果表明，海洋酸化对海洋无脊椎动物在低pH环境下维持贝壳结构的核心影响，可能并非钙化作用的维持受阻，而是能量限制阻碍了支撑结构的生长。