Seawater carbonate chemistry and pulse rate, bell diameter, and righting ability of Cassiopeia sp.

The detrimental effect of ocean acidification (OA) on marine animals with carbonate exoskeletons or shells is an issue drawing increased attention in marine biology and ecology, yet few studies have focused on the impact on gelatinous organisms like scyphozoan medusae. Here, we examined the physiological tolerance of Cassiopea sp., an abundant genus of scyphozoans valuable for their role as bioindicators and for having similarities to other cnidarians, to OA by conducting three, 12-week trials using CO2 diffusers and electronic pH controllers to incrementally lower the water to test pHs of 7.5 and 7.0. The impact of reduced pH on the survival, pulse rate, bell diameter, and reorientation and settlement abilities of Cassiopea sp. medusae were measured weekly. Cassiopea sp. was tolerant to pH 7.5 while further reduction of the pH to 7.0 resulted in 22.22% mortality rate, which was significantly different from the control and treatment pH 7.5. Significant differences between the treatment pH 7.0 and control first occurred on day 23.5 with a 50% reduction in the pulse rate, and on day 36 with a 16.6% reduction in bell diameter, while pH 7.5 had no effect. By the final time point of 66 days in treatment pH 7.0, there was an 87% reduction in pulse rate and a 36% reduction in bell diameter versus control. Reduced pH 7.0 caused bell malformations, inhibited swimming abilities, and deterioration of the oral arm feeding apparatus, but had no effect on the orientation and settlement assay. Observations indicate that asexual reproduction via planuloid production and strobilation was unaffected by pH reduction, though polyps in treatment pH 7.0 gave rise to ephyrae with inverted bells. Combined, findings from this study demonstrate Cassiopea sp. to be resilient to the end of century ocean acidity prediction of pH 7.6, and vulnerable to more severe OA to pH 7.0.

海洋酸化（ocean acidification, OA）对具有碳酸盐外骨骼或外壳的海洋动物所产生的不利影响，是海洋生物学与生态学领域备受关注的研究议题，但目前针对诸如钵水母纲水母（scyphozoan medusae）这类胶状生物的相关研究仍较为稀缺。本研究以倒立水母属（Cassiopea sp.）为研究对象：该类群是分布广泛的钵水母类群，兼具生物指示物价值且与其他刺胞动物（cnidarians）具有诸多相似特征。研究通过二氧化碳扩散器（CO2 diffusers）与电子pH控制器（electronic pH controllers）逐步将水体pH调节至实验设定的7.5与7.0，开展3组为期12周的实验，以此探究其对海洋酸化的生理耐受性。研究人员每周测定了pH降低对倒立水母属水母的存活率、钟体搏动频率、钟体直径以及定向与附着能力的影响。实验结果显示，倒立水母属物种可耐受pH 7.5的水体环境；而当pH进一步降至7.0时，其死亡率达22.22%，与对照组及pH 7.5处理组均存在显著差异。pH 7.0处理组与对照组的首次显著差异分别出现于实验第23.5天（钟体搏动频率下降50%）与第36天（钟体直径缩小16.6%），而pH 7.5处理组未观测到此类影响。至实验第66天的最终观测节点，与对照组相比，pH 7.0处理组的水母钟体搏动频率下降87%，钟体直径缩小36%。pH 7.0的低水环境还会引发钟体畸形、游泳能力受抑以及口腕摄食器官退化，但对定向与附着实验无显著影响。观测结果表明，通过浮浪幼体产生与横裂生殖实现的无性繁殖并未受pH降低的影响，但pH 7.0处理组中的水螅体所发育出的碟状幼体（ephyrae）存在钟体倒置的情况。综合来看，本研究结果表明，倒立水母属物种对本世纪末海洋酸化预测的pH 7.6环境具有较强耐受性，但对pH 7.0的极端酸化环境较为脆弱。