Ocean acidification alters fish-jellyfish symbiosis

Symbiotic relationships are common in nature, and are important for individual fitness and sustaining species populations. Global change is rapidly altering environmental conditions, but, with the exception of coral?microalgae interactions, we know little of how this will affect symbiotic relationships. We here test how the effects of ocean acidification, from rising anthropogenic CO2 emissions, may alter symbiotic interactions between juvenile fish and their jellyfish hosts. Fishes treated with elevated seawater CO2 concentrations, as forecast for the end of the century on a business-as-usual greenhouse gas emission scenario, were negatively affected in their behaviour. The total time that fish (yellowtail scad) spent close to their jellyfish host in a choice arena where they could see and smell their host was approximately three times shorter under future compared with ambient CO2 conditions. Likewise, the mean number of attempts to associate with jellyfish was almost three times lower in CO2-treated compared with control fish, while only 63% (high CO2) versus 86% (control) of all individuals tested initiated an association at all. By contrast, none of three fish species tested were attracted solely to jellyfish olfactory cues under present-day CO2 conditions, suggesting that the altered fish?jellyfish association is not driven by negative effects of ocean acidification on olfaction. Because shelter is not widely available in the open water column and larvae of many (and often commercially important) pelagic species associate with jellyfish for protection against predators, modification of the fish?jellyfish symbiosis might lead to higher mortality and alter species population dynamics, and potentially have flow-on effects for their fisheries.共生关系 (symbiotic relationships) 在自然界中广泛存在，对个体适合度 (individual fitness) 与物种种群的维持至关重要。全球变化正快速改变环境条件，但除珊瑚-微藻互作之外，我们对全球变化将如何影响共生关系知之甚少。本研究旨在检验由人为二氧化碳排放引发的海洋酸化 (ocean acidification)，会如何改变幼鱼与其水母宿主之间的共生互作。按照照常排放的温室气体情景预测的本世纪末升高海水二氧化碳浓度处理组的鱼类，其行为受到了负面影响。在可观察并嗅探宿主的选择实验装置 (choice arena) 中，受试黄尾鲹 (yellowtail scad) 在靠近水母宿主的总时长，在未来二氧化碳浓度条件下约为当前二氧化碳条件下的三分之一。同样，二氧化碳处理组鱼类尝试与水母建立关联的平均次数几乎是对照组的三分之一，而成功发起关联的个体比例，高二氧化碳组仅为63%，对照组则为86%。与之形成对比的是，在当前二氧化碳条件下，三种受试鱼类均未仅被水母的嗅觉线索 (olfactory cues) 所吸引，这表明鱼-水母共生关系的改变并非由海洋酸化对嗅觉的负面影响所驱动。由于开阔大洋水层中庇护场所稀缺，且许多（通常具有商业价值的）大洋洄游性鱼类幼体依赖水母以躲避捕食者，鱼-水母共生关系的改变可能会导致更高的死亡率，改变物种种群动态，并可能对相关渔业产生连锁效应 (flow-on effects)。