Data from: 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）以及物种种群的维持均具有重要意义。全球变化（global change）正快速改变环境条件，但除珊瑚（coral）-微藻（microalgae）共生互作外，我们对全球变化如何影响共生关系的认知仍十分有限。本研究旨在探究由人为二氧化碳排放（anthropogenic CO2 emissions）加剧引发的海洋酸化（ocean acidification），会如何改变幼鱼（juvenile fish）与其水母（jellyfish）宿主之间的共生互作。按照常规温室气体排放情景预测，到本世纪末海水二氧化碳浓度将升高，经此条件处理的幼鱼其行为会受到负面影响。在可观察并感知宿主气味的行为选择箱（choice arena）中，黄尾鲹（yellowtail scad）在水母宿主附近停留的总时长约为当前环境二氧化碳浓度（ambient CO2）条件下的三分之一。同样，经高二氧化碳处理组幼鱼尝试与水母建立共生关联的平均次数较对照组低近三分之二；且仅63%的高二氧化碳处理组个体，相较于对照组的86%，能够至少发起一次共生关联尝试。相较之下，在当前环境二氧化碳条件下，受试的3种鱼类均不会仅被水母的嗅觉线索（olfactory cues）所吸引，这表明鱼-水母共生关联的改变并非由海洋酸化对嗅觉的负面影响所导致。由于开阔水层（open water column）中可获得的庇护场所十分有限，且许多（通常具有重要商业价值的）远洋物种（pelagic species）幼体依赖与水母共生以躲避捕食者，因此鱼-水母共生关系的改变可能会导致幼体死亡率升高，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）。