Abundance of Cnidaria and Ctenophora in the North Atlantic sampled during the G. O. Sars Cruise in May 2013

In recent years a global increase in jellyfish (i.e. Cnidarians and Ctenophores) abundance and a rise in the recurrence of jellyfish outbreak events have been largely debated, but a general consensus on this matter has not been achieved yet. Within this debate, it has been generally recognized that there is a lack of reliable data that could be analyzed and compared to clarify whether indeed jellyfish are increasing throughout the world ocean as a consequence of anthropogenic impact and hydroclimatic variability. During the G.O. Sars cruise jellyfish were collected at different depths in the 0-1000m layer using a standard 1 m**2 Multiple Opening/Closing Net and Environmental Sensing System (MOCNESS) (quantitative data), Harstad and macroplankton trawls (qualitative data). The comparison of records collected with different nets during the G.O. Sars transatlantic cruise shows that different sampling gears might provide very different information on jellyfish diversity. Indeed, the big trawls mostly collect relatively large scyphozoan and hydrozoan species such as Atolla, Pelagia, Praya, Vogtia, while small hydrozoans (e.g. Clytia, Gilia, Muggiaea) and early stages of ctenophora are only caught by the smaller nets.近年来，全球水母（即刺胞动物门Cnidarians与栉水母动物门Ctenophores）丰度攀升、水母暴发事件发生频次增加的议题引发了广泛探讨，但学界尚未就此形成普遍共识。在这场讨论中，学界普遍认为目前缺乏可供分析对比的可靠数据，难以明确全球海域内的水母种群是否确因人类活动影响与水文气候变异而呈现增长态势。在G.O. Sars科考航次期间，研究人员使用标准1平方米多开口闭合网与环境传感系统（Multiple Opening/Closing Net and Environmental Sensing System，简称MOCNESS），在0至1000米水层的不同深度采集水母样本，获取定量数据；同时采用Harstad拖网与大型浮游生物拖网开展采样，获得定性数据。对G.O. Sars跨大西洋航次中不同网具采集的记录进行对比后可发现，不同采样装备所获取的水母多样性相关信息可能存在显著差异。事实上，大型拖网主要捕获体型相对较大的钵水母类（Scyphozoa）和水螅虫类（Hydrozoa）物种，例如艾拉水母属（Atolla）、霞水母属（Pelagia）、管水母属（Praya）、沃格特水母属（Vogtia）；而小型水螅虫类（如克里亚水母属Clytia、吉利亚水母属Gilia、穆吉亚水母属Muggiaea）以及栉水母动物的幼体阶段，则仅能通过小型网具采集得到。