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The impact of microbiome in animal physiology is well appreciated, but characterization of animal-microbe symbiosis in marine environments remains a growing need. This study characterizes the microbial communities associated with the moon jellyfish Aurelia coerulea, first isolated from the East Pacific Ocean and has since been utilized as an experimental system. We find that the microbiome of this Pacific Aurelia culture is dominated by two taxa, a Mollicutes and Rickettsiales. The microbiome is stable across life stages, although composition varies. Mining the host sequencing data, we assembled the bacterial metagenome-assembled genomes (MAGs). The bacterial MAGs are highly reduced, and predict a high metabolic dependence on the host. Analysis using multiple metrics suggest that both bacteria are likely new species. We therefore propose the names Ca. Mariplasma lunae (Mollicutes) and Ca. Marinirickettsia aquamalans (Rickettsiales). Finally, comparison with studies of Aurelia from other geographical populations suggests the association with Ca. Mariplasma lunae occurs in Aurelia from multiple geographical locations. The low-diversity microbiome of Aurelia provides a relatively simple system to study host-microbe interactions.微生物组（microbiome）对动物生理的影响已得到广泛认可，但海洋环境中动物-微生物共生关系的解析仍有待进一步深入研究。本研究针对与海月水母（Aurelia coerulea）共生的微生物群落展开解析，该菌株最初分离自东太平洋，后续被用作实验模型体系。研究发现，该太平洋海月水母培养体系的微生物组以两个类群为主，分别隶属于柔膜菌纲（Mollicutes）与立克次体目（Rickettsiales）。尽管其群落组成存在差异，但该微生物组在海月水母的各个生活阶段均保持稳定。通过挖掘宿主测序数据，我们组装获得了细菌宏基因组组装基因组（metagenome-assembled genomes, MAGs）。这些细菌MAGs的基因组规模高度缩减，推测其在代谢层面高度依赖宿主。多维度指标分析结果显示，这两种细菌大概率均为新物种。据此，我们提议将这两种细菌分别命名为Ca. Mariplasma lunae（柔膜菌纲）与Ca. Marinirickettsia aquamalans（立克次体目）。最后，通过对比其他地理种群海月水母的相关研究，我们发现Ca. Mariplasma lunae与海月水母的共生关系在多个地理分布的海月水母种群中均存在。海月水母的低多样性微生物组为宿主-微生物互作研究提供了一个相对简洁的实验模型体系。