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The upside-down jellyfish holobiont,Cassiopea xamachana, is a useful model system for tri-partite interactions between the cnidarian host, the photosymbiont, and the bacterial microbiome. While the interaction between the host and photosymbiont has been well studied, less is understood of the associated bacterial community. To date, the bacterial microbiome of wildC. xamachanahas remained largely uncharacterized. Thus, wild medusae (n=6) and larvae (n=3) were collected from two sites in the Florida Keys. Bacterial community composition was characterized via amplicon sequencing of the 16S rRNA gene V4 region. The medusa bacterial community was dominated by members of the Alphaproteobacteria and Gammaproteobacteria, while Planctomycetota, Actinomycetota, Bacteroidota, and Bacillota were also present, among others. Community composition was consistent between locations and across medusa structures (oral arm, bell, and gonad). The larval bacterial community clustered apart from the medusa community in beta diversity analysis and was characterized by the presence of several Pseudomonadota taxa that were not present in the medusa, including theAlteromonas,Pseudoalteromonas, andThalassobiusgenera. A bacterial isolate library encompassing much of the amplicon sequencing diversity was also developed and tested via metabolic assays in a separate culture-dependent analysis of isolates from medusa bells, oral arms, and laplets. Most characteristics were not correlated with host sex or medusa structure, but gelatinase production was more common in laplet isolates, while lactose fermentation was more common in female oral arm isolates. TheEndozoicomonasgenus was dominant in both amplicon sequencing and in our isolate library, and was equally prevalent across all medusa structures and in both sexes. Understanding the bacterial component of theC. xamachanaholobiont will allow us to further develop this important model cnidarian holobiont. 

This study was designed to investigate the impact of heat stress on the physiological changes and mortality rates of different life stages of the rhizostome jellyfish species Cassiopea xamachana, including planula larvae, scyphistomae (polyps), and medusae. Both larval and scyphistoma stages of C. xamachana are relatively tolerant to high temperatures, but both experience nearly 100% mortality at 36 °C. Increasing temperatures also induced stage-specific effects. Settlement rates of artificially induced larvae were near 100% at lower temperatures but decreased at 34–36 °C; larvae were dead at 36 °C. When scyphistomae of C. xamachana were subjected to a gradual increase in temperature from 28 to 38 °C, polyp size declined steadily in starved animals, with animals showing clear signs of temperature stress between 35 and 36 °C. Small medusae of C. xamachana pulsed more than larger medusae and tended to have peak pulse rates at higher temperatures (~35 °C) compared to larger medusae (~29–33 °C), though the latter was not significant. At a temperature of 39 °C, all the medusae exhibited signs of heat stress, including pulsing erratically (generally lower) rather than steady rhythmic pulsations, releasing copious amounts of mucus, and having withdrawn oral arms. Temperature data presented here, and in the literature, show that pulsing C. xamachana medusae exhibit a bell-shaped curve, with temperatures over 38 °C being detrimental and becoming lethal at 40 °C. Based on the findings of this study, it is proposed that the medusa stage of C. xamachana has a higher tolerance for elevated temperatures compared to both the larvae and the polyps. Predictions of global climate change indicate that populations of C. xamachana will likely face longer and hotter summer periods, leading to increased population sizes. However, higher temperatures pose a greater risk to the survival of the species as they increase mortality in the polyp and larval stages compared to the medusa stage.