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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. 

The regenerative capacity of Scyphozoans (Phylum Cnidaria) has been relatively understudied. The model organism Cas- siopea xamachana hosts photosynthetic dinoflagellate symbionts in the host’s motile amoebocyte cells. A handful of studies have reported regeneration in the polyps of C. xamachana, but the mechanisms underlying regeneration have not been fully explored. Despite undergoing drastic developmental changes when symbiotic, the effect of symbiont presence and species on host regeneration has never been explored. C. xamachana polyps were decapitated when aposymbiotic, and symbiotic with both a homologous and a heterologous symbiont species. Regeneration and asexual budding were observed, and EdU labeling was performed to observe patterns of cell proliferation in regenerating polyps. The presence of symbionts increased likelihood to regenerate, yet symbiont species did not affect success of regeneration. No blastema or dividing cells were observed, implying cell proliferation is not the primary mechanism behind regeneration in polyps of C. xamachana.