2.1.6. Pelagiidae

Within the Pelagiidae family, jellyfish from genus *Chrysaora* have been most extensively studied, while only one study on the microbiota of *Pelagia noctulica* was published, focusing on a single specific bacterium, the fish pathogen *Tenacibaculum maritimum* (see Section 3.6 and Reference [33]). We did not find any studies on jellyfish from the genera *Mawia* and *Sanderia*.

The microbiome of *Chrysaora* was analyzed by Lee et al. [40], focusing on the benthic life stages of *Chrysaora plocamia*. In particular, forms involved in propagation through cys<sup>t</sup> formation, i.e., the mother polyp, its dormant cysts (podocysts), and polyps recently excysted from podocysts, were studied. The microbial community was analyzed using Illumina MiSeq sequencing of the V1–V2 region of the 16S rRNA gene. *Chrysaora plocamia* was collected along the Chilean–Peruvian Humboldt Current System (along the coast of Northern Chile) at different sampling times, accounting for temporal/seasonal variability. Polyps were grown in the lab from planulae subtracted from oral arms of collected jellyfish and produced podocysts, often followed by strobilation. No comparison with the bacterial community of the ambient seawater community was made, however, which would have allowed us to understand the degree of specialization of jellyfish-associated microbiome.

At a broader taxonomic level, the microbiomes of all life stages of *C. plocamia* were dominated by *Gammaproteobacteria* (mainly *Alteromonadales, Legionellales*, and *Methylococcales*), followed by *Alphaproteobacteria* (mostly *Rhizobiales*), *Bacteroidetes* (*Flavobacteriaceae*), and *Planctomycetes* [40]. Only *Betaproteobacteria* were significantly more abundant in the polyps of *Chrysaora* as compared

to isolated podocyst. The microbiome of *Chrysaora* clustered according to the life stage rather than by sampling location and time, with polyp bacterial communities being significantly different from their podocyst and excyst. Thus, Lee et al. [40] speculated that the polyp-specific microbiome might be essential for their sessile lifestyle and important for the initiation of later developmental stages. Furthermore, the microbiome of the polyps was more similar to the next developmental stage, the excyst, than to the podocyst, which, as Lee et al. [40] argued, demonstrates an early stage in the successive restructuring of microbial communities from podocysts to polyps. Bacteria that were more abundant in the cys<sup>t</sup> stages than in polyps included chemolithoautotrophs of the genera *Nitrospira, Nitrospina, Thiogranum*, and *Desulfovermiculus* and nitrogen fixers of the order *Rhizobiales*. It has been suggested that the podocyst capsule harbors a specific beneficial assemblage of microbes to sustain the viability of podocysts [40]. This specific assemblage consists of bacteria affiliated with *Bacteroidetes, Planctomycetes, Alpha-, Beta-*, and *Gammaproteobacteria* and *Chloroflexi*.

One of the most interesting results of Lee et al. [40] was that within the jellyfish-associated microbiome, half of the detected bacteria were closely related to microbial communities found associated with seaweed, sponges, and sea squirts, all of which interestingly have a benthic lifestyle. Bacteria also had closest relatives known to be drivers of major elemental cycles, including methanotrophs. Interestingly, a number of these bacteria were also present in the microbiome of *A. aurita* as determined by Weiland-Brauer et al. [35]. However, it is important to note that for the comparison, bacteria associated with benthic and not medusa stages from the *Aurelia* study should be considered. Whether or not this was taken into account is not clear. Furthermore, Lee et al. [40] speculated that these shared bacteria might play an essential role in the scyphozoan life cycle, especially since the two jellyfish species originate from different marine regions. However, the microbiome of *Chrysaora* exhibited higher diversity than *A. aurita*, potentially explained by the fact that some of the studied life stages of *Aurelia* originated from polyps grown in the laboratory, most likely in an environment with lower microbial diversity than under *in situ* conditions. However, we would like to stress that Lee et al. [40] used 1 μm prefiltered seawater as the medium for their laboratory experiments, which could have introduced ambient seawater bacteria and altered the diversity of the jellyfish-associated microbial community. Also, the studies used different sequencing platforms, sequencing depths, and primers to amplify bacterial 16S rRNA genes, hence comparisons between these two jellyfish species should be done with caution.

Another representative of the *Chrysaora* genus, *Chrysaora hysoscella*, collected in the German Bight was studied in Reference [39]. The pluses and minuses of this study are discussed above (see the paragraph on *Cyanea lamarckii*). For this jellyfish species no significant differences between the microbiomes associated with different body parts were found. The microbiome of the umbrella exhibited the highest variability and richness, while the gonads revealed the lowest richness of all body parts. The microbiomes associated with different life stages indicated a strong selective colonization process, with decreasing diversity from larvae to adult medusae. Unfortunately, the methodological approach used in this study did not provide any insight into the phylogenetic composition of the associated microbiome.
