2.1.9. Hydrozoa

Among cnidarians, *Hydrozoa* is the only taxon with freshwater species and formation of colonies combining both polyp-zoids and medusa-zoids; however, one stage or the other, more frequently medusa, is reduced or absent. They are found in nearly every marine habitat type and their diversity is significantly higher than that of Scyphomedusae, the former comprising more than 3800 species. Chronologically, to our knowledge, the first report of bacteria associated with Cnidaria was published in 1978 by Margulis et al. [31], who reported a large number of rod-shaped Gram-negative bacteria with a single polar flagellum, found in gastrodermal cells of healthy freshwater green hydras *Hydra viridis*. Afterward, the microbiota associated with the freshwater *Hydra* was extensively studied (reviewed in Reference [19]). This review focuses on the marine hydromedusan species. Marine Hydrozoa *Phialella quadrata* (*Leptothecata*) and *Muggiaea atlantica* (*Siphonophorae*) were investigated by Ferguson et al. [32] and Fringuelli et al. [34] and recognized as a vector and potential natural reservoir of *Tenacibaculum maritimum*, a bacterial pathogen frequently found in farmed fish. *Tubularia indivisia* (*Anthothecata*) was investigated by Schuett and Doepke ([38]; see first paragraph on Cyaneidae for details of this study), showing that its tentacles are associated with potential pathogenic endobiotic bacteria, such as *Cobetia marina*, *Colwellia aestuarii*, *Endozoiciminas elysicola, Vibrio aestuarianus, Bacillus subtilis*, and *Ilyobacter psychrophilus*. Daley et al. [36] investigated the microbiome of *Nemopsis bachei* (Anthothecata) parallel to the *A. aurita* analysis (see paragraph on Ulmaridae for details of this study). Bacteria found to be associated with *N. bachei* were affiliated with *Gammaproteobacteria* (*Vibrionales, Oceanospirillales, Enterobacteriales*, and *Alteromonadales*), with *Alphaproteobacteria* (*Rickettsiales* and *Rhizobiales*), with *Flavobacteria* (e.g., *Tenacibaculum maritimum*), with *Cyanobacteria* (e.g., *Synechoccous*), and with *Firmicutes*.
