Physalia gonodendra are not yet sexually mature when released.

The blue bottle genus Physalia is one of the well-known siphonophore belonging to the Cnidaria, Hydrozoa. Physalia is also known as a ferocious predator, occasionally stinging and fatally wounding humans, but key details of its life cycle and reproductive biology are unclear. Physalia have separate sexes, and sexual reproduction occurs through the release of complex structures called gonodendra that contain many gonophores that will release either eggs or sperm. It is not known how mature the gonophores are when the gonodendra are released. In this study, we aim to characterize germ cell maturation by conducting histological, cytological, and gene expression analyses of the gonodendron of Physalia utriculus from Japan. We found a layered structure of the gonophore, consistent with other studies; however, gametes were not found even in gonophores that were within the released gonodendra. Moreover, haploid cells were not detected by flow cytometry. Analysis of the expression of putative germ cell marker and meiosis related genes showed high expression in the gonophore. These results strongly suggest that germ cells do not mature until after gonodendra are released. These findings provide valuable insights into the reproductive ecology and life cycle of Physalia.