Reproduction of the venomous "blue bottle" jellyfish partly clarified by Tokai University

A joint research team comprising Junior Associate Professor Kohei Oguchi of the Department of Biology, Undergraduate School of Biological Sciences at Tokai University, Technical Specialist Hisanori Kohtsuka, Misaki Marine Biological Station, Graduate School of Science at the University of Tokyo, Aquarium Keeper Gaku Yamamoto of Enoshima Aquarium, and Professor Casey W. Dunn of Yale University in the United States, has revealed that germ cells in the gonodendra released by the venomous blue bottle jellyfish are immature at the time of release. The results were published in Scientific Reports.

The blue bottle is widely distributed throughout the world, is highly venomous, and is known to sting humans at beaches and other locations. It comprises species of the genus Physalia of the order Siphonophorae of the class Hydrozoa of the phylum Cnidaria, which are members of the jellyfish. The blue bottle individuals drifted by strong southerly winds are every summer and fall washed up on sandy beaches and coastal areas of Kanagawa and other prefectures. It has long been known as a dangerous creature, but it has remained largely unknown how and when its reproduction occurs due to various obstacles, such as extreme difficulty in long-term rearing and no cases of finding sperm or eggs worldwide.

The research team focused on the gonodendron, which has been considered a reproductive organ, to elucidate the reproductive process of the blue bottle. Within the gonodendron, there is a structure called the gonophore, which has been considered to produce gametes such as eggs and sperm. However, no definite gametes or germ cells have been found previously. Therefore, the research team performed the morphological and histological observation on the gonodendra and the gonophores within them.

The results confirmed that the gonophore had a complex structure consisting of three layers: epithelial cell layer, germ cell layer, and gastrodermic cell layer. However, no definite gametes (eggs or sperm) could be identified in the gonophores observed microscopically. In addition, nuclear phase diagnosis through flow cytometry revealed that haploid cells, indicative of gametes, were not detected in the gonophore and that all cells were diploids.

This result indicates that the germ cells have not reached maturity when the gonodendron was detached from the main body. Gene expression analysis for germ cell markers and meiosis I-related genes (Dmc1, Mnd1, Msh4/5, and Sycp1) to determine the stage of germ cell maturation showed that these genes were highly expressed in the gonophores. This result strongly suggests that the germ cells are in the early stages of meiosis when the gonodendron is detached from the main body and may undergo maturation after release.

The blue bottle has often received attention because it causes human damage. Its fragile structure and difficulty in long-term rearing have been obstacles in elucidating the development and ecology of the blue bottle, which remain largely unknown. Moving forward, the research group hopes to determine when and where the gonodendra are detached from the main body and how the detached gonodendra completes the maturation process and releases gametes. The results of this study are expected to spark similar research worldwide, contributing to the elucidation of the whole life cycle of the blue bottle.

Journal Information
Publication: Scientific Reports
Title: Physalia gonodendra are not yet sexually mature when released
DOI: 10.1038/s41598-024-73611-5