A Comparative Study of Asexual Reproduction in Three Jellyfish, Aurelia aurita s.1., Chrysaora melanaster and Cyanea nozakii (Cnidaria: Scyphozoa: Semaeostomae), with Special Reference to the Role of Podocysts

The large gelatinous zooplankton such as cnidarian medusae and ctenophores have increased their abundance and caused problematic blooms in many coastal waters worldwide.In particular, in east Asian seas around Japan, China and Korea, where environmental conditions have been increasingly deteriorated by human impact, several cnidarian jellyfish species have become frequently blooming to cause severe damage to fisheries and other human economic sectors.In the light of increased jellyfish population outbreaks or blooms, it is necessary to understand the mechanisms to cause such phenomena.Because it is widely recognized that the asexual reproduction during benthic polyp stage may play a very important role to determine the medusa abundance in the following season, studies dealing with polyps have increased in recent years.However, few studies have been made on the role of podocysts, one of asexual reproduction modes of many semaeostome and rhizostome jellyfish species.The podocyts are a cuticle-covered-cell-mass formed by polyps and they are capable of dormancy until excystment into new active polyps.Therefore, in this study, I aimed to reveal the ecological role of podocysts in three semaeostome jellyfish species, viz.Aurelia aurita s.l., Chrysaora melanaster and Cyanea nozakii, which are frequently blooming not only in Japanese coastal waters but also in Chinese and Korean waters, by conducting mainly laboratory experiments to determine the effects of different environmental factors (i.e.temperature, salinity, dissolved oxygen concentration and food supply) on their encystment, dormancy and excystment in addition to some histological studies.In A. aurita, there are several modes of asexual reproduction by polyps, viz.budding and longitudinal fission, in addition to podocyst formation.The podocysts were never formed by well-fed polyps but only formed by un-fed and poorly-fed polyps (food regime: ≤4.8 µg C polyp - Contents Chapter 1.General introduction Global expansion of jellyfish blooms ……………………………………………...1 Causes of jellyfish blooms………………………………………………………….4Life cycle of scyphozoan jellyfish………………………………………………… 5 Brief overview of podocyst studies………………………………………………...6 Ecological significance of podocysts ………………………………………………8 Objectives of this study……………………………………………………………..8 Chapter 2. Asexual reproduction of the moon jellyfish Aurelia aurita s.l. with special reference to the role of podocysts Introduction………………………………………………………………………....10 Materials and methods………………………………………………………………12 Results………………………………………………………………………………17 Discussion…………………………………………………………………………...22 Chapter 3. Asexual reproduction of the Japanese sea nettle Chrysaora melanaster with special reference to the role of podocysts Introduction………………………………………………………………………….26 Materials and methods……………………………………………………………….29 Results……………………………………………………………………………….32 Discussion…………………………………………………………………………...37 Chapter 4. Asexual reproduction of Cyanea nozakii with special reference to the role of podocysts Introduction………………………………………………………………………….41 Materials and methods……………………………………………………………….43 Results………………………………………………………………………………..45 Discussion……………………………………………………………………………48 Chapter 5. General discussion………………………………………………………….52 Acknowledgements…………………………………………………………………….. 58