Relationship between mass occurrence of scyphozoan jellyfish Aurelia aurita and hypoxia in bollom layer

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The continuous changes in the number of newly established polyps of Aurelia aurita on settling plates under natural conditions were obsreved in Tokyo Bay, Japan. A sharp decline in survivorship of newly settled polyps was observed within the first few days, however, survivorship of polyps settled in autumn increased by budding after two months. The number of discs in each strobila varied from 1 to 6, however, most of the strobilae formed single discs. The percentage rations of the total number of ephyrae to the initial number of polyps on settling plates were generally lower than 10%, but the highest ratio of ca. 600% was estimated for the polyps settled in autumn. It is considered that most of the liberated ephyrae originate from the polyps settled in autumn in Tokyo Bay. This study suggests that the occurrence of ripe medusae with planula larvae during autumn contributes to the success of settlement and growth of the polyp stage in Tokyo Bay.The decline of the survivorship of A.aurita polyps is caused by the space competetion or the feeding of the other sessile organisms such as Mytilus galloprovincialis. The abundance of M.galloprovincialis become high in summer, and little space is found for polyp settlement. However, there are no M.galloprovincialis in the bollom hypoxic layer, and some aggregations of A.aurita polyps was observed. Aurelia aurita polyps is tolerable even in in the water of low dissolved oxygen (ca.2ml/l), and finaly the ephyrae were liberated. The highest reproduction season of female medusae is summer, and it is suggested that planula larvae and polyps effectively utilize the bottom hypoxic layer to avoid the settlement of the other sessile organisms. It is also considered that the development of hypoxia is one of the factor to induce the mass occurrence of A.aurita medusae.