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

在日本东京湾，观察了自然条件下沉降板上auria aurita新形成珊瑚虫数量的连续变化。新定居的水螅体的存活率在最初几天内急剧下降，然而，秋天定居的水螅体的存活率在两个月后出芽而增加。每颗球果的球果数从1个到6个不等，但多数球果为单球果。在沉淀盘中，苔藓虫总数占初始息肉数的比例普遍低于10%，但在秋季沉淀的息肉中，这一比例最高，约为600%。据认为，大部分被释放的前肢来源于秋天在东京湾定居的珊瑚虫。该研究表明，秋季成熟水母和浮藻幼虫的出现有助于东京湾水螅期的成功定居和生长。水螅的存活率下降是由于空间竞争或其他无底生物（如Mytilus galloprovincialis）的摄食造成的。在夏季，加洛省密螺旋体的丰度较高，水螅体的栖息空间很小。然而，在球孢缺氧层中未发现加洛省m.g ulloprovincialis，并观察到一些耳蚜息肉聚集。即使在溶解氧较低的水中（约0.2 ml/l），耳纹水母息肉也能耐受，并最终释放出卟啉。雌性水母的最高繁殖季节是夏季，说明浮藻幼虫和水螅体有效地利用了底部的缺氧层，避免了其他无根生物的定居。同时认为缺氧的发展是诱导水母群发生的因素之一。

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.