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

在日本东京湾，观察到在自然条件下，在沉淀板上新建立的极光水蚤数量的连续变化。在最初的几天内，新定居的息肉存活率急剧下降，然而，秋季定居的息肉在两个月后萌发，存活率增加。每个球果中的圆盘数目从1到6个不等，但大多数球茎形成单个圆盘。沉淀板上的肺泡总数与初始数量的比例一般都在10%以下，但秋季的比例最高，约为600%。据认为，大部分被释放的肺泡起源于秋季在东京湾定居的息肉。这项研究表明，秋季成熟水母和浮萍幼虫的出现有助于东京湾水母阶段的定居和生长。金鱼成活率的下降是由于空间竞争或其他固着生物如紫贻贝的取食造成的。夏季高氏沼虾丰度较高，息肉栖息地较小。然而，在Bollom缺氧层中没有发现高氏支原体，并观察到了一些金龟子息肉的聚集。即使在低溶解氧(约2ml/L)的水中也能耐受，并最终释放出肺泡。雌性水母的繁殖高峰期为夏季，这表明浮萍幼虫和水母能有效地利用底层缺氧层，以避免其他固着生物的定居。同时认为，低氧的发展是导致水母黑线姬蜂大量发生的因素之一。

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.