Causative factors and conditions of the population explosion of the moon jelly Aurelia aurita in Tokyo Bay

通过野外调查和实验室实验，研究了东京湾上半部水母种群发生的影响因素和条件。在野外，从1993年4月起连续三年，每月进行一到两次水母取样和水文条件测量。测定了水母大小和扁平、叶斑发生的季节变化。基于这些信息，在每年的几个月里，在不同的温度和食物条件下进行了浮萍和水蚤的饲养试验。结果揭示了球茎的形成过程和通过萌发进行的有性繁殖。研究结果还表明，温度和摄食条件对幼体的生长和解体时间有很大的影响，温度降低后，幼虫的芽数随着摄食时间的增加而增加。前一年秋季前落户的息肉在1月份释放肺泡，而秋季落户的在2~4月发生肺泡，在息肉初期适宜的较高温度和食物密度下，息肉生长迅速，数量增加。当温度降低到15ﾟC或更低时，就会产生频闪现象。如果降温后食物条件仍然较好，则从大量的息肉中释放出大尺寸的叶状体。在这些特殊的条件下，金龟子种群数量的激增可能是因为它具有很高的繁殖潜力。

Factors and conditions to relate mass occurrence of the moon jelly Aurelia aurita in the upper part of Tokyo Bay have been examined through field survey and laboratory experiment. In the field sampling of the jelly and measurement of hydrographic conditions were carried out once or twice a month during three consecutive years from April 1993. Seasonal variation of the size of medusae and occurrence of planulae and ephyrae were determined. Based on these information, rearing experiments of planulae and polyps were carried out under different temperature and food regimes during a few months in each year. As the result, processes of formation of the strobilation and a sexual reproduction by budding were revealed. Other findings included the size of polyps to commence strobilation, duration of strobilation, production rate of strobilae, and number and size of ephyrae.Growth of polyps and timing of the ephyla's liberation are strongly influenced with temperature and feeding condition.Number of buds of polyps increase with increasing feeding duration after the temperature lowered. The polyps settled before autumn of the previous year liberate ephyrae in January, whereas those settled in autumn produce ephyrae in February to April.With favorable higher temperature and food density during the early polyp stage, the polyps growth and increase the number rapidly. The strobilation is induced when the temperature lowered to 15ﾟC or less. If food condition is still preferable after the lowering temperature, large size ephylae are liberated from large numbers of polyps. The population explosion of A.aurita could be attributed to its high reproductive potential under these special conditions.