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A population explosion of the giant jellyfish Nemopilema nomurai (Scyphozoa: Rhizo- stomeae) occurred in the Sea of Japan in 2002 and 2003, causing severe damage to fisheries. There was a further population explosion in 2004, but on a much smaller scale. In both years, young medusae began to appear in the Tsushima and Korea Strait in July and August, followed by a north- ward expatriation in the Tsushima Current to the northern Sea of Japan. We obtained scyphistomae of this species by artificial fertilization, and reared them to the young medusa stage in the laboratory. Asexual reproduction of scyphistomae occurred by means of podocyst formation. A thermal increase from 13 to 23°C induced strobilation, followed by liberation of 3 to 7 ephyrae from a strobila. The ephyrae developed to metephyrae having a complex canal system and characteristically possessing long reddish purple filiform appendages. By 40 d post-liberation, the metephyrae had grown to medusae in which the central mouth had closed and been replaced by numerous mouthlets on both oral wings and scapulets. The results from both laboratory-rearing experiments and field investiga- tions were combined to construct the seasonal life cycle and geographical distribution of N. nomurai in eastern Asian waters. Possible causes for the mass occurrence of N. nomurai, which has tended to be more frequent in the last decade, are discussed.

Unusual population explosion of the giant jellyfish Nemopilema nomurai (Scyphozoa: Rhizostomeae) in East Asian waters