Tokyo Bay is one of the most eutrophicated bays in Japan, and mass occurrences of scyphozoan jellyfish Aurelia aurita have often been observed with progress of eutrophication. In the coastal environment, the abundance of substrates suitable for A. aurita polyps are obviously increasing by reclamation. However, most suitable substrates for settling, except in the bottom-layer in the innermost part of Tokyo Bay, are occupied by other sessile organisms such as mussels, barnacles or ascidians. The bottom layer is characterized by low DO concentrations (≦2.0 ml O₂ L⁻¹) that turn to hypoxia during summer, resulting in low recruitment and low growth of other sessile organisms. To elucidate the mechanism by which mass occurrence of jellyfish occurs, adaptative behavior to hypoxia in planula larvae and the polyp stage was experimentally studied. In the present study, planula settlement was promoted by a decrease in DO concentrations, and apparent growth of polyps and production of daughter polyps by asexual budding were also observed, even in the polyps cultured in hypoxic waters. Moreover A. aurita polyps cultured under hypoxic conditions began strobilation and ephyrae were liberated after the DO concentration levels were restored. It is suggested that the occurrence of hypoxic waters is rather favorable for the survival, growth and asexual reproduction of A. aurita polyps, and it can directly contribute to the consequent production of A. aurita ephyrae in the next spring within Tokyo Bay.

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The moon jellyfish Aurelia aurita has increased in abundance in coastal waters around Japan during recent decades. Since the moon jellyfish is highly tolerant of low dissolved oxygen concentrations, predation impacts by moon jellyfish on zooplankton can increase during summer hypoxia in coastal waters, which is often caused by anthropogenic effects such as an increase in nutritional loading from the land. Laboratory experiments were conducted in order to test the hypothesis that summer hypoxia leads to qualitative changes in predator-prey interactions between moon jellyfish and larval fish. Larvae of a common coastal fish, red sea bream Pagrus major (2.9, 4.1, 6.2 and 8.6 mm in standard length), were used for the experiments. Predation rates (% of larvae preyed on by a moon jellyfish per 10 min.) were examined at four oxygen concentrations (1, 2, 4 and 5.8 mg L⁻¹) in 10 L tanks (4 replicates). Size-selective predation was observed at the two highest oxygen concentrations (4 and 5.8 mg L⁻¹): about half of the 6.2 and 8.6 mm larvae survived the 10 min. trials while more than 85% of the 2.9 and 4.1 mm larvae were captured. Larval body size did not affect the predation rates at the two lowest oxygen concentrations (1 and 2 mg L⁻¹): more than 90% of larvae in all size classes were caught. These results indicate that trophic flow from ichthyoplankton to moon jellyfish increases during summer hypoxia in coastal waters and a qualitative change in predator-prey interaction, i.e., shift from size-selective to non-size-selective predation occurs at oxygen concentrations <2 mg L⁻¹.

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Mass aggregations of the giant jellyfish Cyanea nozakii have occurred in China since the end of the 20th century. In particular, C. nozakii bloomed abnormally in Liaodong Bay during July and August of 2004 and the catch of edible Rhopilema esculentum was greatly reduced. In order to clarify the causes leading to mass occurrences of C. nozakii, the reproductive cycle and morphological characters of this species were compared with other common jellyfish species using data from both laboratory–rearing experiments and field investigations. The results showed that, in particular, settling of planulae and asexual reproductive strategies of scyphystomae made C. nozakii more capable of thriving under unfavorable physical conditions than R. esculentum. The ephyrae of Nemopilema nomurai, R. esculentum, C. nozakii, Aurelia aurita, Rhopilema hispidum and Rhopilema asamushi were differentiated by different shapes of lappets, rhopalar clefts, gastric filaments and nematocyst batteries. In addition, the life cycle of C. nozakii was compared with earlier reports on the life cycle of Cyanea capillata, R. esculentum, Rhizostoma pulmo, Stomolophus meleagris and N. nomurai, and the morphology of adult C. nozakii was compared and contrasted with species of the genus Cyanea (C. capillata, C. ferruginea and C. purpurea).

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The rhizostome jellyfish Nemopilema nomurai, which is endemic to the East Asian Marginal Seas (i.e. the Bohai, Yellow, East China and Japan Seas), is unique both by its enormous body size (ca. 2 m maximum bell diameter and 200 kg wet weight) and propensity for occasional population explosionss. Massive blooms of this species have historically been reported only once per ca. 40 years (i.e. in 1920, 1958 and 1995), but have become increasingly frequent recently (i.e. in 2002, 2003, 2005 and 2006). Both accumulated knowledge on the spatiotemporal distributions and physical modeling of the water circulation show that the medusae are released from benthic polyps during April–June in the Yellow Sea and East China Sea, and transported by the Tsushima Current to the Japan Sea. The bloom in 2005 might be the largest ever in history; as many as 3–5×10⁸ medusae passed through the Tsushima Strait daily during the summer and there were more than 100,000 complaints from commercial fishermen. The recent blooms of N. nomurai may have been caused by environmental changes, such as increased water temperature, eutrophication, coastal modification, and over-fishing in Chinese coastal waters. Frequent jellyfish blooms can apparently be a threat to the fisheries sustainability of the East Asian Marginal Seas, one of the world's most productive fisheries grounds.

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