No abstract

During the last decade, an Indo-Pacific scyphomedusa, Rhopilema nomadica, penetrated the eastern Mediterranean. Annual blooms of the medusa near the Israeli coast were observed during the summer months of the years 1989 to 1992. R. nomadjca migrated northward from the Israeli coast towards Lebanon and Syria and has not been found elsewhere in the Mediterranean. Temperature dependence of the metamorphosis of R. nornadica polyps into strobila was studied in the laboratory. Rapid strobilation occurred between 18 and 20°C and a decline of strobilation was observed when the temperature was raised to 24 or 26OC. When water temperature was below 16"C, polyps did not strobilate, and below 13 "C, polyps did not feed, resulting in a gradual loss of polyp vitahty. Con~parison of the temperature regime in the Levant Basin and the dependence of polyp strobilat~on on temperature suggest that the spring rise in water temperature causes a rapid strobilation, whereas winter and summer water temperatures inhibit or diminish reproduction. Laboratory studies support the possibility that synchronization and annual occurrence of R. nomadica is controlled by seasonal changes in water temperature regimes, leading to rapid strobilation and release of ephyrae during springtime. The sensitivity of the polyps to low temperatures enables us to predict that the future dispersal of Rhopilerna nornadica may be limited to the eastern Mediterranean. This study emphasizes the importance of the effect of 1 abiotic factor on the physiology and the migration pattern of R. nomadica. This can be a good model for understanding the behavior and limiting factors of Indo-Pacific jellyfish migrating into non-tropical seas.