Comparison of life cycles and morphology of Cyanea nozakii and other scyphozoans

Abstract

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).