On the biology of Aurelia aurlta (L.): 2. Major factors regulating the occurrence of ephyrae and young medusae in the Gullmar Fjord, western Sweden

The cosmopolitan scyphomedusa Aurelia aurita is the most common jellyfish in the coastal waters of Sweden. The effects on the pelagic ecosystem caused by predation, in connection with mass occurrences of Aurelia, are believed to be considerable. However, the causes behind the great variations in Aurelia abundance are not fully understood. In this paper, the results from a study focusing on the mechanisms regulating the occurrence of Aurelia are presented. The settling of planulae, strobilation of scyphistomae and release of ephyrae have been followed on ceramic settling plates which were brought to the laboratory for analysis at regular intervals. Weekly to bi-weekly hauls with Bongo nets have been used to study the abundance of ephyrae and young medusae in both surface and deep water layers. The results have then been correlated with the local hydrography and irradiation. The results from 1983–1984 confirm the findings of 1982–1983, that the main period of ephyrae release is in October–November and not during the spring as was previously thought. The results also confirm our hypothesis that the ephyrae rapidly descend towards deeper water layers and overwinter in a state similar to diapause. Following some trigger mechanism, the ephyrae and young medusae ascend towards the surface layers in April and from then on grow at a very rapid rate. The time of ephyrae release, the appearance of young medusae and rapid growth was identical in the 2 years of investigation. However, the extent of ephyrae release differed by two orders of magnitude.