Environmental effects on asexual reproduction rates of the scyphozoan Aurelia labiata

Problem outbreaks of jellyfish and warming of the Earth's climate are both being reported at unprecedented rates. Models forecast continued changes in temperature, salinity, and solar radiation (insolation) in the world's oceans as consequences of global warming. Many species with a swimming jellyfish stage also have a benthic stage that asexually produces buds and new jellyfish (ephyrae). This perennial benthic stage probably determines the numbers of jellyfish in the population. In this study, polyps of the moon jellyfish Aurelia labiata from Puget Sound, Washington, USA, were tested in 9 combinations of temperature (7, 10, 15 degrees C) and salinity (20, 27, 34) in the dark, and in 9 combinations of photoperiod (12, 8, and 4 h d(-1)) and light intensity (1 screen, 2 screens, opaque) at ambient salinity (27) and temperature (15 degrees C). Another experiment tested polyps in treatments of 10, 15, and 20 degrees C. Survival of the initial polyps in all treatments was high (83 to 100%). Temperature, salinity, and their combination dramatically affected the numbers of ephyrae produced (from nearly 0 at 7 degrees C to 42 ephyrae polyp(-1) at 15 degrees C), the percentages of ephyrae out of total asexual reproduction (<= 12 % at 7 degrees C to 89% at 20 degrees C), and the delay before ephyra production (>81 d at 7 degrees C but only 39 to 46 d at 15 degrees C). Thus, all results showed that more jellyfish were produced with increasing temperature. Long photoperiod and highest light intensity greatly accelerated strobilation, with polyps in 12 h light strobilating 30 to 40 d before those in other treatments. Polyps receiving the most light strobilated most frequently. In situ conditions showed that light increased much more rapidly than temperature before strobilisation, suggesting that light may be the more important signal. I suggest that the light-sensitive hormone melatonin, or a precursor like serotonin, coordinates the timing of strobilation in A. labiata with the seasonal light cycle.