Strobilation

Abstract: Large population fluctuations of jellyfish occur over a variety of temporal scales, from weekly to seasonal, inter-annual and even decadal, with some regions of the world reported to be experiencing persistent seasonal bloom events. Recent jellyfish research has focussed on understanding the causes and consequences of these population changes, with the vast majority of studies considering the effect of changing environmental variables only on the pelagic medusa. But many of the bloom-forming species are members of the Scyphozoa with complex metagenic life cycles consisting of a sexually reproducing pelagic medusa and asexually reproducing benthic polyp. Recruitment success during the juvenile (planula, polyp and ephyrae) stages of the life cycle can have a major effect on the abundance of the adult (medusa) population, but until very recently, little was known about the ecology of the polyp or scyphistoma phase of the scyphozoan life cycle. The aim of this review is to synthesise the current state of knowledge of polyp ecology by examining (1) the recruitment and metamorphosis of planulae larvae into polyps, (2) survival and longevity of polyps, (3) expansion of polyp populations via asexual propagation and (4) strobilation and recruitment of ephyrae (juvenile medusae). Where possible, comparisons are made with the life histories of other bentho-pelagic marine invertebrates so that further inferences can be made. Differences between tropical and temperate species are highlighted and related to climate change, and populations of the same species (in particular Aurelia aurita) inhabiting different habitats within its geographic range are compared. The roles that polyps play in ensuring the long-term survival of jellyfish populations as well as in the formation of bloom populations are considered, and recommendations for future research are presented.

Abstract: 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 jel- lyfish (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°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°C). Another experiment tested polyps in treat- ments of 10, 15, and 20°C. Survival of the initial polyps in all treatments was high (83 to 100%). Tem- perature, salinity, and their combination dramatically affected the numbers of ephyrae produced (from nearly 0 at 7°C to 42 ephyrae polyp -1 at 15°C), the percentages of ephyrae out of total asexual reproduction (≤12% at 7°C to 89% at 20°C), and the delay before ephyra production (> 81 d at 7°C but only 39 to 46 d at 15°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 sug- gest that the light-sensitive hormone melatonin, or a precursor like serotonin, coordinates the timing of strobilation in A. labiata with the seasonal light cycle.

Abstract: Outbreaks of jellyfish are reported worldwide, yet the environmental factors that control the sizes of jellyfish populations are not well understood. The scyphomedusan Chrysaora quinquecirrha occurs in the mesohaline portion of Chesapeake Bay each summer. Population sizes of the medusae show dramatic annual variations that are correlated with salinity and temperature. We measured the total numbers of ephyrae and polyps produced by benthic polyps of C. quinquecirrha in laboratory experiments lasting 42 d, and found that temperature (15, 20, 25°C) was not a statistically significant factor at low salinities (5 to 20‰); however, ephyra production increased significantly with increasing temperature at high salinities (20 to 35‰). Conversely, each 5°C decrease in temperature delayed strobilation (ephyra production) by about 1 wk. Salinity significantly affected the numbers of ephyrae and polyps produced in all experiments. Ephyra and polyp production was lower at both low (<11‰) and high salinities (≥25‰) than at intermediate salinities. Also, more ephyrae, but not polyps, were produced with more available prey. Medusa numbers were 2 orders of magnitude lower in July 1996 when water temperatures, salinities, and zooplankton densities in Chesapeake Bay all were lower than in July 1995. The effects of these factors are important in understanding the changes caused by human activities in near-shore ecosystems, including effects of global warming, eutrophication, and reduction of commercial species.