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In many regions of the world the abundance and distribution of jellyfish is increasing. These changes have important consequences for marine ecosystems and the services that they provide to humans. It is a fundamental goal of marine ecologists to develop understanding of how jellyfish populations respond to environmental change. Two key processes are involved in the regulation of jellyfish populations: local demographics (i.e. production and mortality) and dispersal (i.e. immigration and emigration). A failure to discriminate between the contributions of these two fundamental processes can obscure the specific environmental factors that drive changes in jellyfish populations, impeding understanding. This thesis aims (1) to assess recent changes in the abundance and distribution of Muggiaea atlantica in the coastal waters of the United Kingdom; and to (2) investigate the influence of environmental variability on both the biological (i.e. demography) and physical (i.e. dispersal) processes that drive these changes. This study utilised data collected from a network of coastal monitoring stations, together with data on a range of local and regional hydroclimatic environmental factors. I show that since the late 1960s, there has been a progressive northward extension of this species’ distribution from its centre of population in the Bay of Biscay and Iberian Coast region. These changes involved the establishment of a resident population in the Western English Channel (WEC) and the subsequent development of transient seasonal populations on the east and west coasts of Scotland. In the WEC the M. atlantica population was capable of overwintering, sustaining a resident population irrespective of immigration. This population was modulated primarily by the availability of suitable local environmental conditions that influenced demography (temperature and food availability) and secondarily by changes to water circulation patterns that influenced its dispersal. In contrast, in Scotland M. atlantica was incapable of overwintering and its population was primarily modulated by immigration and secondarily by the availability of suitable local environmental conditions. On the west coast of Scotland, the European Slope Current (ESC) was identified as the source of immigration, whereas on the east coast the mechanism of immigration was not identified. Despite the importance of the ESC as a transport mechanism from the WEC region, a direct source-sink relationship between the WEC and Scottish coastal waters was not clearly demonstrated, suggesting the importance of a network of sources. Key environmental requirements for local M. atlantica production were consistent between the localities, these were: a critical limit of 10 °C for reproduction, with an optimum of 13–15°C, together with the availability of small copepod prey. Changes in the timing of this critical temperate modulated trophic phasing with copepod prey, influencing the phenology of the blooms of M. atlantica. This thesis provides the basis for developing a mechanistic understanding of the factors that modulate distribution in a species of jellyfish. The principles and methods used could be applied to better understand changes in the abundance and distribution of M. atlantica in other areas, and also for different species of jellyfish.

This work set out to study the population dynamics of jellyfish in European seas, employing population genetics and plankton surveys on a diverse range of gelatinous zooplankton. The European population of P. noctiluca was found to ba genetically unstructured. Within-aggregation inbreeding was detected suggestive of a reproductive role of aggregations in this species. A vestigial divergence into two clades was detected from the mitochondrial data. Gene flow is sufficient that these two clades are not geographically structured, or supported by nuclear markers. The results of the study of R. octopus population genetics were indicative of recent population expansion, probably during post-glacial colonisation of the current distribution. Populations of R. octopus are somewhat separated both geographically and genetically, indicating low gene flow between populations and high site fidelity The population genetics of jellyfish species may be closely linked with their biology. Meroplanktonic, metagenic species such as R. octopus have a more constrained niche due to their benthic life stages, and also may persist as asexually reproducing benthic colonies between years. These species are likely to have differentiated or differentiating populations, even when the motility of the species suggests high gene flow potential. Plankton sampling Is an effective method to study ephyral release as a proxy for strobilation. In Aurelia aurlta, the current findings contrast with published literature, where the consensus Is that strobilation initiates with rising temperatures. This study records ephyrae nine weeks prior to, and 2°C greater than, the thermal minimum, Increasing temperatures cannot be solely responsible for triggering strobilation. The wider gelatinous zooplankton community in Strangford Lough was found to be diverse, 33 taxa were recorded over 38 weeks. A census of the gelatinous zooplankton population was produced. The study examines the contribution of gelatinous species’ biology and ecology on population dynamics.

Jellyfish (Phylum Cnidaria, Class Scyphozoa) play important roles in pelagic ecosystems as predators and prey. Seasonally they form blooms facilitating reproductive success, but that are at times problematic for human enterprise. Medusa abundance has been correlated with environmental variables in several instances. However, the direct mechanisms for changes in medusa abundance are unclear. As global sea surface temperatures continue to change there is increasing concern that warming may enhance conditions favourable for the generation of jellyfish medusae. It is important to understand the ways in which temperature affects all life history stages of jellyfish if we are to begin to understand factors associated with jellyfish bloom formations, but how temperature and salinity affects life history stages of scyphozoan jellyfish from British waters remains largely unknown. In Chapter 1 I provide a general introduction to some key issues important to the formation of jellyfish blooms. In Chapter 2 I present results for experiments testing the effects of temperature on settlement and metamorphosis of planulae larvae of Cyanea capillata, Cyanea lamarckii, Chyrsaora hysoscella, and Aurelia aurita. Chapter 3 reports on the effects of temperature and salinity on survival, and asexual reproduction of scyphistomae of the same species. Chapter 4 reports on the effects of temperature and salinity on growth of newly released ephyrae of each of the above mentioned species, as well as the effects of starvation on survivorship on ephyrae of A. aurita originating from two distinct populations of scyphistomae. In Chapter 5 I provide a brief summary of significant findings for each life history stage, their theoretical implications when taken together, and next steps for future research. I also offer recommendations for ecosystem managers with an eye toward affecting the numbers of near-shore jellyfish medusae generated each season in the waters surrounding the British Isles.

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