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.

Nausithoe aurea Silveira & Morandini, 1997 é uma espécie metagenética e dióica com fecundação externa. Os oócitos são liberados continuamente (55 dias em laboratório), porém com grandes variações no número a cada dia. No desenvolvimento embrionário a clivagem, após o estágio de 8 células, passa de holoblástica e igual para pseudoespiral. A gastrulação ocorre por ingressão multipolar e inicia-se aproximadamente 24 horas após a fecundação. A estrutura histológica geral das gônadas assemelha-se a outros Scyphozoa, onde os gonócitos proliferam a partir da gastroderme, migram e diferenciam-se na mesogléia. Na gônada masculina as células germinativas formam camadas razoavelmente distintas e constituem folículos testiculares. Na gônada feminina os oócitos surgem da zona germinativa na gastroderme e apresentam um gradiente de maturação a partir deste ponto (cortes no sentido oral-aboral). Os oócitos encontram-se livres na mesogléia da gônada, sem associação com outras células. A relação espacial entre a musculatura circular, as gônadas e o sulco coronal, é uma característica a ser usada na sistemática do gênero Nausithoe Kölliker, 1853. / Nausithoe aurea Silveira & Morandini, 1997 is a metagenetic and dioecious species with external fertilization. The oocytes are released continuously (55 days in laboratory), but with great variations in the daily number. In the embryonic development the cleavage, after the 8 cells stage, changes from holoblastic and adequal to pseudospiral. The gastrulation occurs through multipolar ingression and begin 24 hours after fertilization. The general histological structure of the gonads resembles other Scyphozoa, in which the gonocytes proliferate from the gastrodermis, migrate and differentiate in the mesoglea. In the male gonad the germ cells are arranged in distinctive layers and form follicles (cysts). In the female gonad the oocytes develop from the germinative zone in the gastrodermis and present a maturing gradient from this point on (oral-aboral sections). The oocytes are free in the gonad mesoglea, without association to any cell. The spatial relation of the coronal musculature, gonads and coronal groove, is a character to be used in the systematics of the genus Nausithoe Kölliker, 1853.

Nausithoe aurea Silveira & Morandini, 1997 é uma espécie metagenética e dióica com fecundação externa. Os oócitos são liberados continuamente (55 dias em laboratório), porém com grandes variações no número a cada dia. No desenvolvimento embrionário a clivagem, após o estágio de 8 células, passa de holoblástica e igual para pseudoespiral. A gastrulação ocorre por ingressão multipolar e inicia-se aproximadamente 24 horas após a fecundação. A estrutura histológica geral das gônadas assemelha-se a outros Scyphozoa, onde os gonócitos proliferam a partir da gastroderme, migram e diferenciam-se na mesogléia. Na gônada masculina as células germinativas formam camadas razoavelmente distintas e constituem folículos testiculares. Na gônada feminina os oócitos surgem da zona germinativa na gastroderme e apresentam um gradiente de maturação a partir deste ponto (cortes no sentido oral-aboral). Os oócitos encontram-se livres na mesogléia da gônada, sem associação com outras células. A relação espacial entre a musculatura circular, as gônadas e o sulco coronal, é uma característica a ser usada na sistemática do gênero Nausithoe Kölliker, 1853. / Nausithoe aurea Silveira & Morandini, 1997 is a metagenetic and dioecious species with external fertilization. The oocytes are released continuously (55 days in laboratory), but with great variations in the daily number. In the embryonic development the cleavage, after the 8 cells stage, changes from holoblastic and adequal to pseudospiral. The gastrulation occurs through multipolar ingression and begin 24 hours after fertilization. The general histological structure of the gonads resembles other Scyphozoa, in which the gonocytes proliferate from the gastrodermis, migrate and differentiate in the mesoglea. In the male gonad the germ cells are arranged in distinctive layers and form follicles (cysts). In the female gonad the oocytes develop from the germinative zone in the gastrodermis and present a maturing gradient from this point on (oral-aboral sections). The oocytes are free in the gonad mesoglea, without association to any cell. The spatial relation of the coronal musculature, gonads and coronal groove, is a character to be used in the systematics of the genus Nausithoe Kölliker, 1853.