The oocyte is a cell of unique importance for all sexual reproducing animals, providing a haploid ge-nome, all the cellular constituents and the ground plan for embryonic development. Tight regulation of cytoplasmic, cytoskeletal and nuclear events during successive steps of oogenesis is thus essen-tial to avoid both infertility and defects in embryo development. Much of this regulation remains un-derstudied, with a significant limiting factor being experimental access to oocytes developing within the female gonad. An exciting route to overcome such limitations is currently being opened by alter-native model organisms, notably ones selected from marine environments, offering a huge range of natural diversity. Marine organisms are typically well suited for imaging approaches, however particu-lar constraints relating to their cool, salt-water environment currently restrict the application of ad-vanced imaging technologies. Our interdisciplinary project aims to overcome these constraints by developing an innovative, tailored light sheet imaging system. We will use this to exploit a jellyfish model, Clytia hemisphaerica, that has high but as yet untapped potential for experimental manipulation and live imaging of oogenesis with-in the completely transparent animal or in autonomously functioning isolated ovaries. Our research plan includes two technical innovation work-packages and two that address oocyte animal-vegetal polarity establishment, a key process that presages the larval body plan: In WP1 we will develop adaptable custom light sheet microscopes with integrated image acquisition and analysis workflows tailored for marine organisms. In WP2 we will build a suite of molecular tools for expression of tagged proteins and mRNAs during Clytia oogenesis, including transgenic lines. In WP3 we will use these tools in live Clytia ovaries to visualise and dissect functionally how the microtubule cytoskeleton establishes animal-vegetal polarity during the growth phase of oogenesis, focussing on the reposi-tioning of the nucleus and localisation of Fz1 mRNA, one of three key Wnt-pathway axis determi-nants. In WP4 we will address the mechanisms driving segregation of the two other determinant mRNAs, Fz3 and Wnt3, to opposite cortical domains during oocyte meiotic maturation. For WPs 3 and 4 we will image the cytoskeleton, mRNAs and other key cellular components live in 3D with high spa-tial and temporal resolution through oocyte growth and maturation. We will combine these with mo-lecular perturbations using inhibitors, morpholinos and genetic tools, as well as physical micromanipu-lations. Our findings will reveal key conserved mechanisms and also illuminate their evolutionary history. More widely, this project will make a significant contribution to oogenesis research by establishing Clytia as a powerful model system, and provide tailored light sheet microscopes to exploit the potential of ma-rine model organisms for mechanistic studies in biological research.