The current proposal seeks to evaluate the effects of highly unsaturated fatty acids (HUFA) on the development of vision in marine fish larvae. The study will consider the effect of natural diets with differing contents of HUFA on the development of the retinal tissue in larvae of the flatfishes Pleuronectes platessa and Paralichthys orbignyanus.HUFA play an important role as an energy storage and membrane components, including photoreceptor cells in the retina. Marine fish have a limited capacity to synthesize HUFA and must acquire them trough the food. The need is critical in fish larvae because of the rapid growth and the critical development of specialized tissues. We expect that larvae that feed on prey with high content of docosahexaenoic acid (DHA) will have a better configuration of the retina and lipid composition than larvae raised on a low-DHA diet and that this should affect their vision, feeding capability and fitness.The project will be carried out at the Department of Marine Ecology, Göteborg University at Kristineberg Marine Research Station during the two-year incoming phase and in Faculty of Science, Universidad de la República during the one-year returning phase. The objective of the present study is to assess the effect of diets with differing contents of HUFA on the development of the retinal tissue in larvae of flatfishes from different estuarine systems: Gullmarsfjord (Sweden) and micro-estuaries along coast of Uruguay. Specific objectives are: 1) To analyse the evolution of the retinal tissue structure in larvae of P. platessa (in laboratory) and P. orbignyanus (in situ) at different stages of development subject to diets of differing HUFA contents. 2) To characterise the HUFA profiles in the retina of P. platessa and P. orbignyanus larvae at different stages of development and subject to diets of differing HUFA contents. 3) To test if the quality of food affects the timing of first appearance of rod cells in larvae of P. platessa.