Agbali, M. and Smith, C. 2012. Long‐term fitness consequences for offspring of female oviposition decisions in the rose bitterling (Rhodeus ocellatus). —Acta Zoologica (Stockholm) 93: 367–372.AbstractFemale oviposition decisions can have immediate consequences for offspring fitness as well as longer‐term effects, either through irreversible disruption to normal development, inducing alternative adult phenotypes, or through ‘coping’ whereby challenges to development are compensated in the short term, but carry a fitness cost in the adult. We investigated the long‐term impacts of oviposition decisions on offspring fitness parameters in the rose bitterling fish (Rhodeus ocellatus). Bitterling lay their eggs in the gills of living freshwater mussels, which complete development inside the mussel gill chamber. Bitterling embryo mortality in mussels is density dependent, and consequently, there are immediate costs associated with using mussels containing high numbers of eggs and embryos. We raised embryos at high and low densities for the first 4 weeks of development and then at a common low density until sexual maturity. Our results showed no effect of density on offspring survival or growth. However, there was an impact of density on age and size at maturity, with embryos raised at high densities becoming sexually mature earlier and at a smaller size. This effect may arise through embryos coping with low oxygen conditions during development. The results are discussed in the context of development and mating system evolution.