Global climate change threatens the oceans as anthropogenic carbon dioxide causes ocean acidification and reduced carbonate saturation. Future projections indicate under saturation of aragonite, and potentially calcite, in the oceans by 2100. Calcifying organisms are those most at risk from such ocean acidification, as carbonate is vital in the biomineralisation of their calcium carbonate protective shells. This study highlights the importance of multi-generational studies to investigate how marine organisms can potentially adapt to future projected global climate change. Mytilus edulis is an economically important marine calcifier vulnerable to decreasing carbonate saturation as their shells comprise two calcium carbonate polymorphs: aragonite and calcite. M. edulis specimens were cultured under current and projected pCO(2) (380, 550, 750 and 1000 mu atm), following 6 months of experimental culture, adults produced second generation juvenile mussels. juvenile mussel shells were examined for structural and crystallographic orientation of aragonite and calcite. At 1000 mu atm pCO(2), juvenile mussels spawned and grown under this high pCO(2) do not produce aragonite which is more vulnerable to carbonate under-saturation than calcite. Calcite and aragonite were produced at 380, 550 and 750 mu atm pCO(2). Electron back scatter diffraction analyses reveal less constraint in crystallographic orientation with increased pCO(2). Shell formation is maintained, although the nacre crystals appear corroded and crystals are not so closely layered together. The differences in ultrastructure and crystallography in shells formed by juveniles spawned from adults in high pCO(2) conditions may prove instrumental in their ability to survive ocean acidification. (C) 2014 Elsevier Inc. All rights reserved.