Psammechinus miliaris were collected by scuba divers at 3-10m in February 2010 from Rubha Garbh, Loch Creran, Scotland. The urchins were transported in aerated cool boxes to Bangor, where they were transferred into flow-through aquaria at Bangor University’s School of Ocean Sciences, Anglesey. The animals were held for two months prior to the start of the experiment to acclimate them to normal (control) pH aquarium conditions (~ pH 8). They were fed to satiation (approximately 5% mean body weight) twice a week on an artificial diet largely composed of mussels (Mytilus edulis) and macroalgae (Laminaria digitata). After the two months, urchins with a test diameter over 25mm were chosen as broodstock for the OA trial and moved into an OA flow through mesocosm. To avoid animal stress, all tanks in the flow through system contained control pH sea water, which was gradually replaced by reduced pH sea water until the desired pH of 7.72 was reached in the experimental tanks. Treatments used in this study were derived from the IPCC predictions using a ‘business-as-usual’ scenario. Present day control sea water, imported directly from the Menai Straits, measured ~ pHNIST 8.06, with the year 2100 pH level calculated at ~ pHNIST 7.72 (referred to as End-Century in this study). Adults were cultured in either control conditions or End-Century pH for two months prior to spawning in June of the same year.