Overall, our results highlight the effect of diet on skeletal carbonate composition of sea urchins. They also provide an insight in the mechanisms at nowadays pCO2 levels, both in terms of the mechanism of mineral incorporation (from the water or through the diet) and bottom-up processes (algal diet) affecting fish predation rate (top-down process) in rocky subtidal communities, modulating test robustness of sea urchins and consequently their vulnerability to predation.