Skeletal Mg/Ca ratios of well-preserved fossil echinoderms have been used to reconstruct past Mg/Ca ratio in seawater up to the Phanerozoic, taking into account the known temperature effect on this ratio. This study investigates the effects of salinity and growth rate on Mg/Ca and Sr/Ca ratios in starfish calcite skeletons grown in experimental conditions. Both ratios are not related to growth rate: on the contrary, both are positively related to salinity. This effect induces an error on the reconstructed Mg/Ca ratio in seawater that may reach 46%. An intriguing inverse relation between skeletal Sr/Ca ratio and temperature was recorded. The salinity effects are presumably due to physiological regulation processes.