| Summary: | The geochemistry of the umbo of the right valve of Crassostrea gigas from three Biscay coast sites (Getxo, Plentzia and Zierbena) has been analyzed to determinate a seasonal and ontogenetic control on the distribution of the molar ratios Mg/Ca, Sr/Ca and Na/Ca (mmol/mol). The ratio Mg/Ca shows a good correlation with the marine water temperature, defining a sinusoidal profile correlated with dark zones (produced in warm periods) and bright zones (produced in cool periods). The molar ratio Sr/Ca can be correlated with the water temperature but other factors, like ontogenetic or replacements in the unit cell, have to be considerate. The ratio Na/Ca shows a descendent behaviour from the younger stage to the older one, confirming an ontogenetic control
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