The variability coefficients for the housekeeping genes 18S rRNAand actb were higher than 25%. Moreover, the transcription ofthese genes varied with experimental temperature, with lower 18SrRNA/actb ratio in mussels kept at 18◦C than those kept at 26◦C(Student’s t-test, p < 0.05) (Fig. 5A). Consequently, results were expressed in absolute numbers of transcription level per gene (arbi-trary units), according to Thermal stress provoked an increase in hex transcription level(Fig. 6A). In contrast, exposure to Cd provoked a decrease in hex gene transcription level that was less marked at 26◦C thanat 18◦C (Fig. 6A). It is worth mentioning that hex gene tran-scription level was highly variable in control mussels, variabilitybeing markedly reduced after Cd exposure (Fig. 6A). Likewise,gusb gene was markedly up-regulated as a result of thermal Fig