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