Mercury is one of the most dangerous environmental pollutants. In this work, we analysed the effects of exposure of to 1, 10 and 100 pM HgCl for 24 h on the gonadal morphology and on the expression level of three stress genes: , and . In this tissue we also evaluated the level of steroidogenic enzymes 3β-HSD and 17β-HSD and the expression of PL protein genes. Finally, we determined difference in sperm chromatin accessibility to micrococcal nuclease. We found alterations in gonadal morphology especially after exposure to 10 and 100 pM HgCl and hypo-expression of the three stress genes, particularly for . Furthermore, decreased labelling with both 3β-HSD and 17β-HSD antibodies was observed following exposure to 1 and 10 pM HgCl and complete absence at 100 pM HgCl exposure. Gonads of mussels exposed to all HgCl doses showed decreased expression of PL protein genes especially for PLIII. Finally, micrococcal nuclease digestions showed that all doses of HgCl exposure resulted in increased sperm chromatin accessibility to this enzyme, indicative of improper sperm chromatin structure. All of these changes provide preliminary data of the potential toxicity of mercury on the reproductive health of this mussel.