Time course of the onset of molecular (red), cellular (green) and tissue-specific alterations (blue) in Littorina littorea exposed to sub-lethal low (CdL) (A) and high (CdH) (B) Cd concentrations through 21 days of exposure, followed by a 12 days period of recovery. Abbreviations: MT mRNA: MT mRNA copy numbers, VvLYS: Volume Density of lysosomes in digestive cells of midgut gland tubules, DCI: Digestive cell integrity of midgut gland tubules; CTCA: presence of connective tissue calcium cells in the midgut gland, VVBAS: Volume density of basophilic cells in the epithelium of midgut gland tubules, DTI: Digestive tract integrity. CTDr: Connective tissue to digestive tubule ratio.