Pharmaceuticals, such as antidepressants are constantly released into the aquatic environment. Consequently, fluoxetine (FLX) and venlafaxine (VEN), the active molecules of Prozac(C) and Effexor(C), are detected up to several ng/L in freshwater and marine coastal waters. Both compounds act on the serotoninergic system, which may result in behavioural impairment, especially of juvenile animals presumed to be more susceptible to low concentrations. The objective of this study was to determine if environmental concentrations of FLX alone or combined with VEN could have any effect on innate burying behaviour in two juvenile marine invertebrates, i.e., Sepia officinalis and Carcinus maenas. Juvenile cuttlefish were exposed from hatching to 30 days post hatch to either FLX alone (i.e. 5 ng/L) or in mixture with VEN (i.e. 2.5 ng/L as well as 5 ng/L of each antidepressant). Juvenile crabs (< 2 cm carapace width) were exposed for a period of 34 days to 5 ng/L of FLX and a mixture of 5 ng/L of FLX and VEN each. Several parameters of sand digging behaviour were analysed weekly in both species. The occurrence of sand digging behaviour decreased in cuttlefish exposed to a mixture of FLX and VEN at the lowest concentration (2.5 ng/L each). At the mixture of 5 ng/L VEN and FLX each, a better body covering in juvenile crabs was observed. In both species, innate behaviour was modified under exposure to mixtures of FLX and VEN at environmentally realistic concentrations. These alterations were observed at an early developmental stage, when animals are particularly prone to predation. Hence, the modification of behavioural traits by exposure to pseudo-persistant antidepressants may decrease the survival of these two species in the long term.