Effects of reduced salinities on dry weight (DW) and biochemical composition (total lipid and protein contents) of zoea 1 larvae were evaluated in four decapod crustacean species differing in salinity tolerance (Cancer pagurus, Homarus gammarus, Carcinus maenas, Chasmagnathus granulata). The larvae were exposed to two different reduced salinities (15[permil] and 25[permil] in C. granulata, 20[permil] and 25[permil] in the other species) for a long (ca. 50% of the zoea 1 moulting cycle) or a short period (16 h, starting at ca. 40% of the moulting cycle), while a control group was continually maintained in seawater (32[permil]).In general, the increments in dry weight, lipid and protein content were lower at the reduced salinities than in the control groups. In the zoea 1 of H. gammarus (stenohaline) and C. pagurus (most probably also stenohaline), the lipid and protein contents varied greatly among treatments: larvae exposed to low salinities exhibited very low lipid and protein contents at the end of the experiments compared to the controls. In some cases, there were negative growth increments, i.e. the larvae had, after the experimental exposure, lower lipid and protein contents than at the beginning of the experiment. C. maenas (moderately euryhaline) showed a lower variation in protein and lipid content than the above species. The zoea 1 of C. granulata (fairly euryhaline) showed the lowest variability in dry weight, protein and lipid content. Since salinity tolerance (eury- v. stenohalinity) is associated with the osmoregulatory capacity, our results suggest a relationship between the capability for osmoregulation and the degree of change in the biochemical composition of larvae exposed to variable salinities.Besides larval growth of these species should be affected by natural reductions of salinity occurring in coastal areas at different time scales. These effects may be potentially important for population dynamics since they should influence the number and quality of larvae reaching metamorphosis.