In first-stage zoeae of four decapod species, we tested if changes in biomass (dry mass, W; lipid; protein) under hypo-osmotic salinity stress are related to larval salinity tolerance and known or presumable osmoregulatory capabilities. As model species, we compared the stenohaline marine osmoconformer Hyas araneus with three euryhaline osmoregulators, the estuarine species Sesarma curacaoense, armases roberti, and Perisesarma fasciatum. In all four species, the zoeae were reared in seawater (32 PSU, control) and at a reduced salinity of 25 PSU, the euryhaline species also at 10 PSU; additionally, a 20 PSU treatment was included in H. araneus and P. fasciatum. Biomass was measured when 50% of the time of moult-cycle duration in seawater had elapsed. In H. araneus, W was maximum in seawater, declining at reduced salinities (minimum at 20 PSU). In all three euryhaline species, W was maximum at 25 PSU and minimum at 10 PSU. The lipid fraction showed no significant response in P. fasciatum, but a decreasing tendency with decreasing salinity in all other species. The protein content remained in all species practically constant, with a slight (but significant) peak only in zoeae of P. fasciatum exposed to 25 PSU. In conclusion, reduced zoeal growth occurred in H. araneus at slightly or moderately reduced salinities (20-25 PSU), but in estuarine species only at a very low salinity (10 PSU). The patterns of change in larval biomass (particularly in W and lipid) under salinity stress concur with interspecific variation in early larval salinity tolerance and capability of osmoregulation.