Tolerance boundaries of four crustacean species (Phallocryotus spinosa, Moina salina, Arctodiaptomus salinus and Ctenodaphnia mediterranea) to salinity and temperature were evaluated by studying the species in their natural environment (salted temporary swamp of Lake Zima near Marrakech, Morocco) and in the laboratory. The seasonal density evolution over the year for the four species in their natural environment reveals three phases (youth, maturity and old). The dwell time of each species depends on the environmental factor values as well as on their rate of change. Laboratory study revealed high euryhaline and eurithermal ability for the four species, with a difference in the optimal salinity range from species to species. The ranges are situated between 5 and 25% for C. mediterranea, 15 and 30% for A. salinus, 15 and 60% for M. Salina and 30 and 90% for P. spinosa. Such a range difference, measured under controlled conditions (laboratory), may well explain the variability in the behaviour of the four species in their natural environment. The tolerance range difference from species to species, together with the instability of the environmental conditions of the milieu, seem to be responsible for the quantitative and qualitative changes of the host fauna, which occur within the year and from year to year.