the individuals previously kept in about 34°/ooS. Similar responses 

 were found in Penaeus aztecus acclimated to both salinity and temper- 

 ature variables by changing these factors gradually from control (Ven- 

 kataramiah et al. 1974). 



We have no knowledge of the rates at which young brown 

 shrimp can acclimate to salinity and temperature changes; but this 

 information is important for the U.S. Army Corps of Engineers in 

 maximizing their planning and permitting activities. On the basis 

 of our previous studies, recommendations have been made to the Corps 

 of Engineers to the effect that their floodwater discharges should 

 not suddenly lower the salinities and temperatures in coastal waters 

 at the time of postlarval migration (Ref. Venkataramiah et al. 1974, 

 pp. 124-125). In view of the ecological implications it was decided 

 to study the capacity for acclimation in brown shrimp P. aztecus 

 under conditions of sudden changes in salinity and temperature in- 

 stead of gradual changes. 



Besides the salinity and temperature variation the estuarine 

 invertebrates may also face a situation where the ion composition 

 of the medium and the relative proportion of other solutes may under- 

 go significant modifications. Lethal salinity effects due to changes 

 in relative proportions of solutes seldom occur in pure marine hab- 

 itats. However, in fresh and brackish waters, particularly in 5 

 and 8°/ooS ranges, such variations may reach critical levels. For 

 this reason this salinity range has been considered by some workers 

 as a significant ecophysiological boundary line, characterized by 

 the presence of a minimum number of species (Kinne 1971) . Lobza 

 (1945) observed pronounced ion ratio changes to occur in salinities 

 below 4°/oo. Sudden changes in important hydrochemical characteris- 

 tics have been reported by Kirsch (1956) in the water samples ob- 

 tained near the Bute and Knight inlets (British Columbia) between 

 4°/oo and 7°/ooS. The ion ratios in North Caspian Sea salinities 



30 



