Tolerances to Environmental Factors 



David V. Aldrich 



The behavioral phase of this project started with the exposure of 

 small (13-28 mm. total length) brown shrimp to continuous linear salinity 

 gradients. These gradients are of the vertical type, in which density differ- 

 ences associated with differences in salinity tend to maintain stratification 

 within the water column. Using evaporation -concentrated sea water and dis- 

 tilled water, continuous gradients 

 ranging from l%o to 70%o are estab- 

 lished in a 7 -liter column 4 feet high. 

 (See photo. ) After a shrimp is in- 

 troduced into the column at a salinity 

 level equivalent to that of the medium 

 from which it came, the animal's 

 movements up and down the gradient 

 are observed. To minimize observer 

 bias in this operation, 100 determina- 

 tions of a shrimp's level in the gra- 

 dient are taken regularly at preestab- 

 lished 15-second intervals. Prelim- 

 inary tests showed that (1) such sa- 

 linity gradients are extremely stable 

 and (2) the shrimp, although usually 

 found on the bottom of experimental 

 tanks, are quite capable of swimming 

 continuously at levels above the bot- 

 tom for 12 hours or longer. In pro- 

 viding no resting place for the animal 

 within each salinity range, this type 

 of apparatus continually forces the 

 shrimp to swim to remain within a 

 salinity range of its choice, fre- 

 quently encountering and avoiding 

 the levels which lie above and below 

 this range. Thus, results should 

 suggest avoidance limits, which may 

 be significant in regard to the nat- 

 ural distribution of these animals. 



The grouped results of 13 

 such experiments using juvenile 

 brown shrimp acclimated in a sa- 

 linity of about 27fo« clearly show that 

 the shrimp occupied low salinity 

 ranges within the gradient column 



Determining salinity gradient 

 with conductivity meter. 



57 



