This possibility was shown in the isosmocity of blood osmotic 

 and chloride levels with the external salinities. In the 70 mm 

 juveniles blood chloride concentration was isosmotic at about 17°/ooS 

 level (Venkataramiah et al. 1974). In the present experiments the 

 chlorides were isosmotic at 20-22°/ooS level and the osmotic concen- 

 tration at 22-25°/ooS level. The chloride isosmotic level agrees 

 favorably with the isosmotic level of 700 to 800 mOsm (about 24 to 

 28°/ooS) in Penaeus aztecus reported by McFarland and Lee [1963). 

 The small variations in the isosmotic levels between these two 

 studies can be attributed to the larger sized shrimp (>100 mm) of 

 McFarland and Lee which were collected from a higher salinity (690 

 mOsm or about 23.7°/ooS). 



Other studies from our laboratory have shown that salinity 

 tolerance is also a function of size in brown shrimp. While fol- 

 lowing the development of salinity tolerance in postlarval brown 

 shrimp Biesiot (1975) observed that the tolerance range expanded 

 gradually from the time the postlarvae were 6 mm long (10 days 

 old). From 19 days of age (9-10 mm length) the postlarvae grew 

 faster in 18°/ooS than in either 25 or 32°/ooS. In earlier studies 

 salinity tolerance of postlarvae (20 mm) and juveniles (size 1: 21 

 to 45 mm range; size 2: 50 to 75 mm range) was studied by Venkatara- 

 miah et al. (1974). It was found that postlarvae survived longer 

 than juveniles in the extremely high salinity 59.5°/oo. In low 

 salinities 0.34 and 3.4°/oo juveniles (size 1) survived longer than 

 the others. The salinity tolerance expanded gradually and reached 

 a widest range in late postlarvae. From this stage onward the 

 tolerance range decreased gradually as they became older. Chew 

 (1975) carried bat comparative respiratory studies with two size 

 groups of shrimp of 60 and 100 mm mean length by direct transfer 

 and by acclimation to a series of salinities. The respiratory 

 rates showed significant differences between these groups in re- 

 sponse to salinity changes. The responses of larger shrimp (100 mm) 



285 



