preference to low-salinity water. The distribution of P. aztecus 

 shows that it is a warmwater species. The northern limit of penaeid 

 shrimp is approximately Cape Hatteras, North Carolina, near the north- 

 ern limit of the Carolinian (subtropical) zone of biogeography. 

 These animals are quickly killed by cold waves (Gunter and Hilde- 

 brand 1954) while the hardier crustacean cohort, the blue crab 

 Callinectes sapidus, with a northern distribution to Canada, has 

 not been recorded as a casualty. Gunter (1950) had observed cor- 

 relation between the seasonal variations in the abundance of penaeid 

 shrimp (and other invertebrates) and the annual temperature cycle 

 but not with salinity changes or any other phenomenon. Therefore, 

 he concluded that temperature is apparently a much more important 

 factor than salinity in the general cyclic movements of marine 

 animals in the South Atlantic and the Gulf of Mexico. In a later 

 publication Gunter (1967) stated that although salinity is a limit- 

 ing factor in the geographic distribution of penaeid shrimp, it is 

 certainly not the sole limiting factor. 



From controlled experiments we have observed that it is indeed 

 not a single environmental factor which apparently governs the dis- 

 tribution of brown shrimp but a combination of factors. Two of 

 these, namely salinity and temperature, counteract, support, and 

 modify the physiological effects of each other. During these 

 studies three different size groups of brown shrimp (13-20 mm, 21- 

 45 mm, and 50-75 mm length ranges) were tested in eleven salinities 

 in a 0.34 to 59.5°/ooS range, at temperatures of 21°, 26° and 31°C 

 (Venkataramiah et al. 1974). Over this range none of the experi- 

 mental shrimp died in salinities from 8.5 to 34.0°/oo and conse- 

 quently this range was designated as the normal salinity tolerance 

 range. The normal salinity tolerance range decreased progressively 

 as the test temperatures were altered from 26°C to 21° and 31°C. 



Background temperature is an important factor in the survival 

 of shrimp when both salinity and temperature changes occur. Brown 



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