BISHOP ET AL.: OXYGEN CONSUMPTION OF BROWN SHRIMP 



individual consumes more oxygen than a smaller 

 one, but its rate of oxygen consumption per unit 

 mass is less (Mill 1972). In our study this generali- 

 zation was found for shrimp only at 20%o S. Al- 

 though it is not known why this difference was 

 evident at only one salinity, it should be noted that 

 among the six salinity-size treatment combina- 

 tions, the lowest as well as the highest metabolic 

 rates occurred at 20%o S (Figure 4). It is possible 

 that the 3.7 g shrimp were more active than 

 "routine" in the test chamber and that the 6.7 g 

 shrimp were less active than "routine." Tests for 

 both sizes at each temperature were conducted 

 within a few days of each other, and we believe that 

 the time element was not responsible for the ob- 

 served difference. Each salinity-size combination 

 is the average of approximately 30 tests, and the 

 possibility of obtaining the results by chance is 

 small. The data in Table 8 indicate decreasing 

 metabolic rate (per unit mass) with increasing 

 size, although extreme variability exists. 



0.50 



0.45 



u> 



E 



0.40 



0.35 



0.30 



• 3.7 g P. gztecus 



• 6.7 g £. gztecus 



10 



20 



SALINITY (%o) 



30 



Figure 4. — Mean oxygen consumption rate (average for all test 

 temperatures) of 3.7 and 6.7 gPenaeus aztecus at salinities of 10, 

 20, and 30%o. 



As shrimp increase in size in the estuary, they 

 move to higher, more stable salinities (Weymouth 

 et al. 1933; Gunter 1945, 1950; Williams 1960; 

 Bishop and Shealy'^). This movement may be, in 



'Bishop, J. M., and M.H.Shealy.Jr 1977. Biological obser- 

 vations on commercial penaeid shrimps caught by bottom trawl 

 in South Carolina estuaries, February 1973 - January 

 1975. S.C. Wildl. Mar. Resour. Dep., Mar Res. Div., Tech. Rep. 

 25, 97 p. 



part, a response to a decrease in osmoregulatory 

 ability with increasing size. Only two sizes of 

 shrimp were tested, and both sizes were obtained 

 from the same locality ( often from the same trawl 

 tow). Thus osmoregulation differences would not 

 be anticipated to be large. The larger shrimp ap- 

 pear to be better regulators in hyperosmotic salin- 

 ity and at high temperatures. The slopes of the 

 hemolymph data over test salinities at 33° C for 

 the 3.7 and 6.7 g shrimp were 0.47 and 0.33, indi- 

 cating that the larger shrimp maintained 

 homoiosmoticity to a better degree than did the 

 smaller shrimp at a temperature approaching an 

 environmental extreme (Figure 2). 



Some of our test conditions and those of Wil- 

 liams (1960) are nearly identical, and hemolymph 

 data from shrimp acclimated to similar conditions 

 are comparable. Hemolymph data from both 3.7 

 and 6.7 g shrimp were averaged to be compatible 

 with Williams' (1960) juvenile P. aztecus (42-100 

 mm TL). At 28° C and 10, 20, and 30%o S, we 

 obtained average hemolymph osmolalities of 619, 

 689, and 785 mOsm, respectively, whereas Wil- 

 liams obtained values approximating 657, 804, 

 and 825 mOsm. Williams' values are somewhat 

 higher than ours, but physiological differences in 

 populations, analytical techniques, or acclimation 

 history of test animals could be responsible. 



Because small shrimp (3.7 g) may encounter 

 highly variable salinities, they may be capable of 

 tolerating relatively variable hemolymph os- 

 molalities and their osmoregulatory processes 

 may not be as capable of homoiosmoregulation as 

 those of larger shrimp. This implies that varying 

 salinities would be more expensive energetically 

 for larger shrimp and partially responsible for 

 their offshore movement prior to maturity. 



Temperature Effects on 

 Oxygen Consumption and Osmoregulation 



Metabolic rate of most poikilotherms is related 

 to temperature (Prosser 1973). The lowest test 

 temperature that we used (18° C) approached as 

 closely as our facilities permitted the 16° C at 

 which P. aztecus is reported to exhibit little growth 

 (St. Amant et al. 1966). The highest test tempera- 

 ture approaches the shrimp's lethal limit (Zein- 

 Eldin and Griffith 1969) and is seldom experienced 

 in Louisiana estuaries. The oxygen consumption 

 rates of shrimp increased linearly as temperature 

 increased, and rates for both sizes increased in a 

 similar manner (Figure 1). 



751 



