BISHOP ET AL.: OXYGEN CONSUMPTION OF BROWN SHRIMP 



significant differences were found among any of 

 the period interactions (Table 2 ); thus the effects of 

 size, salinity, and temperature are independent of 

 acclimation time. The average oxygen consump- 

 tion data for each treatment combination in Table 

 3 and Figure 1 are slightly higher than would be 

 expected for shrimp completely acclimated to the 

 test chamber, however. Egusa ( 1961) found that the 

 oxygen consumption rate ofP.japonicus stabilized 

 after about 3 h. Acclimation time to test conditions 

 may have been reduced if fine-grained substrate 

 had been included in the test chamber. Penaeid 

 shrimp exhibit arrhythmic activity when they 

 cannot bury in substrate (Racek 1959; Moller and 

 Jones 1975). 



Because shrimp may be both oxygen conformers 

 and regulators, crowding could profoundly influ- 

 ence their oxygen consumption by increasing the 

 extent of activity. We found no significant oxygen 

 consumption rate differences between one and two 

 3.7 gP. aztecus or among one, two, three, and four 

 6.7 gP. aztecus when compared on a per gram wet 

 mass basis, and believe that testing two shrimp 

 simultaneously did not appreciably affect their 

 oxygen consumption rates. Subrahmanyam ( 1976) 

 noticed no differences in activity when testing 

 pink shrimp singly or in pairs. 



Salinity Effects on 

 Oxygen Consumption and Osmoregulation 



The influence of salinity on the life habits of 

 penaeid shrimp has received considerable atten- 

 tion (Panikkar 1951, 1968; Gunter and Hildebrand 

 1954; Zein-Eldin 1963; Gunter et al. 1964; Parker 

 1970). Panikkar (1951) suggested that high salin- 

 ity may be necessary for ovarian development, but 

 its importance still remains unknown. Life cycles 

 of the three penaeid shrimp important commer- 

 cially in the Gulf are similar (Williams 1965), but 

 juvenile white shrimp, P. setiferus, are reported to 

 prefer salinities <10%o; juvenile brown shrimp, 

 salinities between 10 and 20%©; and juvenile pink 

 shrimp, salinities >18%o (Gunter et al. 1964). 

 Adaptation to low salinities is highly developed in 

 young penaeids, and juveniles are more widely 

 distributed in estuaries than are adults. Thus, os- 

 moregulatory capabilities may influence emigra- 

 tion of subadults from estuaries (Panikkar 1968). 

 Zein-Eldin (1963) obtained good growth and sur- 

 vival for postlarval P. aztecus at 2, 5, 10, 25, and 

 40%o, and concluded that salinity per se may not 

 directly affect growth during the estuarine por- 



tion of their life cycle. These postlarvae were 

 grown only to sizes <0.2 g (Zein-Eldin 1963 ), so the 

 effects of low salinity on growth rate during a 

 substantial portion of their life cycle remains un- 

 investigated. 



Brown shrimp were hyperosmotic regulators in 

 10 and 20%o S and hypoosmotic regulators in 30%o 

 S. Depending on salinity and temperature, 

 hemolymph osmolality was maintained at con- 

 centrations approximating 600-900 mOsm (Fig- 

 ure 2). These results agree with those of Williams 

 (1960) and McFarland and Lee (1963). Thus P. az- 

 tecus cannot be considered a perfect regulator, but 

 it differs substantially from nonregulators. 

 Panikkar (1968) considered homoiosmotic regula- 

 tion to be one of the most advanced capabilities of 

 marine invertebrates. 



Oxygen consumption would be expected to in- 

 crease for osmoregulators as the osmotic differ- 

 ence between the shrimp's hemolymph and its en- 

 vironment increased because metabolism would 

 increase to maintain a constant hemolymph con- 

 centration. Energy expenditure for osmoregula- 

 tion depends on the species and is related to tem- 

 perature as well as other variables (see reviews 

 by Kinne 1964, 1966, 1967). 



There is conflicting evidence as to whether im- 

 portant energy expenditures are necessary to 

 maintain homoiosmotic hemolymph (Schwabe 

 1933; Lofts 1956; Rao 1958; Dehnel 1960). In our 

 tests hemolymph osmolalities of P. aztecus were 

 significantly affected by salinity, but the energy 

 expenditures for osmoregulation after acclimation 

 were small in relation to total metabolic rate. 

 Other studies on euryhaline decapods show that 

 salinity does not have pronounced effects on oxy- 

 gen consumption if the experimental animals are 

 acclimated to the test salinities and if test 

 salinities are not too extreme (Lofts 1956; Rao 

 1958; Kader 1962; Kutty et al. 1971). 



Venkataramiah et al. (footnote 4) acclimated 

 brown shrimp to 15%o S at 25° C and measured 

 oxygen consumption rates after salinity was 

 changed to 2, 5, 10, 15, 25, and 36%o. Metabolic 

 rates increased initially, but generally tended to- 

 ward that of acclimation conditions after a day 

 unless deviations from acclimation salinity were 

 substantial, i.e., 2, 5, and 36%o. Salinity changes in 

 the respirometer were made over a 1-h period, 

 however, and may have been too rapid and (or) 

 extreme for the shrimp's capacity to adjust. Ven- 

 kataramiah et al. (footnote 4) found that blood 

 hemolymph required 6 h to achieve osmotic stabil- 



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