EFFECTS OF ACUTE SALINITY STRESS ON OXYGEN CONSUMPTION AND AMMONIA EXCRETION RATES OF THE MARINE SHRIMP METAPENAEUS MONOCEROS

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1 JOURNAL OF CRUSTACEAN BIOLOGY, 22(1): 45 52, 2002 EFFECTS OF ACUTE SALINITY STRESS ON OXYGEN CONSUMPTION AND AMMONIA EXCRETION RATES OF THE MARINE SHRIMP METAPENAEUS MONOCEROS Bindu R. Pillai and A. D. Diwan Central Marine Fisheries Research Institute, P.O. Box 1603, Tatapuram, Kochi , Kerala, India; present address (BRP): Central Institute of Freshwater Aquaculture, Kausalyaganga, Bhubaneswar , Orissa, India; (ADD) Central Institute of Fisheries Education, Versova, Mumbai , India (corresponding author (BRP) ABSTRACT The present investigation was undertaken to study the effect of an abrupt change in the salinity of the medium on the oxygen consumption and ammonia-n excretion of the marine penaeid shrimp Metapenaeus monoceros (Fabricius). Results showed that in both low-saline (5 ) as well as highsaline (35 ) acclimated shrimps the respiratory rates were significantly lower in midrange salinities (20 and 25 ) and significantly higher in both low (5, 10, and 15 ) and high (30 and 35 ) salinities. A significant increase in ammonia-n excretion was observed when high-saline acclimated shrimps were abruptly exposed to different grades of low-saline media. Exposure to different grades of high-saline media on the other hand induced a significant decrease in ammonia excretion rate of low-saline acclimated shrimps. The O:N ratio (ratio of oxygen consumed to nitrogen excreted in atomic equivalents) showed a decreasing trend when the high-saline acclimated shrimps were abruptly exposed to low-saline media, indicating a shift towards protein dominated metabolism. A reverse trend could be observed in the O:N ratio when the shrimps were exposed to high-saline media indicating a shift towards lipid dominated metabolism in high salinities. Thus, there appears to be a shift in energy substrate utilization in these shrimps from protein dominated metabolism in low salinities (5, 10, 15 and 20 ) to lipid/carbohydrate dominated metabolism in high salinities (25, 30 and 35 ). Metapenaeus monoceros, a tropical penaeid with wide distribution in the Indo-Pacific region is considered a candidate species for aquaculture. Although it is a smaller species when compared to tiger shrimp Penaeus monodon (Fabricius), it nevertheless has several good attributes for aquaculture such as tolerance to a wide range of salinities, resistance to handling and transportation stress, and good market acceptability. These shrimps, like other penaeids, have an estuarine phase in their life cycle and thus get exposed to wide fluctuations in salinity. Because salinity is an important environmental variable that has significant influence on the energy budget of estuarine and marine animals, knowledge of the energy requirement of the animal under different salinity regimes will be useful to aquaculturists in formulating improved culture practices for maximizing growth and returns. A great deal of research has been carried out to study the effect of salinity on the metabolic rate of crustaceans (Rao, 1958; Bishop et al., 1980; Stern et al., 1984; Lei et al., 1989; Chen and Chia, 1996; Rosas et al., 1999); however, few studies deal with the energy requirement under different salinity regimes (Cheng and Fang, 1986; Guerin and Stickle, 1992). Similarly, the effect of salinity on the rate of crustacean ammonia excretion has extensively been studied (Regnault, 1984; Taylor et al., 1987; Diwan and Baskaran, 1992; Chen and Chia, 1996). Further, researchers are also looking into the influence of environmental factors, including salinity, on the relationship between oxygen consumed and excreted nitrogen (atomic O:N ratio), as it can provide information on the changes in energy substrate utilization under various environmental regimes (Mayzaud and Conover, 1988). The present study was carried out with the objective of understanding the metabolic responses of M. monoceros, a candidate species for aquaculture, in response to abrupt salinity changes in the medium. Variations in metabolic response (measured as oxygen con- 45

2 46 JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 22, NO. 1, 2002 sumption and ammonia excretion) of M. monoceros over time were studied when the shrimps were subjected to acute salinity stress. Subsequent estimation of the O:N ratio was used to clarify the nature of the metabolic substrate oxidized to meet the energetic requirement during salinity stress. MATERIALS AND METHODS Animal Collection and Maintenance Live specimens of M. monoceros (65 ± 5 mm total length, measured from the tip of the rostrum to the tip of the telson; 2.25 ± 0.25 g) were collected from a brackish water canal in Vypin Island, Kochi, India and transported to the laboratory. The shrimps were transported in polythene bags filled one-third with the water from the collection site and two-thirds with oxygen. The salinity at the collection site was 12. In the laboratory, the shrimps were acclimated to either low-saline (5 ) or high-saline media (35 ) for 7 d prior to experimentation. Acclimation was carried out in 1,000-l fibreglass reinforced plastic (FRP) tanks containing 800 l of the acclimation media. The tanks were provided with 24 h aeration. During this period the shrimps were fed with clam meat (ad libitum), and the tanks were cleaned; one-third of the medium was changed daily to maintain water quality. Experimental Protocol Time course variations (1 48 h) of oxygen consumption and ammonia excretion rates were determined simultaneously on individual shrimps which were abruptly transferred from acclimation media to test media. Salinities of 5, 10, 15, 20, 25, 30, and 35 were used as the test media. Salinity was measured using refractometer (Atago, Japan). Dilute sea water was prepared by adding fresh water to the stored sea water (32 34 ) and concentrated sea water (35 ) was prepared by adding hypersaline water made by freezing of sea water. Oxygen Consumption and Ammonia Excretion The rate of oxygen consumption was measured by using a modified Fry s annular respirometer described by Kutty et al. (1971). A 200-l capacity FRP tank was used as an overhead tank containing the test media, and was provided with 24 h aeration to maintain the oxygen concentration of the media near air saturation. For experimentation a selected intermoult shrimp acclimated to either low-saline or high-saline media was introduced into the annular chamber of the respirometer, and the test media was recirculated for 30 min to reduce the effect of handling stress. Recirculation was then cut off and water samples were taken just before the closure to estimate the initial levels of dissolved oxygen and ammonia nitrogen. Final samples were taken after 60 min of closure following which the respirometer was flushed with well aerated water for 15 min to remove the static water completely. Subsequently, the initial and final water samples were collected at 3, 5, 7, 24, 26, 28, 30, and 48 h respectively. Upon completion of each experiment the shrimp was blotted dry and weighed accurately using an electronic balance. Dissolved oxygen content in the water samples was estimated using Winkler s method and is expressed as µmolo 2 g wet-wt. 1 h 1. Ammonia was assayed colorimetrically following the method of Solorzano (1969).The Fig. 1. Rate of oxygen consumption (µmolo 2 g wetwt. 1 h 1 ) of low-saline acclimated Metapenaeus monoceros as a function of time after abrupt transfer to different grades of high-saline media. Each symbol represents mean (n = 5). colour intensity was measured using a UV-VIS spectrophotometer (ATI UNICAM, UK) and the results are expressed as µmol NH 4 -N g wet-wt. 1 h 1. All the experiments were carried out at room temperature (29 ± 1 C). The ratio of mean oxygen consumption by atom of oxygen to mean ammonia excretion by atom of nitrogen (O:N) at each salinity and time interval was calculated following Corner and Cowey (1968). The mean oxygen consumption rate at a particular salinity was converted to energy equivalent by using oxycalorific value of J ml O 2 1 (Crisp, 1971). The data were evaluated using a two-way ANOVA to determine whether salinity and exposure time affected the oxygen consumption and ammonia excretion rate. Duncan s multiple range test was used to identify the significant differences between the treatments. Student s t-test was used to find out significant differences between the mean MO 2 at a particular salinity between low-saline and high-saline acclimated shrimps. RESULTS Time course changes in the rate of oxygen consumption (MO 2 ) of low-saline acclimated shrimps when transferred to different grades of high-saline media (10 35 ) are presented in Fig. 1. An overall decrease in oxygen consumption could be observed following the transfer to high salinity. The decreasing trend in MO 2 could be observed from 3 5 h on-

3 PILLAI AND DIWAN: SALINITY STRESS IN M. MONOCEROS 47 Fig. 2. Energy expenditure rate (J/h) of low-saline acclimated shrimps upon abrupt transfer to high-saline media. Each value represents mean ± SE (n = 9). Values bearing same superscript do not differ significantly (P < 0.05) wards after transfer, and the levels were stabilized by 5 7 h after transfer. Lowest levels of oxygen consumption were recorded in 25 and 20. Statistical analysis indicated that both salinity and exposure time had significant effect (P < 0.01) on the rate of oxygen consumption of low-saline acclimated shrimps. Energy expenditure was maximum in 5 media, even though the shrimp were acclimated to that salinity for a period of seven days (Fig. 2). It was significantly less in 20 and 25 media (P < 0.01). Abrupt transfer of high-saline acclimated shrimps to low-saline media did not give any overall picture of MO 2 (Fig. 3). A decreasing trend could be observed in MO 2 when these shrimps were transferred to 25, 20, and 15 media from the acclimation media. The decreasing trend could be observed from 3 h after transfer, and the levels were stabilized at the new level by 5 7 h after transfer to low salinity. No significant changes in MO 2 could be observed upon transfer to 10 media. However, transfer to 5 media resulted in a significant increase (P < 0.01) in the MO 2 of these shrimps. Over the time period, however, a decline in oxygen consumption could Fig. 3. Rate of oxygen consumption (µmolo 2 g wetwt. 1 h 1 ) of high-saline acclimated Metapenaeus monoceros as a function of time after abrupt transfer to different grades of low-saline media. Each symbol represents mean (n = 5). be observed until 7 h, and the levels did not show any significant change from 7 48 h. Statistical analysis revealed significant effect of salinity and time on MO 2. In high-saline acclimated shrimps, the energy expenditure rate was found to be minimum in 20 and 25 (Fig. 4). The mean MO 2 of low-saline and high-saline acclimated shrimps at a particular test salinity did not differ significantly (Table 1, t (10) = 0.28, P > 0.05). Time course changes in ammonia excretion rate of low-saline acclimated shrimps when abruptly transferred to different grades of high-saline media is shown in Fig. 5. An overall decreasing trend could be observed in ammonia excretion rate of these shrimps upon transfer to different grades of high-saline media. The decreasing trend was evident 1 3 h after transfer and continued until 7 h. Subsequently, there was a transient increase in the ammonia excretion after 24 h, and the levels stabilized by 26 h. Statistical analysis revealed significant effect of salinity and time on ammonia excretion rate. An overall increasing trend could be observed in the ammonia excretion rate when

4 48 JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 22, NO. 1, 2002 Table 1. Mean oxygen consumption rate (MO 2 ) and mean ammonia-n excretion rate (MAE) of Metapenaeus monoceros in different test salinities. Each value represents mean ± SE; n = 9. MO 2 (µmol g wet-wt. 1 h 1 ) MAE (µmol g wet-wt. 1 h 1 ) Low-saline High-saline Low-saline High-saline Salinity ( ) acclimated shrimps acclimated shrimps acclimated shrimps acclimated shrimps ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.04 high-saline acclimated shrimps were abruptly transferred to different grades of low-saline media (Fig. 6). The increasing trend could be observed from 1 h after transfer; however, over time the ammonia excretion showed a decline until 7 24 h, and the levels stabilized by 24 h after transfer. Salinity and exposure time had significant effect on the ammonia excretion rate as revealed by statistical analysis. The mean ammonia excretion rate of highsaline and low-saline acclimated shrimps at a particular test salinity did not differ significantly (Table 1, t (10) = 0.31, P > 0.05). Abrupt transfer to different grades of highsaline media resulted in a significant increase in the O:N ratio of low-saline acclimated shrimps (Fig. 7). On the other hand, abrupt transfer to different grades of low-saline media resulted in a significant decrease in the O:N ratio of high-saline acclimated shrimps (Fig. 8). DISCUSSION The present investigation revealed that both low- as well as high-saline acclimated shrimps showed lower respiratory rates in Fig. 4. Energy expenditure rate (J/h) of high-saline acclimated shrimps upon abrupt transfer to low-saline media. Each value represents mean ± SE (n = 9). Values bearing same superscript do not differ significantly (P < 0.05). Fig. 5. The ammonia excretion rate (µmol NH 4 -N g wetwt. 1 h 1 ) of the low-saline acclimated Metapenaeus monoceros as a function of time after abrupt transfer to different grades of high-saline media. Each symbol represents mean (n = 5).

5 PILLAI AND DIWAN: SALINITY STRESS IN M. MONOCEROS 49 Fig. 6. The ammonia excretion rate (µmol NH 4 -N g wetwt. 1 h 1 ) of the high-saline acclimated Metapenaeus monoceros as a function of time after abrupt transfer to different grades of low-saline media. Each symbol represents mean (n = 5). Fig. 7. The O:N ratio for low-saline acclimated Metapenaeus monoceros as a function of time after abrupt transfer to different grades of high-saline media. Each symbol represents mean (n = 5). Fig. 8. The O:N ratio for high-saline acclimated Metapenaeus monoceros as a function of time after abrupt transfer to different grades of low-saline media. Each symbol represents mean (n = 5). midrange salinities (20 and 25 ) and higher respiratory rates in low (5, 10, and 15 ) as well as high (30 and 35 ) salinities. Such characteristic responses to salinity have been reported for various crustaceans (Rao, 1958; McNamara and Moreira, 1987; Chen and Nan, 1993). Chen and Nan (1993) reported that in juveniles of P. chinensis Osbeck the respiratory rate was significantly lower at 20 and 25 media than at 15 and 30 after 6, 12, 18 and 24 h. In a brackish water population of Callinectes sapidus Rathbun juveniles, the total energy expenditure was significantly higher at 2.5 and 35 than at other salinities (Guerin and Stickle, 1992). Dalla Via (1986) reported that the oxygen consumption by juveniles of P. japonicus Bate increased rapidly to 300% of the initial value after a few hours with a change in salinity from 37 to 10. The energy expenditure rate of both low- and highsaline acclimated M. monoceros indicated that the shrimp spends significantly less energy in 20 and 25 when compared to other salinities. The increase in metabolic rate in low as well as high salinities in crustaceans reported in the present study and in many pre-

6 50 JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 22, NO. 1, 2002 vious studies has been attributed to the increased energy requirement for osmoregulation. Metapenaeus monoceros is a highly euryhaline species and is reported to tolerate the highest ranges of salinity from fresh water to hypersaline environment (Panikkar, 1968). Like other penaeids this species is also a hyper- hyporegulator of its body fluids. It has been shown that in euryhaline crustaceans maintenance of a hyperosmotic extracellular fluid in low salinities requires the expenditure of energy (Gilles, 1979). Hyperosmoregulation in dilute media relies on the ability to absorb ions against a concentration gradient, which involves expenditure of energy. Other predominant energy consuming phenomena operating in low saline media are reported to be an increase in oxidative deamination of free amino acids (McNamara and Moreira, 1987). In midrange salinities, as the gradient between haemolymph and external osmolal concentration becomes progressively reduced with increasing external salinity up to isosmotic state, the rate of the above processes also will diminish, thereby decreasing the oxygen consumption rate. In higher salinities, the osmoregulators maintain their extracellular fluid concentration hypo-osmotic to the external media, which again involves an expenditure of energy, thereby increasing the metabolic rate in high salinities. The energy consuming processes operating in high salinities are reported to be the active removal of ions, degradation of haemolymph proteins into free amino acids, and their subsequent transport to tissues (Gilles and Pequeux, 1983). An increase in ammonia excretion rate has been reported in many euryhaline decapod crustaceans upon exposure to low-saline media (Harris and Andrews, 1985; Taylor et al., 1987; Lei et al., 1989; Diwan and Baskaran, 1992). Results obtained in the present study are in concurrence with these earlier reports. It is now widely accepted that the enhanced ammonia excretion rate observed under lowsaline stress is a result of an increase in free amino acid catabolism, which is one of the mechanisms by which intracellular osmolarity is maintained (Gilles, 1979; Taylor et al., 1987). The increased ammonia excretion under low saline conditions has also been linked to the active uptake of sodium ions as NH 4 + has been proposed to act as the counter ion in the regulation of haemolymph sodium concentration (Kirschner, 1979). Lei et al. (1989) opined that while part of the increase in ammonia excretion under low-saline exposure may be due to ion pump exchange of NH 4 + for medium Na +, it may also reflect an accelerated catabolism of amino acids or other nitrogenous compounds. The effect of a salinity increase upon nitrogen excretion, however, appears to be more complex as inconsistent responses have been reported (Regnault, 1984). In the present study, exposure of low saline acclimated shrimps to high-saline media resulted in a significant decrease in ammonia excretion rate. Armstrong et al. (1981) also reported a significant reduction in ammonia excretion rate in Macrobrachium rosenbergii (De Man) when transferred from fresh water to 24. Diwan and Baskaran (1992) similarly reported a decrease in ammonia excretion rate of P. indicus Milne Edwards when transferred from low-saline (10 ) to high-saline (40 ) media, and suggested that such a decrease may be due to reduction in catabolism of free amino acids or due to an increase in polypeptide synthesis. Chen and Nan (1993) also reported a significant decrease in ammonia excretion rate in P. chinensis when transferred from 15 to 30 media. Chen and Chia (1996) reported that ammonia-n excretion of the crab Scylla serrata (Forskål) decreased with higher salinity levels in the range of The decreased rate of ammonia excretion, upon exposure to high-saline media observed in the present study in M. monoceros, may be due the reduction in rate of catabolism of free amino acids together with the increase in polypeptide synthesis. Corner and Cowey (1968) reported that the ratios of oxygen consumption to ammonia-n excretion in atomic equivalents are useful for the evaluation of the characteristics of nutrients utilized by animals and can provide information on changes in energy substrate utilization under various environmental regimes. An O:N ratio higher than 24 indicates lipiddominated metabolism, and an O:N ratio lower than 24 indicates a protein-dominated metabolism (Taniguchi, 1975). A high rate of protein relative to carbohydrate and lipid catabolism results in a low O:N ratio, which is generally indicative of a stressed condition. In the present study a significant increase in O:N ratio was observed when low-saline acclimated shrimps were transferred to high-

7 PILLAI AND DIWAN: SALINITY STRESS IN M. MONOCEROS 51 saline media indicating a shift toward lipiddominated metabolism under hypersaline stress. On the other hand, there was a significant decrease in O:N ratio when high-saline acclimated shrimps were transferred to lowsaline media, indicating a shift towards protein-dominated metabolism under hyposaline stress. In both low-saline as well as highsaline acclimated shrimps the mean O:N ratio was higher than 24 in high salinities (25, 30 and 35 ) and lower than 24 in low salinities (5, 10, 15 and 20 ). The observed changes in O:N ratio thus clearly indicate a shift in energy substrate utilization in response to variations in salinity of the medium. Lei et al. (1989) reported a decrease in O:N ratio from 27.7 at 35 to 8.1 at 15 in juvenile P. monodon. In P. chinensis, Chen and Nan (1993) reported an increase in O:N ratio with an increase in salinity of the environment. In Crangon crangon Linnaeus, an O:N ratio of 27 was reported to be reflective of mostly lipid catabolism (Regnault, 1984). Upon continued stress, the O:N ratio in the above species declined to 10, indicating that only proteins were catabolised. In the American lobster Homarus americanus Milne Edwards, a reduction in O:N ratio from 26.7 to 22.1 was reported to represent an increase in protein catabolism with respect to lipid/carbohydrate catabolism (Capuzzo and Lancaster, 1979). In the amphipod Gammarus wilkitzkii, the pooled O:N ratio for 24-h osmotic stress varied from 21 under hypo-osmotic stress to 37 under hyperosmotic stress (Aarset and Aunas, 1990). Stern et al. (1984) reported that the energy substrate utilization in M. rosenbergii changed as a function of osmotic stress. In fresh water and slightly brackish water, the O:N ratio for this prawn reflected lipid- and or carbohydrate-dominated metabolism, and in more saline water (15 and 24 ), there was a marked decrease in O:N ratio indicating that prawns grown in these media increased protein catabolism relative to lipid catabolism. Thus, from the earlier literature and from the present results, it could be seen that in many estuarine and marine animals the O:N ratio is higher in high salinities and low in low salinities indicating a shift from lipiddominated metabolism in high salinities to protein-dominated metabolism in low salinities. Protein catabolism for energy is less efficient than lipid/carbohydrate catabolism, and it also indicates a stressed condition. From the aquaculture point of view, protein catabolism for energy will be highly uneconomical and should be avoided, as protein sources for formulated aquafeeds are much more costly than lipid/carbohydrate sources. ACKNOWLEDGEMENTS The authors express their thanks to Dr. P. S. B. R. James, former director, Central Marine Fisheries Research Institute, Kochi, India, for providing facilities. The Indian Council of Agricultural Research, New New Delhi, India, supported the research. LITERATURE CITED Aarset, A. V., and T. Aunas Effects of osmotic stress on oxygen consumption and ammonia excretion of the Arctic sympagic amphipod Gammarus wilkitzkii Marine Ecology Progressive Series 58: Armstrong, D. A., K. Strange, J. Crowe, A. Knight, and M. Simmons High salinity acclimation by the prawn Macrobrachium rosenbergii: uptake of exogenous ammonia and changes in endogenous nitrogen compound. Biological Bulletin 160: Bishop, J. M., J. G. Gosselink, and J. H. Stone Oxygen consumption and haemolymph osmolality of brown shrimp, Penaeus aztecus. Fisheries Bulletin (US) 78: Capuzzo, J. M., and B. A. Lancaster Some physiological and biochemical considerations of larval development in the American lobster Homarus americanus Milne Edwards. Journal of Experimental Marine Biology and Ecology 40: Chen, I. M., and L. S. Fang Metabolic and osmotic responses of Metapenaeus ensis subjected to sudden salinity change. Pp in J. L. Maclean, L. B. Dizon, and L. V. Hosillos, eds. The First Asian Fisheries Forum. Asian Fisheries Society, Manila, Philippines. Chen, J. C., and P. G. Chia Oxygen uptake and nitrogen excretion of juvenile Scylla serrata at different temperature and salinity levels. Journal of Crustacean Biology 16: , and F. H. Nan Changes in oxygen consumption and ammonia-n excretion by Penaeus chinensis Osbeck at different temperature and salinity levels. Journal of Crustacean Biology 13: Corner, E. D. S., and C. B. Cowey Biochemical studies on the production of marine zooplankton. Biological Reviews 43: Crisp, D. J Energy flow measurements. Pp in N. A. Holme and A. O. McIntyre, eds. Methods for the Study of Marine Benthos. Blackwell Scientific Publications, Oxford. Dalla Via, G. J Salinity responses of the juvenile penaeid shrimp Penaeus japonicus. I. Oxygen consumption and estimation of productivity. Aquaculture 55: Diwan, A. D, and V. Baskaran Effect of salinity stress on neurosecretory cells, protein and free amino acid content and rate of ammonia excretion of the prawn Penaeus indicus H. Milne Edwards. Journal of Aquaculture in the Tropics 7: Gilles, R Intracellular organic osmotic effectors. Pp in R. Gilles, ed. Mechanisms of Osmoregulation in Animals. Wiley, New York.

8 52 JOURNAL OF CRUSTACEAN BIOLOGY, VOL. 22, NO. 1, 2002, and A. Pequeux Interactions of chemical and osmotic regulation with the environment. Pp in D. E. Bliss, ed. The Biology of Crustacea. Vol. 8. Academic Press, New York. Guerin, J. L., and W. B. Stickle Effect of salinity gradients on the tolerance and bioenergetics of juvenile blue crabs (Callinectes sapidus) from waters of different environmental salinities. Marine Biology 114: Harris, R. R., and M. B. Andrews Total NPS pool and ammonia net efflux rate changes in Carcinus maenas during acclimation to low environmental salinity. Comparative Biochemistry and Physiology 82A: Kirschner, L. B Control of the extracellular fluid osmolarity, control mechanisms in crustaceans and fishes. Pp in R. Gilles, ed. Mechanisms of Osmoregulation in Animals. Wiley, New York. Kutty, M. N., M. Peer Mohamed, K. Thiagarajan, and A. N. Leonard Modification of Fry s fish activity counter and respirometer. Indian Journal of Experimental Biology 9: Lei, C. H., L. Y. Hsieh, and C. K. Chen Effect of salinity on the oxygen consumption and ammonia nitrogen excretion of young juveniles of the glass shrimp Penaeus monodon Fabricus. Bulletin Institute of Zoology, Academia Sinica 28: Mayzaud, P., and R. J. Conover O:N ratio as a tool to describe zooplankton metabolism. Marine Ecology Progressive Series 45: McNamara, J. C., and G. S. Moreira O 2 consumption and acute salinity exposure in the freshwater shrimp Macrobrachium olfersii (Wiegmann) (Crustacea: Decapoda): whole animal and tissue respiration. Journal of Experimental Marine Biology and Ecology 113: Panikkar, N. K Osmotic behaviour of shrimps and prawns in relation to their biology and culture. FAO Fisheries Reports 2: Rao, K. P Oxygen consumption as a function of size and salinity in Metapenaeus monoceros Fabricius from marine and brackishwater environments. Indian Journal of Experimental Biology 35: Regnault, M Salinity induced changes in ammonia excretion rate of the shrimp Crangon crangon over a winter tidal cycle. Marine Ecology Progress Series 20: Rosas, C., L. Ocampo, G. Gaxiola, A. Sunchez, and L. R. Soto Effect of salinity on survival, growth, and oxygen consumption of post larvae (PL 10 PL 21) of Litopenaeus setiferus. Journal of Crustacean Biology 19: Solorzano, L Determination of ammonia in natural waters by phenol hypochlorite method. Limnology and Oceanography 14: Stern, S., A. Barut, and D. Cohen The effect of salinity and ion composition on oxygen consumption and nitrogen excretion of Macrobrachium rosenbergii (De Man). Comparative Biochemistry and Physiology 79A: Taniguchi, A Production ecology of zooplankton. Pp in S. Motoda, ed. Marine Plankton. Tokai University Press, Tokyo, Japan. Taylor, A. C., J. I. Spicer, and T. Preston The relationship between osmoregulation and nitrogen metabolism in the intertidal prawn, Palaemon elegans. Comparative Biochemistry and Physiology 88A: RECEIVED: 12 September ACCEPTED: 21 June 2001.