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Osmoregulation and Spatial Distribution in Four Species of Mysid Shrimps
Comp. Biochem. Physiol. Vol. 117A, No. 4, pp. 427–431, 1997 Copyright 1997 Elsevier Science Inc.
ISSN 0300-9629/97/$17.00 PII S0300-9629(96)00235-6
Osmoregulation and Spatial Distribution in Four Species of Mysid Shrimps Paul Webb, Tris Wooldridge, and Thomas Schlacher University of Port Elizabeth, Institute for Coastal Research and Department of Zoology, P.O. Box 1600, Port Elizabeth 6000, South Africa ABSTRACT. This study investigates the osmoregulatory capacities of four mysid species, viz. Mesopodopsis wooldridgei, Rhopalophthalmus terranatalis, and Gastrosaccus brevifissura which occur in different zones along salinity gradients in southern African estuaries, and the psammophilic, surf-zone mysid, Gastrosaccus psammodytes. All four species maintain body fluid concentrations at species-specific levels over a range of environmental salinities. Analysis of Covariance (ANCOVA) indicated no statistical difference in body fluid concentrations among juveniles, immatures, males, and females for M. wooldridgei, R. terranatalis, and G. brevifissura. Available information suggests that biological and physical factors other than salinity are more important in determining spatial distribution of these four species in coastal waters. comp biochem physiol 117A;4:427–431, 1997. 1997 Elsevier Science Inc. KEY WORDS. Distribution, estuary, mysids, osmoregulation, salinity, sediment, South Africa, species
INTRODUCTION Mesopodopsis wooldridgei is abundant in estuarine and nearshore marine waters of southern Africa (23). Although present in marine waters, Rhopalophthalmus terranatalis and Gastrosaccus brevifissura are more typical of estuaries while Gastrosaccus psammodytes is restricted to the surf-zone (23,27). Few data are available on the osmoregulatory capabilities of these mysids but all belong to genera which contain exceptionally euryhaline species (2,8,9,11,16,19). This study investigates the osmoregulatory capacities of M. wooldridgei, R. terranatalis, G. brevifissura, and G. psammodytes in relation to their distribution in southern African coastal waters. The estuarine species are reported from habitats having water of widely different salinity values, although distributions do overlap to some extent. This prompted the question whether salinity is a major determinant in establishing observed mysid spatial distribution patterns. Of the three estuarine species studied, only G. brevifissura burrows actively into the sediment and therefore substrate preference by this species was also tested. METHODS Samples from schools of predominantly M. wooldridgei or R. terranatalis were seine-netted at stations covering a range of salinities (7–28‰) in the tidal Gamtoos estuary. Additional mysid samples were collected at a single site in the Address reprint requests to: Prof. P. Webb, Department of Zoology, University of Port Elizabeth, P.O. Box 1600, Port Elizabeth 6000, South Africa. Tel. 127 41 558718; Fax 127 41 564519; E-mail: [email protected].
temporary closed Kabeljous estuary (43.8‰ salinity). Captured animals were immediately placed in a sieve and flushed with fresh water. Excess surface water was removed by vigorously shaking the sieve, after which animals were transferred to 40-ml poly-vinyl-chloride (PVC) containers and frozen on dry ice. At no time did animals defrost in the field or during transport to the laboratory. Water salinity was recorded in situ at each sampling site using a Valeport Series 600 CTD. Water samples were taken from depths at which the animals were captured. Gastrosaccus brevifissura was only found in relatively high numbers at a single site in the Gamtoos estuary (salinity 24‰). Live animals were transported to the laboratory in 20-l containers of estuarine water collected at the sample site. In the laboratory sub-samples of 60–70 live G. brevifissura were transferred to 20-l PVC containers with 10-l of 24‰ estuarine water. The salinity of water in three sets of three containers was gradually reduced over 3 hr to 21, 15, and 10‰ by the addition of low salinity estuarine water (5‰, collected in the upper Gamtoos), while in three other containers salinity was increased to 30‰ by gradually adding seawater. Animals were also kept in three containers of 24‰ estuarine water. Three hours after the desired salinity values had been reached, live animals were collected by gently pouring the water through a medium sieve. Individuals were briefly washed with distilled water, vigorously shaken in the sieve to remove excess water, and then deep frozen. Gastrosaccus psammodytes was collected with a bottomsledge from the lower intertidal of an exposed sandy beach (Maitlands Beach) and transported to the laboratory in sea-
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water collected at the sampling site. In the laboratory, groups of G. psammodytes were transferred to 19 experimental containers having salinity values ranging from 0.5–36‰ and at a temperature of 23 6 1°C. Salinity of the experimental units was determined to the nearest 0.01‰ Cl2 by standard AgNO3 titration following Grasshof (7). Introduction to different salinity values was done using the same techniques employed for G. brevifissura. Prior to measurements of body fluid osmotic concentration, samples were allowed to partly defrost and sorted into juvenile (no secondary sexual characteristics visible), immature (secondary sexual characteristics beginning to develop), and adult male and female categories within each species. Individual animals were placed on filter-paper disks for 5 to 10 min to remove any excess surface water. Five to sixty animals, depending on size of the individuals in each category, were transferred to PVC Eppendorf tubes, homogenised and centrifuged for 6 min. Eight µl of supernatant were analysed in a Wescor 5100 vapour pressure osmometer which was re-calibrated and thermally balanced after every 12 measurements. Water samples collected from the corresponding estuarine sampling sites were also analysed for osmolality. From each sample collected in the field, 1–15 determinations of body fluid osmolality were made for specific age and sex classes. The number of determinations depended on size of the mysid sample collected at a particular site. Linear regressions were fitted to plots of mean body fluid osmolality and environmental osmolality. All individual values obtained in laboratory experiments were included in regression plots since samples were independent by design. Mean body fluid osmolalities were compared by size and sex classes within a species, and between species by analysis of covariance (ANCOVA), specifying environmental osmolality as the co-variate. In substrate preference experiments, five 10-l buckets with 28 cm diameter bases were used as experimental units. The bottom of each container was partitioned into five equal segments. Each of the five different sediment types, classified according to mean grain diameter (Md), was introduced to a segment in each bucket. The sediment types were: coarse sand (Md . 0.6 mm), medium-fine sand (Md 5 0.4 mm), fine sand of marine origin collected from the tidal delta (Md 5 0.21 mm), very fine sand with high mud content collected from the middle reaches of the estuary (Md 5 0.08 mm, 50% mud), and riverine sand deposited in the uppermost reaches of the estuary (Md 5 0.20 mm, 2.3% mud). The volume of sediment in each segment was 250 cm3. The allocation of sediment to a segment, as well as the orientation of the containers relative to natural light, was random. Eight litres of 24‰ estuarine water was added gradually to each bucket which was then stocked with 14– 44 animals. After 15 hr of light exposure, all G. brevifissura had left the water column. Water was gently siphoned off, sediments removed separately from their segments and any
animals present were sieved out and counted under a dissecting microscope. Data obtained were analysed by oneway, fixed effects ANOVA. RESULTS Analysis of Covariance (ANCOVA) indicated no statistical difference in body fluid concentrations among juveniles, immatures, males, and females for M. wooldridgei, R. terranatalis, and G. brevifissura. All four species showed strong osmoregulatory capabilities (Fig. 1) with significant differences in body fluid concentrations between species (Table 1). Body fluid concentrations in M. wooldridgei and R. terranatalis were hyperosmotic to the medium in water osmolalities below 600 mOsm and 845 mOsm, respectively, but hypotonic to more concentrated media. Gastrosaccus brevifissura and G. psammodytes maintained hyperosmotic body fluid concentrations at all salinity values tested, including seawater. Gastrosaccus brevifissura is difficult to capture in large numbers and, because of the small size of these animals, at least 60 individuals were required to make available enough body fluid per sample for osmotic concentration analysis using a vapour pressure osmometer. The number of animals available determined that only eight data points of body fluid osmotic concentration were obtained. These data were not included in the analysis of covariance, since a strongly unbalanced design usually leads to a serious loss of efficiency and/or reliability in the interpretation of such data (20). However, available data suggest that G. brevifissura is also an effective osmoregulator. Gastrosaccus brevifissura showed a clear preference for sand with little or no mud content (r 5 13.13, df 5 4,20; p , 0.001,) in substrate selection experiments. Ninety-five percent of all G. brevifissura selected sandy substrates and Tukey’s Multiple Range Test revealed no significant difference in regard to the grade of sand selected. DISCUSSION Work on osmotic regulation in mysids was first reported for Praunus inermis, which maintained a constant body fluid osmolarity over a 5–32‰ salinity range (3). Remane and Schleiper (17) noted that Mysis oculata tolerated a salinity range from fresh to 100% seawater. McLusky and Heard (14) showed clear patterns of osmotic regulation in Praunus flexuosus and Neomysis integer, with both species exercising hyperosmotic and hypo-osmotic regulation depending on the salinity of the medium in which they were maintained. Bobovich (3) also worked on N. integer and investigated the dynamics of salt loss by animals acclimated in water of various salinities while Bhattacharya (2) showed that water of reduced salinity was a major factor associated with increasing abundance of juvenile Mesopodopsis orientalis. McLusky, Hagerman, and Mitchell (15) observed en-
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FIG. 1. Body-fluid osmolality in Mesopodopsis wooldridgei (A), Rhopalophthalmus terranatalis (B), Gastrosaccus psammo-
dytes (C) and Gastrosaccus brevifissura (D) vs environmental salinity.
hanced hyperosmotic regulation with low salinity acclimation in P. flexuosus, while high salinity acclimation enhanced both hypo-osmotic regulation and high salinity tolerance. Lucu (10) showed that Leptomysis mediterranea lacked the ability to hyperegulate in low salinity seawater and demonstrated limited tolerance to dilute sea water. Several laboratory studies have examined functional responses of Mysidopsis bahia (4,5) while resistance patterns to salinity, temperature, and age on growth in this species have also been studied (12,13). Salinity gradients in southern African estuaries characteristically exhibit a high degree of temporal and spatial variability. The climate in the region is predominantly semi-arid with a mean annual rainfall of less than 500 mm (6). River discharge into estuaries along much of the coast is temporarily variable, and often concentrated in sporadic events. Droughts are common and dry periods are often
punctuated by floods of varying magnitude. Estuarine salinity gradients are therefore highly variable and it is not unexpected that mysids investigated in the present study demonstrated a tolerance to a broad salinity range under experimental conditions. These data also conform to the euryhalinity of the three genera noted by earlier workers (2,8,9,11,16,19). Despite their euryhalinity, M. wooldridgei, R. terranatalis, and G. brevifissura occur in discrete, but overlapping zones along axial salinity gradients in local estuaries (23,24,27). Current data suggest that salinity per se does not play the major role in establishing the observed spatial distribution patterns of these species. Only at times of large scale flushing with fresh water do mysid populations disappear from regions of an estuary which become temporarily oligohaline. In extreme cases, mysids may disappear completely from flooded estuaries, but recolonization is rapid and adults are
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TABLE 1. Linear regression analysis for body fluid osmol-
ality in the four species of mysid shrimps. Comparison of mean body fluid osmolality between mysid species by Analysis of Covariance (ANCOVA) with environmental salinity specified as the co-variance also given Linear regression Species
n
Intercept*
Slope
p†
r2
M. wooldridgei R. terranatalis G. psammodytes G. brevifissura
35 32 37 8
534.2 772.3 838.8 897.1
0.109 0.108 0.356 0.312
0.58 0.25 ,0.001 0.418
0,009 0,047 0,705 0,112
ANCOVA‡ Body fluid osm. Mean 606.78 846.27 1080.28 1101.24
Tukey test SE 24.01 24.01 23.80 51.69
of this species. Similarly, G. psammodytes occurs on beaches having medium-fine sands and exposed to wave action (25). Although restricted to the marine environment, G. psammodytes is an efficient osmoregulator and is able to tolerate salinity values down to 8‰ under experimental conditions (1). Brown and Talbot (1) recorded that G. psammodytes survived a 25% dilution of sea water with no ill effect, while 50% dilution caused some mortality during experiments lasting 42 hr. A survival rate of 25% was recorded after 42 hr in water having a salinity of 8–9‰, although mysids showed immediate stress when subjected to this low salinity. In these experiments, however, mysids were transferred directly from sea water to diluted media. CONCLUSION
* * * —
*All intercepts significantly greater than zero (min. t 5 3.42; df 5 34; p , 0.01). †Probability of slope 5 0 (F-test). All slopes are significantly different to the isosmotic line. ‡F 5 94.63; df 5 2,100; p , 0.0001. Tukey multiple range test. Asterisks indicate a statistically significant difference between the body fluid osmolality of each species. Gastrosaccus brevifissura is not included because of low sample number.
again present within weeks (27). After such flooding events, some mysids may remain in isolated pockets of estuarine water unaffected by the floodwaters, but major re-colonisation of an estuary probably occurs from the sea (23,27). Although salinity must play a role, factors such as water depth, predation, food availability, and substrate type are probably more important in establishing observed spatial distribution patterns. Whereas immature R. terranatalis migrate into surface waters after dark, most of the adult population remains in deeper water, both day and night (26,27) where they are presumably less vulnerable to visually orientated predators. Food availability appears to be a major factor regulating diel shoreward migration patterns of M. wooldridgei in inshore waters of Algoa Bay (21,22) while predation by R. terranatalis is a major determinant of the distribution of M. wooldridgei in estuaries (28). Substrate selection experiments indicate that G. brevifissura strongly selects for sandy sediments. These results support field data, showing that the species, occurs over sandy bottoms near the tidal inlets of the Sundays, Gamtoos, and Swartkops estuaries (23,27). In the Gamtoos estuary, the muddy sediments of the middle reaches give way to sand 17 km upstream from the mouth where salinity values range between 10 and 20‰. Gastrosaccus brevifissura is again abundant in these upper reaches of the estuary (18). Sediment distribution within estuaries therefore appears to be the predominant physical factor affecting the distribution
Although three species of mysids occur in clear zones along salinity gradients in estuaries along the south-east coast of South Africa, salinity per se does not appear to be the major regulating factor of spatial distribution, except under flood conditions. Although sex and size classes showed no difference in osmoregulatory capabilities within species, differences were apparent between species. However, all mysids studied were efficient osmoregulators. Available information suggests that biological and physical factors other than salinity are more important in determining spatial distribution of the four species in coastal waters. These factors include food availability, substrate type, predation, and water depth. Financial assistance was provided through the FRD, Estuaries Joint Venture Programme and the University of Port Elizabeth.
References 1. Brown, A.C.; Talbot, M.C. The ecology of the sandy beaches of the Cape Peninsula, South Africa. Part 3: A study of Gastrosaccus psammodytes Tattersall (Crustacea: Mysidacea). Trans. Roy. Soc. S. Afr. 40:309–333;1972. 2. Bhattycharya, S.S. Salinity and temperature tolerance of juvenile Mesopodopsis orientalis: Laboratory studies. Hydrobiologia 93:23–30;1982. 3. Bobovich, M.A. Osmotic regulation in the brackish-water mysid Neomysis integer (Leach). Sov. J. Ecol. 7:368–370;1976. 4. De Lisle, P.F.; Roberts, Jr., M.H. The effect of acclimation on salinity tolerance of the mysid, Mysidopsis bahia Molenock. Comp. Biochem. Physiol. 85A:383–387;1986. 5. De Lisle, P.F.; Roberts, Jr., M.H. Osmoregulation in the estuarine mysid, Mysidopsis bahia Molenock: Comparison with other mysid species. Comp. Biochem. Physiol. 88A:369–372; 1987. 6. Department of Water Affairs. Management of the water resources of southern Africa. Pretoria: Government Printers; 1986. 7. Grasshoff, K. Methods of seawater analysis. Weinheim-New York: Verlag Chemie; 1976. 8. Greenwood, J.G.; Jones, M.B.; Greenwood, J. Salinity effects on brood maturation of the mysid crustacean Mesopodopsis slabberi. J. Mar. Biol. Assoc. U.K. 69:683–694;1989. 9. Hodge, D. The distribution and ecology of the mysids in the
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11. 12. 13.
14. 15. 16.
17. 18.
Brisbane River. University of Queensland Papers. Department of Zoology 2:91–104;1963. Lucu, C. Sodium balance and salinity tolerance of the mysid Leptomysis mediterranea. In: McLusky, D.S., Berry, A.J. (eds). Physiology and Behavior of Marine Organisms. New York: Pergamon Press; 1978:95–103. Mauchline, J. The biology of mysids and euphausiids. In: Blaxter, J.H.S.; Russell, F.S.; Yonge, M. (eds). Adv. Mar. Biol. 18 London: Academic Press; 1980:1–369. McKenney, Jr., C.L. Resistance patterns to salinity and temperature in an estuarine mysid (Mysidopsis bahia) in relation to its life cycle. Comp. Biochem. Physiol. 109A:199 –208;1994. McKenney, Jr., C.L.; Celestial, D.M. Interactions among salinity, temperature and age on growth of the estuarine mysid Mysidopsis bahia reared in the laboratory through a complete life cycle. 1. Body mass and age-specific growth rate. J. Crust. Biol. 15:169–178;1995. McLusky, D.S.; Heard, V.E.J. Some effects of salinity on the mysid Praunus flexuosis. J. Mar. Biol. Assoc. U.K. 51:709–715; 1971. McLusky, D.S.; Hagerman, L.; Mitchell, P. Effects of salinity acclimation on osmoregulation in Crangon crangon and Praunus flexuosus. Ophelia 21:89–100;1982. Moffat, A.M.; Jones, M.B. Correlation of the distribution of Mesopodopsis slabberi (Crustacea, Mysidacea) with physicochemical gradients in a partially mixed estuary (Tamar, England). Neth. J. Aquat. Ecol. 27:155–162;1993. Remane, A.; Schleiper, C. Die Biologie des Brakwassers Stuttgart: E. Scweizerbartsche, 1958. Schlacher, T.A.; Wooldridge, T.H. Tidal influence on distribution and behaviour of the estuarine opossum shrimp Gastrosaccus brevifissura. In: Dyer, K.R.; Orth, R.J. (eds). Changes in
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19. 20. 21. 22. 23.
24. 25. 26. 27. 28.
fluxes in estuaries: Implications for Science and Management. Denmark: Olsen and Olsen; 1994:307 –312. Tattersall, W.M.; Tattersall O.S. The British Mysidacea. London: Ray Society; 1951. Underwood, A.J. Techniques of analysis of variance in experimental marine biology and ecology. Oceanogr. Mar. Biol. Ann. Rev. 19:513–605;1981. Webb, P.; Wooldridge, T.H. Diel horizontal migration of Mesopodopsis slabberi (Crustacea, Mysidacea) in Algoa Bay, southern Africa. Mar. Ecol. Prog. Ser. 62:73–77;1990. Webb, P.; Perissinotto, R.; Wooldridge, T.H. Feeding of Mesopodopsis slabberi (Crustacea, Mysidacea) on naturally occurring phytoplankton. Mar. Ecol. Prog. Ser. 36:115–123;1987. Wooldridge, T.H. Ecology of beach and surf-zone mysid shrimps in the Eastern Cape, South Africa. In: McLachlan, A.; Erasmus, T. (eds). Sandy beaches as ecosystems. The Hague: Junk Publishers; 1983:449–460. Wooldridge, T.H. Distribution, population dynamics and estimates of production for the estuarine mysid, Rhopalophthalmus terranatalis. Estuar. Cstl. Shelf Sci. 23:205–223;1986. Wooldridge, T.H. The spatial and temporal distribution of mysid shrimps and phytoplankton accumulations in a high energy surf-zone. Vie Milieu 39:127–133;1989. Wooldridge, T.H. Utilization of tidal currents by estuarine zooplankton. Estuar. Cstl. Mar. Sci. 11:107–114;1980. Wooldridge, T.H.; Bailey C. Euryhaline zooplankton of the Sundays estuary and notes on trophic relationships. S. Afr. J. Zool. 17:151–163;1982. Wooldridge, T.H.; Webb P. Predator prey interactions between two species of estuarine mysid shrimps. Mar. Ecol. Prog. Ser. 50:21–28;1988.
ISSN 0300-9629/97/$17.00 PII S0300-9629(96)00235-6
Osmoregulation and Spatial Distribution in Four Species of Mysid Shrimps Paul Webb, Tris Wooldridge, and Thomas Schlacher University of Port Elizabeth, Institute for Coastal Research and Department of Zoology, P.O. Box 1600, Port Elizabeth 6000, South Africa ABSTRACT. This study investigates the osmoregulatory capacities of four mysid species, viz. Mesopodopsis wooldridgei, Rhopalophthalmus terranatalis, and Gastrosaccus brevifissura which occur in different zones along salinity gradients in southern African estuaries, and the psammophilic, surf-zone mysid, Gastrosaccus psammodytes. All four species maintain body fluid concentrations at species-specific levels over a range of environmental salinities. Analysis of Covariance (ANCOVA) indicated no statistical difference in body fluid concentrations among juveniles, immatures, males, and females for M. wooldridgei, R. terranatalis, and G. brevifissura. Available information suggests that biological and physical factors other than salinity are more important in determining spatial distribution of these four species in coastal waters. comp biochem physiol 117A;4:427–431, 1997. 1997 Elsevier Science Inc. KEY WORDS. Distribution, estuary, mysids, osmoregulation, salinity, sediment, South Africa, species
INTRODUCTION Mesopodopsis wooldridgei is abundant in estuarine and nearshore marine waters of southern Africa (23). Although present in marine waters, Rhopalophthalmus terranatalis and Gastrosaccus brevifissura are more typical of estuaries while Gastrosaccus psammodytes is restricted to the surf-zone (23,27). Few data are available on the osmoregulatory capabilities of these mysids but all belong to genera which contain exceptionally euryhaline species (2,8,9,11,16,19). This study investigates the osmoregulatory capacities of M. wooldridgei, R. terranatalis, G. brevifissura, and G. psammodytes in relation to their distribution in southern African coastal waters. The estuarine species are reported from habitats having water of widely different salinity values, although distributions do overlap to some extent. This prompted the question whether salinity is a major determinant in establishing observed mysid spatial distribution patterns. Of the three estuarine species studied, only G. brevifissura burrows actively into the sediment and therefore substrate preference by this species was also tested. METHODS Samples from schools of predominantly M. wooldridgei or R. terranatalis were seine-netted at stations covering a range of salinities (7–28‰) in the tidal Gamtoos estuary. Additional mysid samples were collected at a single site in the Address reprint requests to: Prof. P. Webb, Department of Zoology, University of Port Elizabeth, P.O. Box 1600, Port Elizabeth 6000, South Africa. Tel. 127 41 558718; Fax 127 41 564519; E-mail: [email protected].
temporary closed Kabeljous estuary (43.8‰ salinity). Captured animals were immediately placed in a sieve and flushed with fresh water. Excess surface water was removed by vigorously shaking the sieve, after which animals were transferred to 40-ml poly-vinyl-chloride (PVC) containers and frozen on dry ice. At no time did animals defrost in the field or during transport to the laboratory. Water salinity was recorded in situ at each sampling site using a Valeport Series 600 CTD. Water samples were taken from depths at which the animals were captured. Gastrosaccus brevifissura was only found in relatively high numbers at a single site in the Gamtoos estuary (salinity 24‰). Live animals were transported to the laboratory in 20-l containers of estuarine water collected at the sample site. In the laboratory sub-samples of 60–70 live G. brevifissura were transferred to 20-l PVC containers with 10-l of 24‰ estuarine water. The salinity of water in three sets of three containers was gradually reduced over 3 hr to 21, 15, and 10‰ by the addition of low salinity estuarine water (5‰, collected in the upper Gamtoos), while in three other containers salinity was increased to 30‰ by gradually adding seawater. Animals were also kept in three containers of 24‰ estuarine water. Three hours after the desired salinity values had been reached, live animals were collected by gently pouring the water through a medium sieve. Individuals were briefly washed with distilled water, vigorously shaken in the sieve to remove excess water, and then deep frozen. Gastrosaccus psammodytes was collected with a bottomsledge from the lower intertidal of an exposed sandy beach (Maitlands Beach) and transported to the laboratory in sea-
P. Webb et al.
428
water collected at the sampling site. In the laboratory, groups of G. psammodytes were transferred to 19 experimental containers having salinity values ranging from 0.5–36‰ and at a temperature of 23 6 1°C. Salinity of the experimental units was determined to the nearest 0.01‰ Cl2 by standard AgNO3 titration following Grasshof (7). Introduction to different salinity values was done using the same techniques employed for G. brevifissura. Prior to measurements of body fluid osmotic concentration, samples were allowed to partly defrost and sorted into juvenile (no secondary sexual characteristics visible), immature (secondary sexual characteristics beginning to develop), and adult male and female categories within each species. Individual animals were placed on filter-paper disks for 5 to 10 min to remove any excess surface water. Five to sixty animals, depending on size of the individuals in each category, were transferred to PVC Eppendorf tubes, homogenised and centrifuged for 6 min. Eight µl of supernatant were analysed in a Wescor 5100 vapour pressure osmometer which was re-calibrated and thermally balanced after every 12 measurements. Water samples collected from the corresponding estuarine sampling sites were also analysed for osmolality. From each sample collected in the field, 1–15 determinations of body fluid osmolality were made for specific age and sex classes. The number of determinations depended on size of the mysid sample collected at a particular site. Linear regressions were fitted to plots of mean body fluid osmolality and environmental osmolality. All individual values obtained in laboratory experiments were included in regression plots since samples were independent by design. Mean body fluid osmolalities were compared by size and sex classes within a species, and between species by analysis of covariance (ANCOVA), specifying environmental osmolality as the co-variate. In substrate preference experiments, five 10-l buckets with 28 cm diameter bases were used as experimental units. The bottom of each container was partitioned into five equal segments. Each of the five different sediment types, classified according to mean grain diameter (Md), was introduced to a segment in each bucket. The sediment types were: coarse sand (Md . 0.6 mm), medium-fine sand (Md 5 0.4 mm), fine sand of marine origin collected from the tidal delta (Md 5 0.21 mm), very fine sand with high mud content collected from the middle reaches of the estuary (Md 5 0.08 mm, 50% mud), and riverine sand deposited in the uppermost reaches of the estuary (Md 5 0.20 mm, 2.3% mud). The volume of sediment in each segment was 250 cm3. The allocation of sediment to a segment, as well as the orientation of the containers relative to natural light, was random. Eight litres of 24‰ estuarine water was added gradually to each bucket which was then stocked with 14– 44 animals. After 15 hr of light exposure, all G. brevifissura had left the water column. Water was gently siphoned off, sediments removed separately from their segments and any
animals present were sieved out and counted under a dissecting microscope. Data obtained were analysed by oneway, fixed effects ANOVA. RESULTS Analysis of Covariance (ANCOVA) indicated no statistical difference in body fluid concentrations among juveniles, immatures, males, and females for M. wooldridgei, R. terranatalis, and G. brevifissura. All four species showed strong osmoregulatory capabilities (Fig. 1) with significant differences in body fluid concentrations between species (Table 1). Body fluid concentrations in M. wooldridgei and R. terranatalis were hyperosmotic to the medium in water osmolalities below 600 mOsm and 845 mOsm, respectively, but hypotonic to more concentrated media. Gastrosaccus brevifissura and G. psammodytes maintained hyperosmotic body fluid concentrations at all salinity values tested, including seawater. Gastrosaccus brevifissura is difficult to capture in large numbers and, because of the small size of these animals, at least 60 individuals were required to make available enough body fluid per sample for osmotic concentration analysis using a vapour pressure osmometer. The number of animals available determined that only eight data points of body fluid osmotic concentration were obtained. These data were not included in the analysis of covariance, since a strongly unbalanced design usually leads to a serious loss of efficiency and/or reliability in the interpretation of such data (20). However, available data suggest that G. brevifissura is also an effective osmoregulator. Gastrosaccus brevifissura showed a clear preference for sand with little or no mud content (r 5 13.13, df 5 4,20; p , 0.001,) in substrate selection experiments. Ninety-five percent of all G. brevifissura selected sandy substrates and Tukey’s Multiple Range Test revealed no significant difference in regard to the grade of sand selected. DISCUSSION Work on osmotic regulation in mysids was first reported for Praunus inermis, which maintained a constant body fluid osmolarity over a 5–32‰ salinity range (3). Remane and Schleiper (17) noted that Mysis oculata tolerated a salinity range from fresh to 100% seawater. McLusky and Heard (14) showed clear patterns of osmotic regulation in Praunus flexuosus and Neomysis integer, with both species exercising hyperosmotic and hypo-osmotic regulation depending on the salinity of the medium in which they were maintained. Bobovich (3) also worked on N. integer and investigated the dynamics of salt loss by animals acclimated in water of various salinities while Bhattacharya (2) showed that water of reduced salinity was a major factor associated with increasing abundance of juvenile Mesopodopsis orientalis. McLusky, Hagerman, and Mitchell (15) observed en-
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429
FIG. 1. Body-fluid osmolality in Mesopodopsis wooldridgei (A), Rhopalophthalmus terranatalis (B), Gastrosaccus psammo-
dytes (C) and Gastrosaccus brevifissura (D) vs environmental salinity.
hanced hyperosmotic regulation with low salinity acclimation in P. flexuosus, while high salinity acclimation enhanced both hypo-osmotic regulation and high salinity tolerance. Lucu (10) showed that Leptomysis mediterranea lacked the ability to hyperegulate in low salinity seawater and demonstrated limited tolerance to dilute sea water. Several laboratory studies have examined functional responses of Mysidopsis bahia (4,5) while resistance patterns to salinity, temperature, and age on growth in this species have also been studied (12,13). Salinity gradients in southern African estuaries characteristically exhibit a high degree of temporal and spatial variability. The climate in the region is predominantly semi-arid with a mean annual rainfall of less than 500 mm (6). River discharge into estuaries along much of the coast is temporarily variable, and often concentrated in sporadic events. Droughts are common and dry periods are often
punctuated by floods of varying magnitude. Estuarine salinity gradients are therefore highly variable and it is not unexpected that mysids investigated in the present study demonstrated a tolerance to a broad salinity range under experimental conditions. These data also conform to the euryhalinity of the three genera noted by earlier workers (2,8,9,11,16,19). Despite their euryhalinity, M. wooldridgei, R. terranatalis, and G. brevifissura occur in discrete, but overlapping zones along axial salinity gradients in local estuaries (23,24,27). Current data suggest that salinity per se does not play the major role in establishing the observed spatial distribution patterns of these species. Only at times of large scale flushing with fresh water do mysid populations disappear from regions of an estuary which become temporarily oligohaline. In extreme cases, mysids may disappear completely from flooded estuaries, but recolonization is rapid and adults are
P. Webb et al.
430
TABLE 1. Linear regression analysis for body fluid osmol-
ality in the four species of mysid shrimps. Comparison of mean body fluid osmolality between mysid species by Analysis of Covariance (ANCOVA) with environmental salinity specified as the co-variance also given Linear regression Species
n
Intercept*
Slope
p†
r2
M. wooldridgei R. terranatalis G. psammodytes G. brevifissura
35 32 37 8
534.2 772.3 838.8 897.1
0.109 0.108 0.356 0.312
0.58 0.25 ,0.001 0.418
0,009 0,047 0,705 0,112
ANCOVA‡ Body fluid osm. Mean 606.78 846.27 1080.28 1101.24
Tukey test SE 24.01 24.01 23.80 51.69
of this species. Similarly, G. psammodytes occurs on beaches having medium-fine sands and exposed to wave action (25). Although restricted to the marine environment, G. psammodytes is an efficient osmoregulator and is able to tolerate salinity values down to 8‰ under experimental conditions (1). Brown and Talbot (1) recorded that G. psammodytes survived a 25% dilution of sea water with no ill effect, while 50% dilution caused some mortality during experiments lasting 42 hr. A survival rate of 25% was recorded after 42 hr in water having a salinity of 8–9‰, although mysids showed immediate stress when subjected to this low salinity. In these experiments, however, mysids were transferred directly from sea water to diluted media. CONCLUSION
* * * —
*All intercepts significantly greater than zero (min. t 5 3.42; df 5 34; p , 0.01). †Probability of slope 5 0 (F-test). All slopes are significantly different to the isosmotic line. ‡F 5 94.63; df 5 2,100; p , 0.0001. Tukey multiple range test. Asterisks indicate a statistically significant difference between the body fluid osmolality of each species. Gastrosaccus brevifissura is not included because of low sample number.
again present within weeks (27). After such flooding events, some mysids may remain in isolated pockets of estuarine water unaffected by the floodwaters, but major re-colonisation of an estuary probably occurs from the sea (23,27). Although salinity must play a role, factors such as water depth, predation, food availability, and substrate type are probably more important in establishing observed spatial distribution patterns. Whereas immature R. terranatalis migrate into surface waters after dark, most of the adult population remains in deeper water, both day and night (26,27) where they are presumably less vulnerable to visually orientated predators. Food availability appears to be a major factor regulating diel shoreward migration patterns of M. wooldridgei in inshore waters of Algoa Bay (21,22) while predation by R. terranatalis is a major determinant of the distribution of M. wooldridgei in estuaries (28). Substrate selection experiments indicate that G. brevifissura strongly selects for sandy sediments. These results support field data, showing that the species, occurs over sandy bottoms near the tidal inlets of the Sundays, Gamtoos, and Swartkops estuaries (23,27). In the Gamtoos estuary, the muddy sediments of the middle reaches give way to sand 17 km upstream from the mouth where salinity values range between 10 and 20‰. Gastrosaccus brevifissura is again abundant in these upper reaches of the estuary (18). Sediment distribution within estuaries therefore appears to be the predominant physical factor affecting the distribution
Although three species of mysids occur in clear zones along salinity gradients in estuaries along the south-east coast of South Africa, salinity per se does not appear to be the major regulating factor of spatial distribution, except under flood conditions. Although sex and size classes showed no difference in osmoregulatory capabilities within species, differences were apparent between species. However, all mysids studied were efficient osmoregulators. Available information suggests that biological and physical factors other than salinity are more important in determining spatial distribution of the four species in coastal waters. These factors include food availability, substrate type, predation, and water depth. Financial assistance was provided through the FRD, Estuaries Joint Venture Programme and the University of Port Elizabeth.
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