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A comparison of the capabilities of juvenile and adult Penaeus setiferus and Penaeus stylirostris to regulate the osmotic, sodium, and chloride concentrations in the hemolymph
03O+Yb?Y XI 040677.04102
00 0
A COMPARISON OF THE CAPABILITIES OF JUVENILE AND ADULT PENAEUS SETZFERUS AND PENAEUS STYLIROSTRIS TO REGULATE THE OSMOTIC, SODIUM, AND CHLORIDE CONCENTRATIONS IN THE HEMOLYMPH FRANK Department
Agricultural
L. CASTILLE, JR* and ADDISON L. LAWRENCEI
of Biology,
University of Houston, Houston. TX 77004, IJSA and :Texas Experiment Station and Extension Service, Texas A & M University, P.O. Drawer Q, Port Aransas, TX 78373, U.S.A. (Received
31 Julg 1980)
The capabilities ofjuvenile and mature adult Penaeus setiferus and P. stylirostris to regulate the osmotic. sodium, and chloride concentrations in the hemolymph are compared. 2. In P. set@-us and P. stylirosrris acclimated to salinities of 9.8 and 10.8”,,, respectively, juvenile shrimp are stronger hyperosmotic and hyperionic regulators than adults. However, the reduced regulatory capabilities of adult shrimp are not sufficient to require migration to offshore waters for survival. and hypoionic regulators than 3. At 40.4’:,,, juvenile P. setiferus are more effective hypoosmotic adults. However, there is no difference between the regulatory capabilities of juvenile and adult P. stylirostris at 36.2”,,,,. 4. Differences in hemolymph concentration between juvenile and adult P. setjferus at 23.5”<,,, indicate that the isosmotic and isoionic crossover concentrations are elevated with maturation.
Abstract-l.
INTRODUCTION
to more saline conditions is that higher salinities are necessary for either hatching of the eggs or larval development. In freshwater caridean shrimp such as Macrobrachium, only the females migrate to higher salinities after mating in freshwater (Wickins, 1976). However, in penaeidean shrimp, this pattern of migration has only been observed in Metupenaeus brevicornis and M. bennettae (Dali, 1957; Shaikhmahmud & Tembe, 1960). The purposes of this study are to: (1) compare the osmoregulatory capabilities of juvenile and adult P. set@ws (L) and P. stylirostris Stimpson, and (2) determine whether differences between juveniles and adults can account for migratory behavior by adults.
A common pattern of migratory behavior in euryhaline penaeid shrimp is the return of mature animals to more saline conditions to breed (George & Vedavyasa Rao, 1968; Panikkar, 1968; Perez Farfante, 1969). Since mature shrimp are not found at low salinities and circumstantial evidence indicates that shrimp which are unable to reach suitably saline water do not mature (George & Vedavyasa Rao, 1968; Tuma, 1967; Liao & Huang, 1972), a possible explanation for this return to higher salinities is a change in the osmoregulatory capabilities with maturation. Panikkar (1968) concluded that the wider distribution of juvenile penaeids in brackish water indicates that adaptation to low salinities is more highly developed in the younger stages than in full-sized individuals, particularly those in the reproductive phases. He suggested that the decreased osmoregulatory capability necessitates the migration of adult shrimp back to the sea. This hypothesis is consistent with published determinations of osmotic and ionic regulatory capabilities. Although earlier studies have been performed at different temperatures and have used different acclimation procedures, the ones using juvenile animals (Williams, 1960; Castille & Lawrence, 1980) have found lower isosmotic and isoionic crossover concentrations than those using subadult and adult animals (McFarland & Lee, 1963; Bursey & Lane, 1971). An alternative
explanation
MATERIALS
METHODS
Juvenile P. setiferus were obtained from commercial bait dealers at Galveston, Texas and adults were captured by trawling offshore from Port Aransas, Texas. Juvenile P. srrlirosrris were obtained from the Texas A & M Mariculture Facility at Corpus Christi. Texas and adults from the National Marine Fisheries Service, Galveston, Texas. Animals were maintained and acclimated to the experimental salinities as previously described (Castille & Lawrence. 1980). The experimental salinities were chosen so that the shrimp would be hyperosmotic at the lowest salinity, hypoosmotic at the highest salinity, and near osmotic equilibrium at the intermediate salinity. Although both male and female juvenile shrimp were used, all adult shrimp were sexually mature males. For P. seriferus, the mean length was 85 mm with a standard deviation of 15 mm (23 replicates) for juveniles, and 177 mm with a standard deviation of 7 mm (17 replicates) for adults. For P. srylirosrris, the mean length was 68 mm with a standard deviation of 6 mm (24 replicates) for juveniles, and 174 mm with a standard deviation of 5 mm (10 replicates) for adults.
for the return of adults
* Present address: Texas A & M University, Drawer Q, Port Aransas, TX 78373, U.S.A.
AND
P.O. 677
FRANK L. CASTILLE.JR and ADDISONL. LAWRENCE
678 Table I. The osmotic,
sodium, and chloride concentrations setiferus. Values represent mean k SEM parentheses
Penueus
adult
Salinity
23.5
Sodium concentration
(r~~N/kg )
Chloride concentration (Wkg)
729
683
852
900
1160
f6
t6
+12
+1a
+18
(4)
(7)
(6)
(6)
(7)
(7)
(7)
244
109
303
328
308
378
416
549
Cl0
+4
+3
f2
f6
+13
+8
(4)
(4)
(4)
(5)
(6)
(6)
(7)
(7)
(7)
275
240
133
307
321
365
372
414
631
+2
+8
+2
+3
+3
?12
+10
f18
(6)
(4)
(10)
(6)
(6)
(6)
(7)
(7)
543
285
+4
t13
+6
(6)
(4)
272 i5
+ii (4)
1-8
ception of the sodium concentration in P. setiferus where P < 0.05) than in adults. At the highest salinities, the osmotic, sodium, and chloride concentrations in the hemolymph of P. setiferus juveniles are significantly lower (P < 0.05) than those in adults, but in P. stylirosrris, juveniles and adults do not significantly differ (P > 0.05). At the intermediate salinity of 23.5”,,,,, juvenile P. setiferus are isosmotic to seawater and adults are hyperosmotic. The osmotic, sodium, and chloride concentrations in the hemolymph of juveniles are significantly less (P < 0.001, 0.05, and 0.01 respectively) than those of adults. In P. stylirosrris at the intermediate salinity of 20.2”,,,,, both juveniles and adults regulate hemolymph osmotic and sodium concentrations above seawater levels. Differences in osmotic and sodium concentrations between juveniles and adults are not significant (P > 0.05). However, hemolymph chloride concentrations, which are hypoionic to seawater in both juveniles and adults, are significantly higher (P < 0.01) in adults than in juveniles.
The osmotic and ionic concentrations of the hemolymph and seawater are shown for P. serjferus in Table 1, and for P.stylirostris in Table 2. At the lowest salinities, the osmotic, sodium, and chloride concentrations in the hemolymph of juveniles of both species are significantly greater (P c 0.001 with ex-
2. The osmotic. sodium. and chloride concentrations in the hemolymph Penaeus sfylirosrris. Values represent mean + SEM for the number parentheses Salinity 10.8
Sodium concentration (Wkg)
Chloride concentration (m?Vkg)
20.2 seawater
624
405
314
+5
+6
+3
(8)
(3)
278 ?2
of juvenile and of replicates in
of seawater
'loo
adult
juvenile
(mOs/kg)
seawater
f5
626
RESULTS
Osmotic concentration
o/oo
adult
675
seawater
Hemolymph was sampled by inserting a 5 pl pipet (Drummond Microcaps) into the pericardial cavity. The gill chambers as well as the membrane between the posterior edge of the carapace and the first abdominal segment were dried with absorbent paper prior to sampling to prevent contamination of the hemolymph with seawater. Only intermolt animals were sampled. The osmotic. sodium and chloride concentrations in the hemolymph and seawater were determined as previously described (Castille & Lawrence, 1980). Differences between hemolymph concentrations of juvenile and adult shrimp were analyzed for significance with r-tests for differences between two means.
Table adult
juvenile
seawater
adult
juvenile
40.4
'loo
adult
juvenile
(mOs/kg)
of juvenile and of replicates in
of seawater
9.8 o/o0
Osmotic concentration
in the hemolymph for the number
Juvenile
O/o0
36.2
adult
seawater
665
669
588
834
+5
+a
+2
+10
(5)
(8)
(6)
(51
239
158
300
310
+6
+8
+5
t3
Juvenile
'ho
adult
seawater
811
1046
(8)
'3 (5)
(5)
279
364
344
466
t6
+6
t10
+6
Cl
(7)
(3)
(4)
(5)
(5)
(5)
(7)
(5)
(5)
272
229
172
301
318
371
373
375
562
+4
+4
+6
+2
+5
t2
$5
+3
t8
(7)
(3)
(4)
(7)
(5)
(4)
(7)
(5)
(3)
Osmoregulatory
comparison
DISCUSSION
The differences in hemolymph concentrations at low salinities between juvenile and adult shrimp demonstrate that juveniles are better hyperosmotic and hyperionic regulators than adults. This may explain why adults are not as well adapted to low salinities as juveniles and possibly why mature animals are not found at low salinities. A similar pattern of salinity distribution based on animal size is found in the hermit crab, Pagurus bernhardus. Large hermit crabs are less common than small animals in the euryhaline littoral zone and are much less tolerant of lower salinities than small animals (Davenport, 1972). As well as being stronger hyperosmotic regulators at low salinities, juvenile P. sefiferus are also stronger hypoosmotic regulators at high salinities than adults. This is consistent with observations that post larval and juvenile P. set@us are not adversely affected by hypersaline conditions. In Laguna Madre, an estuary on the south Texas coast, small P. setiferus have been found at salinities as high as 4&O”,,,, (Hildebrand, 1958) and 41.3”<,, (Gunter, 1961). Zein-Eldin (1963) reported that, in laboratory experiments, postlarval P. setiferus survive and grow at salinities up to 40”;,. In contrast to the findings for P. settferus, this study does not reveal any differences in the osmoregulatory capabilities of juvenile and adult P. stylirostris at the salinity of 36.2’:,,. However, it is possible that at higher salinities, differences may become apparent. The data obtained at the intermediate salinity explain why differences have been reported in the determination of the isosmotic crossover concentrations between juvenile and adult animals. The isosmatic crossover of between 830 and 850mOs/kg reported by McFarland & Lee (1963) for P. seti&erus of length in excess of IOOmm greatly exceeds that of 673 mOs/kg reported for 85 mm juveniles by Castille and Lawrence (1980). Since in this study at 23.5”,,,,, juveniles are hypoosmotic to seawater and adults are hyperosmotic, there must be an elevation of the isosmotic crossover salinity with maturation. A similar increase in the isosmotic crossover concentration has been reported in the caridean shrimp, Crnngon sepremspinosa (Haefner, 1969). Juvenile C. septemspinosa at both 5 and 15’C, exhibit isotonicity at a lower salinity (22-23”,,,,) than do adults at 5 (25”(,,,)and 15 C (27-28”,,,,). Results obtained for P. sfy/irostriS at the intermediate salinity are more equivocal. Even though both the osmotic and sodium concentrations in the hemolymph are maintained above the concentrations in seawater at 20.2”, hemolymph concentrations do not differ in juveniles and adults as at 10X’,,,,. In contrast, the chloride concentrations are lower in the hemolymph than seawater at 20.2”,,,, and the adults regulate chloride at a higher concentration in the hemolymph than do juveniles. Yet this pattern is not reflected at the higher salinity of 36.2”,,,, Although this study demonstrates that the capacities of hyperosmotic regulation in mature P. setiferus and P. stylirostris are reduced, adult animals are still capable of hyperosmotic regulation at salinities below the isosmotic concentration. This implies that even though movement out of low salinity waters can be explained by reduced osmoregulatory capabilities,
of juvenile
and adult shrimp
679
migration to offshore waters is not directly necessitated by osmotic regulation. In contrast to Pogurus bernhardus, in which the salinity distribution is also determined by animal size, adult shrimp are able to tolerate dilution of the internal concentrations at the lower experimental salinities. Migration to offshore waters is probably caused by factors other than a direct effect of environmental salinity on adult shrimp. If the highly developed capacity for hyperosmotic regulation is not needed in adults, there is no selective advantage for its maintenance. Acknowledgemenrs-This work is a result of a research program sponsored in part by Texas A & M University Sea Grant College program, supported by the National Oceanic and Atmospheric Administration, Office of Sea Grant, Department of Commerce under Grant No. 47-158-44105 and a Moody Foundation Grant to the University of Houston, Addison L. Lawrence. principal investigator. The authors wish to express their gratitude and appreciation to Nathan Howe, David Mailman, and Brian Middleditch of the University of Houston for reviewing this manuscript prior to submission for publication.
REFERENCES BURSEY C. R. & LANE C. E. (1971) Osmoregulation in the pink shrimp Penaeus duorarum Burkenroad. Camp. Biothem. Physiol. 39A, 483493. CASTILLE F. L., JR & LAWRENCE A. L. (1980) The effect of salinity on the osmotic, sodium, and chloride concentrations in the hemolymph of euryhaline shrimp of the genus Penaeus. Comp. Biochem. Physiol. (In press). DALL W. (1957) Observations on the biology of the greentail prawn, Metapenaeus musrersii (Haswell) (Crustacea Decapoda: Penaeidae). Ausr. J. mar. Freshwut. Res. 9, 111-134. DAVENPORT J. (1972) Effects of size upon salinity tolerance and volume regulation in the hermit crab Pugurus hernhardus. Mar. Biol. 17, 222-227. GEORGE M. J. & VEDAVYASA RAO P. (1968) Observations on the development of the external genitalia in some Indian Penaeid prawns, .I. mar. biol. Ass. Indiu 10, 52-70. GUNTER G. (1961) Habitat of juvenile shrimp (family Penaeidae). Ecology 42, 598-600. HAEFNER P. ‘A., JR (i969) Osmoregulation of Crungon septemspinosa Say (Crustacea: Caridea). Biol. Bull. 137, 438446. HILDEBRAND H. H. (1958) Estudios biologicos prelimirares sobre la Laguna Madre de Tamaulipas. Ciens Mer. 17. 151-173. LIAO I. C. & HUANG T. L. (1972) Experiments on the propagation and culture of prawns Taiwan. In, Cousrcd Aauaculture in the Indo-Pacific Reyion (Edited by PILLAY T.‘V. R.), pp. 328-354. Fishing News (Books) Ltd. London MCFARLAND W. N. 8~ LEE B. D. (1963) Osmotic and ionic concentrations of penaeidean shrimps of the Texas Coast. Bull. Mar. Sci. 13, 391417. PANIKKAR N. K. (1968) Osmotic behavior of shrimps and prawns in relation to their biology and culture. Fish. Rep. F. A. 0. 57(2), 527-538. PEREZ FARFANTE 1. (1969) Western Atlantic shrimps of the genus Penaeus. Fishery Bull. Fish Wild/. Sew. U.S. 67, 461-591. SHAIKHMAHMUDF. S. & TEMBE V. B. (1970) Study of Bombay prawns: the seasonal fluctuation and variation in abundance of the commercially important species of Bombay prawns with a brief note on their size. state of maturity and sex ratio. Indian J. Fish. 7, 69981. TUMA D. J. (1967) A description of the development of
680
FRANK L. CASTILLE,JR and ADDISON L. LAWRENCE
primary and secondary sexual characters in the banana prawn, Penaeus merguiensis de Man (Crustacea: Decapoda: Penaeinae). A&. J. mar. Freshwar. Res. 18, 73-88. WICKINS J. F. (1976) Prawn biology and culture. Oceanogr. Mar. Biol. Ann. Rev. 435-507.
WILLIAMSA. B. (1960) The influence of temperature on osmotic regulation in two species of estuarine shrimps (Penaeus). &cl. Bull. 119, 560-571. ZEIN-ELDIN Z. P. (1963) Effect of salinity on growth of postlarval penaeid shrimp. Bid. Bull. 125, 188-196.
00 0
A COMPARISON OF THE CAPABILITIES OF JUVENILE AND ADULT PENAEUS SETZFERUS AND PENAEUS STYLIROSTRIS TO REGULATE THE OSMOTIC, SODIUM, AND CHLORIDE CONCENTRATIONS IN THE HEMOLYMPH FRANK Department
Agricultural
L. CASTILLE, JR* and ADDISON L. LAWRENCEI
of Biology,
University of Houston, Houston. TX 77004, IJSA and :Texas Experiment Station and Extension Service, Texas A & M University, P.O. Drawer Q, Port Aransas, TX 78373, U.S.A. (Received
31 Julg 1980)
The capabilities ofjuvenile and mature adult Penaeus setiferus and P. stylirostris to regulate the osmotic. sodium, and chloride concentrations in the hemolymph are compared. 2. In P. set@-us and P. stylirosrris acclimated to salinities of 9.8 and 10.8”,,, respectively, juvenile shrimp are stronger hyperosmotic and hyperionic regulators than adults. However, the reduced regulatory capabilities of adult shrimp are not sufficient to require migration to offshore waters for survival. and hypoionic regulators than 3. At 40.4’:,,, juvenile P. setiferus are more effective hypoosmotic adults. However, there is no difference between the regulatory capabilities of juvenile and adult P. stylirostris at 36.2”,,,,. 4. Differences in hemolymph concentration between juvenile and adult P. setjferus at 23.5”<,,, indicate that the isosmotic and isoionic crossover concentrations are elevated with maturation.
Abstract-l.
INTRODUCTION
to more saline conditions is that higher salinities are necessary for either hatching of the eggs or larval development. In freshwater caridean shrimp such as Macrobrachium, only the females migrate to higher salinities after mating in freshwater (Wickins, 1976). However, in penaeidean shrimp, this pattern of migration has only been observed in Metupenaeus brevicornis and M. bennettae (Dali, 1957; Shaikhmahmud & Tembe, 1960). The purposes of this study are to: (1) compare the osmoregulatory capabilities of juvenile and adult P. set@ws (L) and P. stylirostris Stimpson, and (2) determine whether differences between juveniles and adults can account for migratory behavior by adults.
A common pattern of migratory behavior in euryhaline penaeid shrimp is the return of mature animals to more saline conditions to breed (George & Vedavyasa Rao, 1968; Panikkar, 1968; Perez Farfante, 1969). Since mature shrimp are not found at low salinities and circumstantial evidence indicates that shrimp which are unable to reach suitably saline water do not mature (George & Vedavyasa Rao, 1968; Tuma, 1967; Liao & Huang, 1972), a possible explanation for this return to higher salinities is a change in the osmoregulatory capabilities with maturation. Panikkar (1968) concluded that the wider distribution of juvenile penaeids in brackish water indicates that adaptation to low salinities is more highly developed in the younger stages than in full-sized individuals, particularly those in the reproductive phases. He suggested that the decreased osmoregulatory capability necessitates the migration of adult shrimp back to the sea. This hypothesis is consistent with published determinations of osmotic and ionic regulatory capabilities. Although earlier studies have been performed at different temperatures and have used different acclimation procedures, the ones using juvenile animals (Williams, 1960; Castille & Lawrence, 1980) have found lower isosmotic and isoionic crossover concentrations than those using subadult and adult animals (McFarland & Lee, 1963; Bursey & Lane, 1971). An alternative
explanation
MATERIALS
METHODS
Juvenile P. setiferus were obtained from commercial bait dealers at Galveston, Texas and adults were captured by trawling offshore from Port Aransas, Texas. Juvenile P. srrlirosrris were obtained from the Texas A & M Mariculture Facility at Corpus Christi. Texas and adults from the National Marine Fisheries Service, Galveston, Texas. Animals were maintained and acclimated to the experimental salinities as previously described (Castille & Lawrence. 1980). The experimental salinities were chosen so that the shrimp would be hyperosmotic at the lowest salinity, hypoosmotic at the highest salinity, and near osmotic equilibrium at the intermediate salinity. Although both male and female juvenile shrimp were used, all adult shrimp were sexually mature males. For P. seriferus, the mean length was 85 mm with a standard deviation of 15 mm (23 replicates) for juveniles, and 177 mm with a standard deviation of 7 mm (17 replicates) for adults. For P. srylirosrris, the mean length was 68 mm with a standard deviation of 6 mm (24 replicates) for juveniles, and 174 mm with a standard deviation of 5 mm (10 replicates) for adults.
for the return of adults
* Present address: Texas A & M University, Drawer Q, Port Aransas, TX 78373, U.S.A.
AND
P.O. 677
FRANK L. CASTILLE.JR and ADDISONL. LAWRENCE
678 Table I. The osmotic,
sodium, and chloride concentrations setiferus. Values represent mean k SEM parentheses
Penueus
adult
Salinity
23.5
Sodium concentration
(r~~N/kg )
Chloride concentration (Wkg)
729
683
852
900
1160
f6
t6
+12
+1a
+18
(4)
(7)
(6)
(6)
(7)
(7)
(7)
244
109
303
328
308
378
416
549
Cl0
+4
+3
f2
f6
+13
+8
(4)
(4)
(4)
(5)
(6)
(6)
(7)
(7)
(7)
275
240
133
307
321
365
372
414
631
+2
+8
+2
+3
+3
?12
+10
f18
(6)
(4)
(10)
(6)
(6)
(6)
(7)
(7)
543
285
+4
t13
+6
(6)
(4)
272 i5
+ii (4)
1-8
ception of the sodium concentration in P. setiferus where P < 0.05) than in adults. At the highest salinities, the osmotic, sodium, and chloride concentrations in the hemolymph of P. setiferus juveniles are significantly lower (P < 0.05) than those in adults, but in P. stylirosrris, juveniles and adults do not significantly differ (P > 0.05). At the intermediate salinity of 23.5”,,,,, juvenile P. setiferus are isosmotic to seawater and adults are hyperosmotic. The osmotic, sodium, and chloride concentrations in the hemolymph of juveniles are significantly less (P < 0.001, 0.05, and 0.01 respectively) than those of adults. In P. stylirosrris at the intermediate salinity of 20.2”,,,,, both juveniles and adults regulate hemolymph osmotic and sodium concentrations above seawater levels. Differences in osmotic and sodium concentrations between juveniles and adults are not significant (P > 0.05). However, hemolymph chloride concentrations, which are hypoionic to seawater in both juveniles and adults, are significantly higher (P < 0.01) in adults than in juveniles.
The osmotic and ionic concentrations of the hemolymph and seawater are shown for P. serjferus in Table 1, and for P.stylirostris in Table 2. At the lowest salinities, the osmotic, sodium, and chloride concentrations in the hemolymph of juveniles of both species are significantly greater (P c 0.001 with ex-
2. The osmotic. sodium. and chloride concentrations in the hemolymph Penaeus sfylirosrris. Values represent mean + SEM for the number parentheses Salinity 10.8
Sodium concentration (Wkg)
Chloride concentration (m?Vkg)
20.2 seawater
624
405
314
+5
+6
+3
(8)
(3)
278 ?2
of juvenile and of replicates in
of seawater
'loo
adult
juvenile
(mOs/kg)
seawater
f5
626
RESULTS
Osmotic concentration
o/oo
adult
675
seawater
Hemolymph was sampled by inserting a 5 pl pipet (Drummond Microcaps) into the pericardial cavity. The gill chambers as well as the membrane between the posterior edge of the carapace and the first abdominal segment were dried with absorbent paper prior to sampling to prevent contamination of the hemolymph with seawater. Only intermolt animals were sampled. The osmotic. sodium and chloride concentrations in the hemolymph and seawater were determined as previously described (Castille & Lawrence, 1980). Differences between hemolymph concentrations of juvenile and adult shrimp were analyzed for significance with r-tests for differences between two means.
Table adult
juvenile
seawater
adult
juvenile
40.4
'loo
adult
juvenile
(mOs/kg)
of juvenile and of replicates in
of seawater
9.8 o/o0
Osmotic concentration
in the hemolymph for the number
Juvenile
O/o0
36.2
adult
seawater
665
669
588
834
+5
+a
+2
+10
(5)
(8)
(6)
(51
239
158
300
310
+6
+8
+5
t3
Juvenile
'ho
adult
seawater
811
1046
(8)
'3 (5)
(5)
279
364
344
466
t6
+6
t10
+6
Cl
(7)
(3)
(4)
(5)
(5)
(5)
(7)
(5)
(5)
272
229
172
301
318
371
373
375
562
+4
+4
+6
+2
+5
t2
$5
+3
t8
(7)
(3)
(4)
(7)
(5)
(4)
(7)
(5)
(3)
Osmoregulatory
comparison
DISCUSSION
The differences in hemolymph concentrations at low salinities between juvenile and adult shrimp demonstrate that juveniles are better hyperosmotic and hyperionic regulators than adults. This may explain why adults are not as well adapted to low salinities as juveniles and possibly why mature animals are not found at low salinities. A similar pattern of salinity distribution based on animal size is found in the hermit crab, Pagurus bernhardus. Large hermit crabs are less common than small animals in the euryhaline littoral zone and are much less tolerant of lower salinities than small animals (Davenport, 1972). As well as being stronger hyperosmotic regulators at low salinities, juvenile P. sefiferus are also stronger hypoosmotic regulators at high salinities than adults. This is consistent with observations that post larval and juvenile P. set@us are not adversely affected by hypersaline conditions. In Laguna Madre, an estuary on the south Texas coast, small P. setiferus have been found at salinities as high as 4&O”,,,, (Hildebrand, 1958) and 41.3”<,, (Gunter, 1961). Zein-Eldin (1963) reported that, in laboratory experiments, postlarval P. setiferus survive and grow at salinities up to 40”;,. In contrast to the findings for P. settferus, this study does not reveal any differences in the osmoregulatory capabilities of juvenile and adult P. stylirostris at the salinity of 36.2’:,,. However, it is possible that at higher salinities, differences may become apparent. The data obtained at the intermediate salinity explain why differences have been reported in the determination of the isosmotic crossover concentrations between juvenile and adult animals. The isosmatic crossover of between 830 and 850mOs/kg reported by McFarland & Lee (1963) for P. seti&erus of length in excess of IOOmm greatly exceeds that of 673 mOs/kg reported for 85 mm juveniles by Castille and Lawrence (1980). Since in this study at 23.5”,,,,, juveniles are hypoosmotic to seawater and adults are hyperosmotic, there must be an elevation of the isosmotic crossover salinity with maturation. A similar increase in the isosmotic crossover concentration has been reported in the caridean shrimp, Crnngon sepremspinosa (Haefner, 1969). Juvenile C. septemspinosa at both 5 and 15’C, exhibit isotonicity at a lower salinity (22-23”,,,,) than do adults at 5 (25”(,,,)and 15 C (27-28”,,,,). Results obtained for P. sfy/irostriS at the intermediate salinity are more equivocal. Even though both the osmotic and sodium concentrations in the hemolymph are maintained above the concentrations in seawater at 20.2”, hemolymph concentrations do not differ in juveniles and adults as at 10X’,,,,. In contrast, the chloride concentrations are lower in the hemolymph than seawater at 20.2”,,,, and the adults regulate chloride at a higher concentration in the hemolymph than do juveniles. Yet this pattern is not reflected at the higher salinity of 36.2”,,,, Although this study demonstrates that the capacities of hyperosmotic regulation in mature P. setiferus and P. stylirostris are reduced, adult animals are still capable of hyperosmotic regulation at salinities below the isosmotic concentration. This implies that even though movement out of low salinity waters can be explained by reduced osmoregulatory capabilities,
of juvenile
and adult shrimp
679
migration to offshore waters is not directly necessitated by osmotic regulation. In contrast to Pogurus bernhardus, in which the salinity distribution is also determined by animal size, adult shrimp are able to tolerate dilution of the internal concentrations at the lower experimental salinities. Migration to offshore waters is probably caused by factors other than a direct effect of environmental salinity on adult shrimp. If the highly developed capacity for hyperosmotic regulation is not needed in adults, there is no selective advantage for its maintenance. Acknowledgemenrs-This work is a result of a research program sponsored in part by Texas A & M University Sea Grant College program, supported by the National Oceanic and Atmospheric Administration, Office of Sea Grant, Department of Commerce under Grant No. 47-158-44105 and a Moody Foundation Grant to the University of Houston, Addison L. Lawrence. principal investigator. The authors wish to express their gratitude and appreciation to Nathan Howe, David Mailman, and Brian Middleditch of the University of Houston for reviewing this manuscript prior to submission for publication.
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