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Osmoregulation in Corophium volutator—The effect of starvation
Comp. Biochem. Physiol., 1970, Vol. 35, pp. 303 to 306. Pergamon Press. Printed in Great Britain
OSMOREGULATION IN COROPHIUM VOLUTATOR-THE EFFECT OF STARVATION DONALD S. M c L U S K Y Department of Biology, The University, Stirling, Scotland
(Received 5 February 1970) Abstract--1. A significant drop in the blood concentration of Corophium volutator (Amphipoda) has been noted, following removal of animals from their food (mud). 2. No significant change in the oxygen consumption of C. volutator has been noted, followingremoval of animals from food. 3. It is suggested that food may be necessary, at least in part, for osmotic regulation in C. volutator, as a direct supply of ions via the gut. INTRODUCTION PREVIOUS studies on the physiology of Corophium volutator (Pallas) (Crustacea, Amphipoda) in relation to salinity have shown that food is necessary for long-term survival at low salinities (McLusky, 1967), but that short-term maintenance of hyperosmotic conditions in low salinities, and uptake of ions may be accomplished without feeding (McLusky, 1969). It was also found (McLusky, 1969) that when comparison was made of the oxygen consumption of C. volutator of similar size, at the same level of activity and in different salinities, no significant differences were noted between animals acclimated to different salinities. Discussion of these results suggested that either the metabolic cost of osmoregulation was so low as to be unmeasurable, that a shift in energy requirements occurred in low salinities from growth and reproduction into osmoregulation, or that food was necessary for osmoregulation as a direct supply of ions via the gut. Starvation of Branchipus apus and Daphnia magna has been observed to result in a fall in the blood concentration of the animal (Krogh, 1939). Krogh suggested that this result was due, at least in part, to their dependence on food as a source of ions. In view of the observations of the effects of food on the survival time of C. volutator at low salinities, whilst maintaining hyperosmotic conditions in the blood(McLusky, 1967), experiments were undertaken to determine the effect of feeding on the blood concentration. Lockwood (1962) has suggested, however, that the fall in blood concentration of unfed animals noted by Krogh (1939) is due to a fall in the overall metabolic rate. MATERIALS AND METHODS Animals were collected from the Ythan estuary, Aberdeenshire, and maintained in stock tanks with mud in the laboratory at 10~ooand at 10°C. A group of these fed animals, fully 303
304
DONALD S. McLusKY
acclimated to 10%o, was divided into two. One half was then maintained at 10 '~'jo0with food (mud), whilst the other half was maintained in filtered sea water of the same salinity without food (mud). At regular intervals animals were removed and the freezing point of the blood determined, following the method described by McLusky (1967). Oxygen consumption measurements were made of unfed active animals using Scholander micro-volumetric respirometers, as described by McLusky (1969). The mean dry weight of animals used was 1"21 rag, and all animals were within the range 1"0-1"5 nag. RESULTS
Blood concentration It was found that no change occurred in the freezing point of the blood of the control (fed) animals during the experimental period (Fig. 1). T h e freezing point
4I 1"2
Fed .....
(J o i.0
i
Unfed
i,
0.8
0.6
I 0
~ 24-
; 48
7'2
i 96
I 120
I 144
I 168
I 192
I 216
Hours
FIG. 1. The effect of starvation on the freezing point of the blood of C. volutator at 10%o. Fed animals: mean, It; individual, • : Unfed animals: mean, [] ; individual, A.
of the blood of animals without food fell markedly during the first 72 hr and remained approximately constant at a level significantly (t = 4.268; P = <0.01) below that of the fed animals for the duration of the experiment (192 hr). These findings confirm those of K r o g h (1939), in that starvation causes a fall in the blood concentration of the animal, and suggest that food is necessary for the maintenance of hyperosmotic conditions in the blood of C. volutator.
Oxygen consumption Oxygen consumption measurements, as an indicator of metabolic rate, were made of unfed animals in order to determine the validity of Krogh's (1939) and Lockwood's (1962) suggestions. T h e results (Fig. 2) indicate an oxygen consumption level of 2.22/~10z/mg dry wt per hr at the commencement of the experiment. Following maintenance of the experimental animals without food, the oxygen
305
I N COROPHIUM VOLUTATOR
OSMOREGULATION
consumption fell to 1 . 2 0 / ~ 1 0 J m g dry wt per hr after 24 hr and rose to 1.72/zl O J m g dry wt per hr at 120 hr and 1.90/zl 09/rag dry wt per hr at 264 hr. Comparison of these results by means of Student's t-test indicates a significant difference between the initial animals, and those after 24 hr without food (t = 2-737; P = <0.01). There were, however, no significant differences between other groups of animals, 24 hr vs. 120 hr, t = 1.505, P = <0.2; 120 hr vs. 264 hr, t = 0.370, P = <0.7; 0 hr vs. 120 hr, t = 1.193, P = <0.3; 0 hr vs. 264 hr, t = 0.604, P = < 0.6.
5
"~
4
-0
a
0
:
::k 2 %
|
! O
0
1 24
, I 48
I 72
I 96
I 120
] 144
I 168
I J92
I 216
I 240
I 264
Hours
FxG. 2. The effect of starvation on the oxygen consumption of C. volutator at 10~oo. Mean, [] ; individual animals, O. DISCUSSION Observation of the effects of starvation on the maintenance of hyperosmotic conditions and on the survival of animals at low salinities has led Krogh (1939), Robertson (1939) and McGrorty (1970) to suggest that ion uptake via the gut may be an important factor in body-fluid regulation. The present findings on the effect of starvation on the blood concentration of C. volutator, coupled with previous studies (McLusky, 1967) on the effect of feeding on the survival time of C. volutator in low salinities, do lend support to suggestions regarding the role of food as a supplier of ions for osmotic regulation in Crustacea. The observations on the metabolic rate of active C. volutator in relation to starvation indicated a significant drop in oxygen consumption over the first 24 hr, followed by an increase in oxygen consumption during the ensuing 240 hr. Previous findings (McLusky, 1969) indicated for active fed animals of 1.0-1.5 mg dry wt, as in the present experiments, an oxygen consumption of 1.50/zl O~/mg dry wt per hr. Comparison of these earlier findings with those from all animals
306
DONALD S. McLusKY
deprived of food in the present experiments, reveal no significant difference (t -- 0.667, P = < 0.6). T h e significant difference already noted between the initial animals and those after 24 hr starvation may be due to elevation of the initial oxygen consumption caused by the effects of handling and transference from m u d to filtered sea water. It has already been noted that the oxygen consumption at 24, 120 and 264 hr are not significantly different from each other, nor were the results at 120 hr or 264 hr significantly different from the initial results, thus indicating no significant effect of food on oxygen consumption. T h e results of these experiments, indicating a significant drop in blood concentration but no significant drop in oxygen consumption following starvation, lend support to earlier hypotheses that food m a y be necessary, at least in part, for the maintenance of blood concentration in a small hyperosmotic regulator such as C. volutator, as a direct supply of ions via the gut. Acknowledgements--I should like to thank Professor F. G. T. Holliday for reading the manuscript. REFERENCES KROGH A. (1939) Osmotic Regulation in Aquatic Animals, p. 242. Cambridge University Press (reprinted by Dover, New York, 1965). LOCKWOOD A. P. M. (1962) The osmoregulation of Crustacea. Biol. Rev. 37, 257-305. McGRoRTY S. (1970) Salinity as a factor controlling the distribution of Bathyporeia spp. Proc. I I I Symp. Europ. Biol. Mar. Arcachon. (In press.) McLusaY D. S. (1967) Some effects of salinity on the survival, moulting, and growth of Corophium volutator (Amphipoda). J. mar. biol. Ass. U.K. 47, 607-617. McLusKY D. S. (1969) The oxygen consumption of Corophium volutator in relation to salinity. Comp. Biochem. Physiol. 29, 743-753. ROB~RTSON J. D. (1939) The inorganic composition of the body fluids of three marine invertebrates. J. exp. Biol. 16, 387-397. Key Word Index--Corophium volutator; Amphipoda; salinity; osmoregulation; effect ot starvation ; blood concentration ; oxygen consumption.
OSMOREGULATION IN COROPHIUM VOLUTATOR-THE EFFECT OF STARVATION DONALD S. M c L U S K Y Department of Biology, The University, Stirling, Scotland
(Received 5 February 1970) Abstract--1. A significant drop in the blood concentration of Corophium volutator (Amphipoda) has been noted, following removal of animals from their food (mud). 2. No significant change in the oxygen consumption of C. volutator has been noted, followingremoval of animals from food. 3. It is suggested that food may be necessary, at least in part, for osmotic regulation in C. volutator, as a direct supply of ions via the gut. INTRODUCTION PREVIOUS studies on the physiology of Corophium volutator (Pallas) (Crustacea, Amphipoda) in relation to salinity have shown that food is necessary for long-term survival at low salinities (McLusky, 1967), but that short-term maintenance of hyperosmotic conditions in low salinities, and uptake of ions may be accomplished without feeding (McLusky, 1969). It was also found (McLusky, 1969) that when comparison was made of the oxygen consumption of C. volutator of similar size, at the same level of activity and in different salinities, no significant differences were noted between animals acclimated to different salinities. Discussion of these results suggested that either the metabolic cost of osmoregulation was so low as to be unmeasurable, that a shift in energy requirements occurred in low salinities from growth and reproduction into osmoregulation, or that food was necessary for osmoregulation as a direct supply of ions via the gut. Starvation of Branchipus apus and Daphnia magna has been observed to result in a fall in the blood concentration of the animal (Krogh, 1939). Krogh suggested that this result was due, at least in part, to their dependence on food as a source of ions. In view of the observations of the effects of food on the survival time of C. volutator at low salinities, whilst maintaining hyperosmotic conditions in the blood(McLusky, 1967), experiments were undertaken to determine the effect of feeding on the blood concentration. Lockwood (1962) has suggested, however, that the fall in blood concentration of unfed animals noted by Krogh (1939) is due to a fall in the overall metabolic rate. MATERIALS AND METHODS Animals were collected from the Ythan estuary, Aberdeenshire, and maintained in stock tanks with mud in the laboratory at 10~ooand at 10°C. A group of these fed animals, fully 303
304
DONALD S. McLusKY
acclimated to 10%o, was divided into two. One half was then maintained at 10 '~'jo0with food (mud), whilst the other half was maintained in filtered sea water of the same salinity without food (mud). At regular intervals animals were removed and the freezing point of the blood determined, following the method described by McLusky (1967). Oxygen consumption measurements were made of unfed active animals using Scholander micro-volumetric respirometers, as described by McLusky (1969). The mean dry weight of animals used was 1"21 rag, and all animals were within the range 1"0-1"5 nag. RESULTS
Blood concentration It was found that no change occurred in the freezing point of the blood of the control (fed) animals during the experimental period (Fig. 1). T h e freezing point
4I 1"2
Fed .....
(J o i.0
i
Unfed
i,
0.8
0.6
I 0
~ 24-
; 48
7'2
i 96
I 120
I 144
I 168
I 192
I 216
Hours
FIG. 1. The effect of starvation on the freezing point of the blood of C. volutator at 10%o. Fed animals: mean, It; individual, • : Unfed animals: mean, [] ; individual, A.
of the blood of animals without food fell markedly during the first 72 hr and remained approximately constant at a level significantly (t = 4.268; P = <0.01) below that of the fed animals for the duration of the experiment (192 hr). These findings confirm those of K r o g h (1939), in that starvation causes a fall in the blood concentration of the animal, and suggest that food is necessary for the maintenance of hyperosmotic conditions in the blood of C. volutator.
Oxygen consumption Oxygen consumption measurements, as an indicator of metabolic rate, were made of unfed animals in order to determine the validity of Krogh's (1939) and Lockwood's (1962) suggestions. T h e results (Fig. 2) indicate an oxygen consumption level of 2.22/~10z/mg dry wt per hr at the commencement of the experiment. Following maintenance of the experimental animals without food, the oxygen
305
I N COROPHIUM VOLUTATOR
OSMOREGULATION
consumption fell to 1 . 2 0 / ~ 1 0 J m g dry wt per hr after 24 hr and rose to 1.72/zl O J m g dry wt per hr at 120 hr and 1.90/zl 09/rag dry wt per hr at 264 hr. Comparison of these results by means of Student's t-test indicates a significant difference between the initial animals, and those after 24 hr without food (t = 2-737; P = <0.01). There were, however, no significant differences between other groups of animals, 24 hr vs. 120 hr, t = 1.505, P = <0.2; 120 hr vs. 264 hr, t = 0.370, P = <0.7; 0 hr vs. 120 hr, t = 1.193, P = <0.3; 0 hr vs. 264 hr, t = 0.604, P = < 0.6.
5
"~
4
-0
a
0
:
::k 2 %
|
! O
0
1 24
, I 48
I 72
I 96
I 120
] 144
I 168
I J92
I 216
I 240
I 264
Hours
FxG. 2. The effect of starvation on the oxygen consumption of C. volutator at 10~oo. Mean, [] ; individual animals, O. DISCUSSION Observation of the effects of starvation on the maintenance of hyperosmotic conditions and on the survival of animals at low salinities has led Krogh (1939), Robertson (1939) and McGrorty (1970) to suggest that ion uptake via the gut may be an important factor in body-fluid regulation. The present findings on the effect of starvation on the blood concentration of C. volutator, coupled with previous studies (McLusky, 1967) on the effect of feeding on the survival time of C. volutator in low salinities, do lend support to suggestions regarding the role of food as a supplier of ions for osmotic regulation in Crustacea. The observations on the metabolic rate of active C. volutator in relation to starvation indicated a significant drop in oxygen consumption over the first 24 hr, followed by an increase in oxygen consumption during the ensuing 240 hr. Previous findings (McLusky, 1969) indicated for active fed animals of 1.0-1.5 mg dry wt, as in the present experiments, an oxygen consumption of 1.50/zl O~/mg dry wt per hr. Comparison of these earlier findings with those from all animals
306
DONALD S. McLusKY
deprived of food in the present experiments, reveal no significant difference (t -- 0.667, P = < 0.6). T h e significant difference already noted between the initial animals and those after 24 hr starvation may be due to elevation of the initial oxygen consumption caused by the effects of handling and transference from m u d to filtered sea water. It has already been noted that the oxygen consumption at 24, 120 and 264 hr are not significantly different from each other, nor were the results at 120 hr or 264 hr significantly different from the initial results, thus indicating no significant effect of food on oxygen consumption. T h e results of these experiments, indicating a significant drop in blood concentration but no significant drop in oxygen consumption following starvation, lend support to earlier hypotheses that food m a y be necessary, at least in part, for the maintenance of blood concentration in a small hyperosmotic regulator such as C. volutator, as a direct supply of ions via the gut. Acknowledgements--I should like to thank Professor F. G. T. Holliday for reading the manuscript. REFERENCES KROGH A. (1939) Osmotic Regulation in Aquatic Animals, p. 242. Cambridge University Press (reprinted by Dover, New York, 1965). LOCKWOOD A. P. M. (1962) The osmoregulation of Crustacea. Biol. Rev. 37, 257-305. McGRoRTY S. (1970) Salinity as a factor controlling the distribution of Bathyporeia spp. Proc. I I I Symp. Europ. Biol. Mar. Arcachon. (In press.) McLusaY D. S. (1967) Some effects of salinity on the survival, moulting, and growth of Corophium volutator (Amphipoda). J. mar. biol. Ass. U.K. 47, 607-617. McLusKY D. S. (1969) The oxygen consumption of Corophium volutator in relation to salinity. Comp. Biochem. Physiol. 29, 743-753. ROB~RTSON J. D. (1939) The inorganic composition of the body fluids of three marine invertebrates. J. exp. Biol. 16, 387-397. Key Word Index--Corophium volutator; Amphipoda; salinity; osmoregulation; effect ot starvation ; blood concentration ; oxygen consumption.