Diurnal routine O2 consumption at different O2 concentrations by Colossoma macropomum and Colossoma brachypomum (Teleostei: Serrasalmidae)

Camp. Biochem. Physiol. Vol. 89A, No. 4, pp. 675-682, Printed in Great Britain

1988 0

0300-9629/aa 53.00 + 0.00 1988 Pergamon PM pk

DIURNAL ROUTINE O2 CONSUMPTION AT DIFFERENT O2 CONCENTRATIONS BY COLOSSOMA A4ACROPOMUA4 AND COLOSSOMA BRACHYPOMUM (TELEOSTEI: SERRASALMIDAE) ULRICH

Institut

fiir Hydrobiologie

SAINT-PAUL

und Fischereiwissenschaft, Olbersweg FRG. (Telephone: 040-3807-2513)

24, D-2000

Hamburg

50,

(Received 4 August 1987)

Abstract-l. Colossoma macropomum and C. brychypomum were found to reveal a bimodal diurnal pattern of IjoZ which persisted at all levels of 0, concentration. lie, increased around dawn and dusk. This seemed to be a response to the light on/off stimuli rather than being endogenous. 2. When there is a diurnal rhythm with a morning minimum in the 0, concentration, the maximum r’,, of C. macropomum occurred at dawn, while that of C. brachypomum occurred at dusk. 3. Vb, of pond fish was by the factor 0.6-0.8 lower than in fish from natural water bodies. 4. The experimental results suggest that these two species, which have somewhat similar diets, occupy

separate temporal niches in the nutrient-rich habitats of the Amazon floodplains.

INTRODUCTION

Floodplain lakes in Amazonia, which were formed by recent deposition of alluvium, are typical along the sediment-rich white water rivers. The limnological conditions in such lakes change periodically with the natural, seasonal water level fluctuations of as much as 15 m. Changes in the oxygen concentration are particularly pronounced. Some extreme seasonal changes, including large diurnal fluctuations and frequent long periods of oxygen depletion, have been reported (Marlier, 1965; Sioli, 1968; Schmidt, 1973; Junk, 1970, 1980; Kramer et al., 1978; Junk et al., 1983). These floodplain lakes are richer in fish than any other kind of water body in Amazonia. Among others, the serrasalmids Colossoma macropomum (Cuvier 18 18) and C. brachypomum (Cuvier 1818) can populate these habitats with a temporary deficiency or even absence of oxygen. They employ a great variety of mechanisms to adapt to the strongly fluctuating O2 concentration. Patterns of seasonal and diurnal migrations induced by the availability of oxygen have already been observed (Junk et al., 1983; Saint-Paul and Soares, 1987). Haematological changes in response to long-term, habitat induced hypoxia are found for C. macropomum (Saint-Paul, 1984). There are also morphological and ethological adaptations that permit Colossoma species to take advantage of the O2 richness in the surface layer of the water by aquatic surface respiration (ASR) during periods of oxygen depletion (Branson and Hake, 1972; Braum and Junk, 1982). The circumstances suggest low rates of metabolism in species adapted to these environmental conditions. However, the data on one of the serrasalmid species, C. macropomum, show that its routine V, does not differ essentially from that of other tropical fish

species (Saint-Paul, 1983, 1984). However, in tropical habitats, there are daily oscillations in certain factors of which light and oxygen are probably the most conspicuous abiotic variables. That is why average consumption values might be not very helpful for an understanding of the mechanisms of adaptation to the diurnal periodicity in the O2 content of the water. More important is the diurnal oxygen consumption curve for the fish confronted with hypoxic conditions, thus giving an indication how fish react to different 0, concentrations. This study was undertaken to investigate this subject. It is part of an extensive program to investigate the adaptation of serrasalmids in the genus Coiossoma to hypoxia. MATERIALS

AND METHODS

Experimental jsh The experiments were conducted on C. macropomum and C. brachypomum. The fish were sent from Brazil by air freight in 1983 and 1984. They came from a fish hatchery of the Departamento National de Obras contra as Secas (DNOCS) in Fortaleza. On arrival, each fish weighed about 5%log. They were kept in aquaria supplied satisfactorily with oxygen at a temperature of about 25°C. Their feed consisted of a pelletized diet mixture containing 30% crude protein. Determination

of vo,

The respirometer used to determine the voI is depicted in Fig. 1. While maintaining a constant temperature, this device can be adjusted to supply oxygen at different concentrations or to simulate fluctuations in the supply as encountered during a diurnal cycle. Changes in the partial pressure of oxygen in the water were effected chemically through the addition of a sodium sulfite solution to the ambient water (Vanderhorst and Lewis, 1969; Lewis, 1970). Secondary effects of this chemical compound on teleosts are so far unknown. Lewis (1970) 675

ULRICH SAINT-PAUL

676

f

Fig. 1. The flow-through respirometer; I. Supply tub with 0, saturated water, 2. Raised container for O2 saturated water, 3. Mixing chamber for 0, poor water, 4. Stock solution of Na,SO,, 5. Supply tub with OZ poor water, 6. Raised container for 0, poor water, 7. Pressure regulating container for tap water, 8. Levelling chamber, 9. Water bath for the respirometer, IO. Respirometer, 1I. Water level regulator with magnetic valve, 12. Pneumatic pump, 13. Flow rate gauge, 14. Timer for controlling the pneumatic pump, 15. Timer for controlling the magnetic valve, 16. Thermostat, 17. Filter column with activated charcoal and filtering fibers, 18. 0, electrode, 19. 0, probe, 20. Analog-digital converter, 21. Computer, 22. Data recorder, 23. Pump, 24. Water tap, 25. Plastic balls, 26. Aerater.

found no observable difference in behavioural response of cyprinodontids to water freed of oxygen with sulfite and water freed of oxygen with nitrogen. The sulfite concentration in the water does not constitute a significant osmotic challenge to the fish: to reduce the O,-concentration from saturation down to 0.5 mg/l, 70.0 mg Na,SO,/l are necessary. As Na,SO,, the product of chemical oxidation, is the most frequent inorganic S-compound in natural water bodies, a burden on the fish by sulfate seems to be impossible. Normal concentrations found are between 2(t30 mg/l. In tropical water, the concentration can exceed lOOmg/l (H611, 1979). The flow-through respirometer used consisted of three essential components: the supply tanks, the level regulator, and the respiration chamber. The supply tanks (I, 5) consisted of two large PVC tubs, each with a capacity of about 600 I. The continuous filling of the tub with oxygen saturated water was controlled by a conductive fullness regulator (I I). The incoming water (7) was cleaned of suspended matter by passage through an activated charcoal filter column (17). A thermostat (16) adjusted the water temperature to the experimental value with a deviation of +0.5”C. Compressed air was used for aeration (26). The second tub held the oxygen-poor water, previously prepared in a 200 I supply tank (3). The dosage of sodium sulfite solution was added through a pneumatic pump (12). Whenever the water fell below a present level, the tub was refilled from the supply tank. The water in this tub was also adjusted to the experimental temperature. In order to minimize gas exchange, the surface of the tub was covered with plastic balls (25). The oxygen-rich and the oxygen-poor media were pumped into separate elevated containers (2,6), from which they could enter the levelling container (8) by gravitational flow. The proportions of the water mixture could be regulated. The magnetic valve that regulated this could be controlled by a timer (I 5). Thus, it was possible to maintain constant oxygen concentrations or to vary them according to a daily schedule. The surface of the water in the levelling container was also covered with plastic balls. From the levelling container, water flowed into the actual respiration chamber (IO), which was immersed in a water

bath (9). According to the size of the fish, opaque chambers were used with volumes from 0.5 to 6 I. The oxygen concentrations in the inflowing and outflowing water were determined electrochemically using WTW or Eschweiler oxygen probes (19). The data were digitalized, averaged for 5 min periods, and continually recorded by a computer (2&22). The oxygen consumption was recorded for fish that had been starved for 24 hr. In every case, the experimental temperature was 25°C. The actual 0, concentrations corresponding to the experimental saturations were as follows: 80% = 6.44 mg/l, 60% = 4.98 mg/l, 100% = 8.38 mg/l, 40% = 3.19mg/l, 20% = 1.47mg/l. For the adaptation to the oxygen saturation levels of 80, 60, 40 and 20%, a progressive rate of 10% per day was permitted. An experiment usually continued for at least 36 hr. The light phase (L) of the experimental period lasted from 5:30 to l9:OO hr (L/D = 13.5/10.5). The influence of the 0, concentration on the VOXof C. macropomum was determined by tests on the following groups: I5 fish (190.8 + 83.2g) at 100% saturation, IO fish (286.8 + 58.0 g) at 80%, IO fish (294.1 & 27.1 g) at 60%. 9 fish (249.5 & 54.9 g) at 40%, and 9 fish (122.0 f 44.8 g) at 20%. The corresponding experiments on C. brachypomun~ were conducted on the following groups: 10 fish (22.1 + 5.2 g) at 100% saturation, IO fish (118.1 & 18.2g) at 80%, I2 fish (146.5 f 21.3 g) at 60%, and 9 fish (173.5 k 31.5 g) at 40%. Experiments at 20% saturation could not be conducted because individuals of this species were not able to tolerate concentrations this low in the respiration chamber. r’,, was calculated according to the method of Niimi (1978), using the 0, concentrations in the inflowing and outflowing water, the flow rate, and the size of the respirometer. As absolute values could not be used for plotting the diurnal voZ the percent deviation of each datum from the mean for each series had to be calculated. For the analysis of the diurnal patterns, the 0, consumption data of different sized fish were combined to accumulated curves. The resulting curves are therefore based on as many as 4000 individual data. Control experiments in 0, saturated water at 24 hr constant light period were conduced with I I C. macropomum

Teleost respiration Table I. Routine

O,-saturation

611

r’o, of Colossomo macropomum at 25°C in relation to Oz saturation. experiments L/D = 13.5/10.5 (control 24 hr light) Fish weight

Mean PO, (mgO,/hr)

During

the

(g)

SD

n

Time

SD

n

100 control

370.9

98.1

II

Total L-phase D-phase

26.15 26.06 26.27

1.22 1.20 1.25

72 39 33

100

137.1

17.4

IO

Total L-phase D-phase

14.56 15.25 13.74

1.22 I.16 0.64

72 39 33

100

295.0

50.3

4

Total L-phase D-phase

23.73 24.86 21.95

3.81 3.86 3.13

72 39 33

80

286.8

58.0

IO

Total L-phase D-phase

24.00 26. I5 21.46

4.94 3.32 5.36

72 39 33

60

294. I

27.1

IO

Total L-phase D-phase

29.24 31.88 26.12

4.79 3.74 3.98

72 39 33

40

249.5

54.9

9

Total L-phase D-phase

26.16 29.49 28.78

1.75 I.55 1.90

72 39 33

20

122.0

44.8

9

Total L-phase D-phase

15.77 16.50 14.91

1.64 1.65 I.16

72 39 33

(X)

(370.9 f 98.1 g) and 10 C. bruchypomum (984.1 + 69.8 g). Fish were preadapted to the 24 hr L-phases for at least 3 weeks. The influence of diurnal fluctuating O2 concentrations on the lioI was determined by reducing the 0, concentration from saturation down to l-2 mg/l between 24:00 and 6: 00 hr and then allowing it to increase again to saturation until 12:OOhr. A total of 18 experiments were conducted on C. mncropomutn (116.7 k 125.1 g) and 13 corresponding experiments on C. brachypomum (171.3 k 187.8 g) at 25°C. All data were expressed as mean values + SD. Differences between the groups were statistically evaluated by Student’s f-test for two means. Differences within a group were tested with a paired t-test. For not normal distributed data, non parametric tests were applied, such as the Wilcoxon signed rank test for differences between the groups and the MannWhitney U-test for differences within a group. p i 0.05 was considered significant.

Table 2. Routine O,-saturation W)

RESULTS

Routine voz The results on routine vo* of C. macropomum and C. brachypomum are summarized in the Tables 1 and 2, respectively. The diurnal voI within a group of similar sized fish are averaged. As the I& is size-dependent, the calculated standard deviations presented in the tables are larger than of equal sized fish. vos (mean fish weight) for all six experiments with C. macropomum are between 15.77 mg O,/hr (122.0 g) for the 20% experiment and 29.24 mg O,/hr (294.1 g) for the 60% experiment. The data calculated for C. brachypomum of all five experiments are between 9.48 mg OJhr (22.1 g) in 100% saturation and 95.08 mg O,/hr (984.1 g) in the 100% saturation control experiment. Considering only those trials

PO1of Colossoma brachypomum at 25’C in relation to 0, saturation. experiments L/D = 13.5/10.5 (control 24 hr light)

the

Mean vo, (mgO,/hr)

SD

n

Total L-phase D-phase

95.08 95.95 93.98

5.48 6.64 3.15

288 162 126

IO

Total L-phase D-phase

9.48 10.50 8.16

I .62 I .38 0.72

288 162 126

18.2

IO

Total L-phase D-phase

23.61 28.38 17.48

6.75 4.04 4.03

288 162 126

146.5

21.2

I2

Total L-phase D-phase

25.03 28.51 20.56

5.33 3.73 3.41

288 162 126

173.5

31.5

9

Total L-phase D-phase

24.88 27.57 21.43

3.85 2.56 2.07

288 162 126

Fish weight (9)

SD

n

Time

984. I

69.8

IO

100

22. I

5.2

80

118.1

60

40

loo control

During

678

ULRICH SAINT-PAUL

with similar sized fish, highest V,, of 25.03 mg 02/l (146.5 g) is found in the 60% experiment. In all experiments for both species, there is within the groups a highly significant difference (p I 0.001) between all L/D-phases. Only in the control experiment with C. macropomum is no significant difference found. A maximum difference for C. macropomum between L- and D-phase of 5.76mg OJhr is found in the 60% and a minimum of 0.71 mg O,/hr in the 100% experiment. A maximum difference for C. brachypomum of 8.90mg O,/hr is found in the 80% and a minimum difference of 1.34 mg OJhr in the 100% experiment.

@Ji

q

100% satur, control

darkness

Diurnal variations in v,,, For demonstrating diurnal variations in po,, the influence of differently weighing fish on the averaged routine voio,is eliminated by calculating the percent deviation of the individual experimental values from the total mean. Figure 2 shows the accumulated graphic for C. macropomum. Minima and maxima in all temporal curves are clearly evident. They are independent of the O2 concentration. Differences between L- and D-phases are between 3.2% (control) and 13.5% (80%). The poio, during L-phases is significantly greater (p < 0.001) than the diurnal average and during darkness it is significantly less. In Table 3 the results of a statistical evaluation of differences between the groups are presented. There are no significant differences between the diurnal averages of the groups. However,. there are three significant differences in relative VO,between the groups during the L- and five during D-phase. The examination of the diurnal pattern of poio, reveals very pronounced maxima with an increase of up to 20% during the two periods of experimentally induced L/D changes. These peaks occur in both the morning and evening at 0, saturations of 100, 80 and 60%, but only in the evening at 40 and 20%. In order to reveal the influence of light on the diurnal Ijo> variations, a control experiment with no L/D-changes at 100% saturation was conducted. The diurnal variations in vol of fish under constant light conditions differ considerably from the L/Dexperiment (Fig. 2). There is a significant difference between the diurnal averages of both experiments. However, within the control experiment, there is no significant difference between the L- and the Dperiods as could be found for the L/D-experiment.

20. 0-A

7n__ 12 00

18~00

, 24.00

time

6.00

12 00

lhl

Fig. 2. Influence of the 0, concentration on the diurnal fluctuations in the routine vo3 at 25°C of C. macropotnum. The control experiment was conducted at 24 hr constant light.

Comparing the two groups, there is a significant difference between the L-phases and a highly significant difference between the D-phases of both experiments. While the fish in the L/D-experiment show pronounced maxima during both periods of light change in the control, the course of PO2decreases linearly over 18 hr from I2 : 00 to 6 : 00 hr, followed by an increase until 12:00 hr.

Table 3.

Statistical evaluation between the groups of relative I& of C. macropomum at different O2 saturation levels. Values are D-values. asterisks indicate the significance levels ([-_I = no significance, [*I = 95%, [t] = 99%, (11= 99.9% significance) 80

60

__.-. 40

20

C

80

60

A

SO(%)

20

C

80

60

0.270

0.240

0.054

0.008

0.210

i-1

I-l

1-I

IS1

r-1

0.229

-

0.072

0.017

0.0190

E-1

[‘I 0.296 b-1

0.146

I

L-1

l-1

b-1

A

A

-

A

-

60 (S’o)

A

D-phase 40

20

C

1

0.002

W)

(%)

(%) 100 (%)

L-phase 40

0.000

I

1-I

-

0.008

b-1 I [-I

-

ItI 0.015 [‘I

A

-

I L-1 I 1-l 0.000 [Xl

40 (%)

k-1

0.006

RI

1x1

A

RI ~ -

Teleost respiration

679

El

Colossomo brachypomum 100%

gg

satuc.control

darkness

darkness

4oi

-40’ lOO%satw 40 25 0 s-20 g-40 "

60 aJ

g g =

40 20 0 -20

$

-40

129l

18'00

24'00

12:oo

Fig. 4. Influence of periodic diurnal fluctuations in oxygen concentration on the diurnal cycle of routine L$, at 25°C by C. macropomum and C. brachypomum.

40%sotur

40.

6:00

[hl

time

20. O-20. -4o12 00

18.00

2499

brachypomum.

12 00

lhl

time Fig. 3. Influence diurnal fluctuations

#

600

of the 0, concentration on the in the routine lie, at 25°C of C.

The control experiment was conducted at 24 hr constant light.

The cumulated data on diurnal variation of PO2 calculated for C. brachypomum are illustrated in Fig. 3. For this species, there are also pronounced diurnal fluctuations in vo’,,, with minima during the D- and above-average values during the L-phase. The differences are highly significant (p < 0.001) in all experiments, including the control. The results of the statistical evaluation between the groups are presented in Table 4. There is a highly significant difference between the diurnal average of all groups. However, no difference is found between the experiment at 100% saturation and its control. During the L-phases, except for only one comparison (100/40%), there is a highly significant difference

Table 4. Statistical evaluation levels. Values are p-values,

between the groups of relative r’% of C. brachypomum at different 0, saturation asterisks indicate the significance levels ([-I = no significance, [‘I = 95%, [t] = 99%, [i] = 99.9% significance)

Total 80

between all groups. There is also a highly significant difference between all groups during the D-phase. Very pronounced maxima induced by L/D-changes can be detected for the 80 and the 60% experiment. The comparison of the course of vojolat 100% and its control with C. brachypomum reveals that the diurnal differences in Vo, of both phases are not so pronounced in the control and these differences are not significant. The influence of fluctuations in the O2 content of the water on fioio,of C. macropomum is depicted in Fig. 4. To make the illustration clearer, the 0, concentrations are noted. Here again, just as in the experiments conducted at constant 0, concentrations, highly significant differences (p I 0.001) in the diurnal and nocturnal oxygen consumption rates are apparent. In spite of lower oxygen concentrations at the beginning of the L-phase there is a sudden increase of about 60% in the oxygen consumption. This high metabolic rate does not persist throughout the day but declines rapidly after 12:OO hr, when the oxygen concentration has again reached saturation. At about 18:OO hr, towards the end of the L-phase another slight increase in the metabolic rate is observed. With the onset of darkness, this rate sinks to

60

C

80

60

80 (%)

0.000

0.000

0.000

El

El

El

0.000

RI 60(%)

40

C

80

60

0.434

o.ooo it1 0.000 IX1

-

0.000

O.ooO

[$I

18

0.065

0.000

0.000

RI

[$I 0.000 [$I

40

C

W)

W)

(%) lOO(%)

D-phase

L-phase 40

0.000

0.000

[$I

RI

-

-

0.000

0.006

0.000

111

RI

Ul

0.000

0.000

RI

ItI 0.000 IS1

-

-

680

ULRICH SAINT-PAUL

about -20% and remains at a constant low value throughout the entire night. An influence of the decrease in the oxygen concentration after 24:00 hr can scarcely be detected. Between 24: 00 and 6 : 00 hr a slight increase in the oxygen consumption was noted. The mean daytime value is 9.82 + 13.79%, and - 11.92k 7.17% is the mean nocturnal value. Also depicted in Fig. 4 are the corresponding findings for C. brachypomum. The differences between the day and night 0, consumption values are very clear and differ significantly (p I 0.001). Differences in the pattern of 0, consumption between this species and C. macropomum are apparent. With the beginning of illumination during the period of the early morning 0, minimum there is a sudden increase in the O2 consumption amounting to about 20%. During the course of the day after 12: 00 hr, 0, consumption increases rapidly and reaches a maximum at about 6: 00 p.m. After the onset of darkness there is a rapid decrease. The mean diurnal value was 15.75 k 27.13%, while the nocturnal average was - 19.15 f 10.42%. DISCUSSION

Changes in environmental factors such as 0, and light seem to have an appreciable effect on the course of 0, consumption in Colossoma. Generally, Colossoma appears to be more active in terms of Vs during the light period than after dark. Because of the differences in size of both Colossoma species used in the experiments, a direct comparison of their metabolic rate is not always possible. Previous measurements of & by Colossoma showed clearly that its metabolic rate is very similar to other tropical fish species (Saint-Paul, 1983). Comparing those findings with the present results, a distinct difference between both experimentsis found. The 100 and 80% experiments yield lower PO, by the factor 0.6, as do the other experiments by the factor 0.8, than in the previous trials. For C. brachypomum there are no previous data available. However, 0, consumption of C. brachypomum seems to be somewhat greater than of C. macropomum. When trying to explain the difference between l& of both experiments, the different ecological origins of the experimental fish have to be considered. In the previous experiment the fish used were from natural water bodies while in the present one pond fish were used. Winberg (1956) was the first to mention ecologically determined differences in the metabolism of fish. Wild carp and pond carp taken from their normal habitats differ with respect to the level of metabolism. The differences are of similar dimensions than in the present experiment with C. macropomum. Afonich and Sokolova (1984) also found a lower metabolic rate in pond-reared than in wild sturgeons. May be wild fish are less adapted to handling thus leading to a higher metabolic rate than pond fish. It seems to be true that metabolic rates are not only species specific but also depend to a great extent upon the condition under which the fish used in the experiment was reared. Looking at the diurnal variations of r’,,, both Colossoma species are found to reveal a bimodal diurnal pattern which persists at all levels of Or

concentrations. The activity peaks occurring around dawn and dusk might be a response to the light on light off stimuli rather than being endogenous because they were not observed under constant light conditions. A summary on diurnal cycles of activity in marine fish is given by Woodhead (1967), demonstrating that several diurnal types of activities tend to increase in frequency or intensity both when Iight is decreasing and also when it is increasing, at dusk and dawn. Hesthagen (1976) investigated locomotory activity of the black goby under artificial light conditions. He found that fish were mainly active when it was light, but the activity was not completely suppressed during the dark period. By studying the activity patterns of gymnotid electric fish in nature and in the laboratory the endogenous nature of the activity rhythms of fish could be demonstrated (Lissmann and Schwassmann, 1965; Schwassmann, 1971). The endogenous nature of the activity rhythms in fish was shown by Schwassmann (1980) in a review on adaptive significance of biological rhythms. In order to prove the possible presence of endogenous components of such rhythmic activities, which may be entrained or synchronized by daily changes in light, control experiments were conducted at constant light conditions. Significant differences were confirmed for C. macropomum. However, no differences were found for C. brachypomum. The activity peaks in both the morning and the evening could not be observed in the control. It seems to be that both the diurnal variations in O2 uptake and these activity peaks at the time of L/D change are only responses to light on or light off stimuli. From these experiments it can be concluded with reliability that the diurnal variations in po2 are modulated by a change in light conditions and are not due to endogenous rhythms continuing at constant light. Information on the influence of the 0, concentration on diurnal variations of routine Ijo* in fish is not available. In the present investigation the diurnal rhythms persist at all levels of 0, concentrations. Apparently, the O2 concentration of the water has no influence on the diurnal po* curve. This confirms the information on the crepuscular activity patterns of both species, previously obtained through ecological investigations, which characterize C. macropomum as sunrise-active (Goulding, 1980; Saint-Paul, 1982, 1983) and C. brachypomum as sunset-active. The experiments of Groot (1964) confirm for the flounder that it is by no means certain that activity rhythms in the laboratory are the same as those in nature. That is why the present results are considered as a good confirmation of the ecological information so far available for Colossoma species. Because voZ also reflects the activity of an animal, the constant diurnal rhythm in 0, uptake for C. macropomum that persists at all levels of O2 concentrations support the previous observations on locomotory behavior relative to 0, concentration. Down to 1.O mg 0,/l no change in locomotory activity could be demonstrated. Only when the O2 concentration reaches 0.5 mg/l does the fish come to the surface to begin ASR, which is detected as an increase in movement (Binder, 1986; Saint-Paul and Soares, 1987). However, 0, concentrations in the present experiment were always above 1.O mg/l, so that an in-

Teleost

fluence of the O2 concentration on respiratory activity can be excluded. In the case of C. brachypomum, no statistically demonstrable influence of the 0, concentration on the locomotory activity was detected (Binder, 1986). This species constantly shows a higher level of activity than C. macropomum. The determination of l& during the diurnal fluctuations in the O2 concentration confirms that C. macropomum displays an active peak in the morning when light is switched on. In spite of the minimal O2 concentration that prevails at this time, there is a significant increase in PO>which remains at that high level until about 12:00 hr. These findings correspond to those of diurnal distribution and behavioral responses of Colossoma to hypoxia in an Amazon floodplain lake (Saint-Paul and Soares, 1987). It was demonstrated that fish react to diurnal fluctuations in the 0, concentration by migrations between the zone of macrophyte cover and the open water of a floodplain lake, in which richer oxygen supplies are available. In spite of the fact that Colossoma, in contrast to the other species investigated, return to the zone of macrophyte cover after longer lasting periods of oxygen depletion this period signifies for the fish an increase in activity. These ecological findings from a previous investigation are confirmed by the present experiments on diurnal patterns of 0, uptake. In the case of C. brachypomum, the situation seems to be somewhat different. The experimental lowering of the 0, concentration to a minimum in the early morning brought about no detectable increase in the metabolic rate. The increase was observed to begin at 15 : 00 hr and continued until 18 : 00 hr. Only after the onset of darkness did the rate fall again to a significantly lower level. However, the determinations made at constant O2 concentrations also usually revealed higher levels of metabolism in the second half of the day than in the first. In conclusion, it can be stated that neither of the species investigated adapt to temporary, diurnal reductions in the 0, supply through a reduction in 0, consumption. This is also suggested by observations that C. macropomum seeks out the habitats with particularly low 0, concentrations at sunrise in search of food. In this way, it distinguishes itself from other fish species which either emigrate from habitats with unfavorable 0, concentrations or modify their behaviour to reduce their oxygen consumption. This is confirmed by numerous examples in the literature, such as those of Umbra limi and Culaea inconstans (Klinger et al., 1982). For the Colossoma species, this form of adaptation is advantageous because it permits them to visit nutrient-rich habitats that must be avoided by other species less tolerant of hypoxia. This reduces both the predator pressure and competition. Unlike C. macropomum, C. brachypomum seems to reach its activity peak shortly before sunset. This assumption was reinforced by indirect observations showing that these fish are most readily caught in the evening with gill nets, into which they have to swim actively. Since the diet of both of these species is quite similar (Goulding, 1980), it seems logical from a biological viewpoint that they should visit their feeding grounds in the flooded varzea at different times of the day.

respiration

681

Acknowledgemenrs--I

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