Metabolic rate, hypoxia tolerance and aquatic surface respiration of some lacustrine and riverine African cichlid fishes (Pisces: Cichlidae)

Camp. Biochem. Physiol. Vol. 107A. No. 2, pp. 40341 I, 1994 Copyright 0 1994 Elsevier Science Ltd

Pergamon

Printed in Great Britain. All rights reserved 0300-9629/94

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Metabolic rate, hypoxia tolerance and aquatic surface respiration of some lacustrine and riverine African cichlid fishes (Pisces: Cichlidae) Erik Verheyen, Ronny Blust and Walter Decleir Ldboratorium voor Biochemie en Algemene Dierkunde Groenenborgerlaan 171, 2020 Antwerp, Belgium

Universiteit

Antwerpen

(R.U.C.A.),

The oxygen uptake rate (V02 as pg O,/g/hr), the critical partial oxygen pressure (PC,,) and the presence/absence of aquatic surface behavior (ASR) under hypoxia was studied for 10 cichlid species from different habitats (Oreochromis niloticus, Steatocranus tinanti, Astatotilapia burtoni, Dimidiochromis compressiceps, Nimbochromis uenustus, Melanochromis auratus, Tropheus moorii, Neolamprologus brichardi, Julidochromis marlieri and Eretmodus cyanostictus). After correcting for differences in bodyweight, the differences in VO, were shown to be not statistically significant between cichlids from lacustrine and fluvial habitats. The capacity to regulate VO, as pg O,/g/hr under gradually increasing hypoxia (P,, < PC,,) differs markedly among the studied species. Under the given experimental conditions two species (Julidochromis marlieri, Neolamprologus brichardi) were conformers at 0 < Pot < 150 torr. Six species that naturally occur in well aerated water were not able to perform ASR when subjected to severe hypoxia (O.&-l.5 ppm 0,) (Julidochromis marlieri, Neolamprologus brick&, Eretmodus cyanostictus, Tropheus moorii, Melanochromis auratus, Sreatocrunus tinanti). Key words: Cichlidae;

Respiration;

Hypoxia

tolerance;

Aquatic surface respiration;

Adaptive

radiation. Camp. Biochem. Physiol. -

107A, 403411,

1994.

Introduction Cichlids are freshwater fishes that are widely distributed in the tropical areas of the world. They display widespread adaptive radiation in the great East African lakes and are remarkable because of their high degree of endemism, their morphological and ecological diversity (Fryer and Iles, 1972). Many morphological, ethological, ecological and genetic studies have bearing on the evolutionary processes that underlie the extreme diversity of these lacustrine cichlid species flocks (e.g. Bare1 et al., 1989; Crapon de Caprona and to: Erik Verheyen, Royal Belgian Institute of Natural Sciences, Section Biochemical systematics and Taxonomy, Vautierstraat 29, B-1040 Brussels, Belgium. Fax: 32-2-646-44-33. Received 1 April 1993; accepted 7 May 1993 Correspondence

Fritzsch, 1984; Goldschmidt et al., 1990; Meyer et al., 1990). However, except for one study on the oxygen requirements of two closely related species (Galis and Smith, 1979), little is known about the physiological aspects of the adaptive radiation observed in lacustrine and/or riverine cichlids. Yet, it is clear that the intralacustrine adaptive radiation of cichlids involves respiratory adaptations (Hoogerhoud et al., 1983) what suggests that the metabolic rates and hypoxia tolerances of these fishes are related with their life styles and habitats. The present study investigates the oxygen requirements of African cichlids from different lacustrine and fluviatile habitats by measuring their oxygen consumption and hypoxia tolerance under laboratory conditions. First, we investigated whether cichlids from aerated and 403

Erik Verheyen

404

hypoxic habitats have different metabolic rates by measuring their routine oxygen uptake rates (VOz as pg O,/g/hr). Second, we estimated the hypoxia tolerance of these fishes by measuring their capacity to regulate VO, under hypoxic conditions. This is achieved by determining the species-specific critical oxygen tensions (PC,,), defined as the partial dissolved oxygen pressure (PoJ below which VO, ceases to be independent of ambient P,, (Beamish, 1964a). In addition, the behavioral responses (aquatic surface respiration, ASR) of these fishes under experimentally lowered dissolved oxygen levels were observed. For obligatory water breathing fishes, the use of oxygen rich water from the upper water layers is a common response. By keeping their mouth just below the water-surface, many fishes are capable to survive extended periods of hypoxic conditions that would otherwise be lethal (Kramer, 1983). In this paper we report on our results on the VO,, hypoxia tolerance measured as the PC, and ASR of five lacustrine cichlids from we1f aerated rocky littoral habitats, three other lacustrine cichlids living above sandy and/or and two non-lacustrine muddy substrate species; one of these is hypoxia tolerant, the other lives under rocks in aerated water of rapids. These data are supplemented with relevant literature data on respiration and hypoxia tolerance of other, mostly tilapiine, cichlids.

Materials and Methods The jishes

We studied 10 cichlid species of which eight are endemic to Lake Tanganyika or Lake

Table I. List of the studied species, their natural

Natural

II

Melanochromis auratus Tropheus moorii Eretmodus cyanostictus Neolamprologus brichardi Julidochromis marlieri Steatocranus tinanti Astatotilapia burroni

III

Dimidiochromis compressiceps Nimochromis venustus Oreochromis niloticus

Lake Malawi Lake Tanganyika Lake Tanganyika Lake Tanganyika Lake Tanganyika Stanley Pool Lake Tanganyika surr. rivers Lake Malawi Lake Malawi Syria/Nile and Congo basin

I

Oreochromis niloficus

Oreochromis mossambicus Pseudocrenilabrus mulricolor

Malawi. The Tanganyika cichlids are: Astatotilapis burtoni (three specimens), Tropheus moorii (six), Eretmodus cyanostictus (four), Neolamprologus brichardi (four) and Julidochromis marlieri (four). The Malawi cichlids are Dimidiochromis compressiceps (four), Nimbochromis venustus (five) and Melanochromis auratus (four). Two other species that we used are Oreochromis niloticus (19) and Steatocranus tinanti (eight). All these specimens were obtained from imported stocks. According to the oxygen availability in their natural environment, we divided the studied species into three groups (Table 1): (I) lacustrine species from oxygenated rocky littoral habitats and one species from torrenticole oxygenated fluviatile waters (Eretmodus cyanostictus, Julidochromis marlieri, Neolamprologus brichardi, Melanochromis auratus and Steatocranus tinanti), (II) other lacustrine cichlids (Astatotilapia burtoni, Dimidiochromis compressiceps, Nimbochromis venustus) who live in less aerated

water (over sand/mud between vegetation) and (III) a single species that prevails in hypoxic habitats (Oreochromis niloticus). The results for the third group are supplemented with literature data on Oreochromis niloticus (Mishrigi and Kubo, 1978; Ross and Ross, 1983; Farmer and Beamish, 1969), Oreochromis mossambicus (Caulton, 1978) and Pseudocrenilabrus multicolor (Schierwater and Mrowka, 1987). All the used specimens were kept in aquaria (250 1) with filtered, thermostated and aerated tapwater (water hardness 180 mg CaCO,, pH 8.2-8.4, 25X, Po, > 90% saturation) for at least two weeks prior to the measurements. The fishes were fed every day with commercial fish food, regularly supplemented with live Artemia.

distribution, habitats, the number used in this study

Species

Group

et al.

distribution

and

Literature data Mishrigi and Kubo, 1978 Ross and Ross, 1983 Farmer and Beamish, 1969 Caulton, 1978 Schierwater and Mrowka, 1987

(N) and weight range (g) of the specimens

Habitat

N

Weight

Rocks Rocks Rocks Rocks Rocks Rapids/under rocks Sand/vegetation

4 6 4 4 8 8 3

4.8-16.4 5.0-10.9 4.964 4.0-5.5 2.4-3.9 2.24.0 1.1-15.2

4 5 19

5.7-11.1 1.74.2 2.1-250.3

8 3

43000 50-100 40-I 10 50-100 24

Sand/vegetation Sand/vegetation Rivers/ponds lacustrine

I 3 9

(g)

Respiration and hypoxia in cichlid fish

0,electrode

Respiration measurements The respiration measurements were performed with a respirometer that can be used as an open and/or closed system and that has been described elsewhere (Verheyen et al., 1985). The volume of the test chamber was adapted to the size of the used fishes to approximate 50 g body weight/l. The fishes were shielded from visual stimuli by the walls of the waterbath surrounding the respirometer. In addltion, the fishes were allowed to acclimatize to the experimental conditions for at least 18 hr and the measurements were always made at approximately the same hour of the day ( 11.30 a.m.). The declining dissolved oxygen concentration was monitored with oxygen electrodes (Delta Scientific, Syland 2110) linked to a paper recorder. The declining Po2, corrected with the blank run, was converted to the oxygen uptake rate of the tested fish (VO, as pg O,/g/hr) as a function of time and of the dissolved oxygen levels between which the measurements were taken. The oxygen uptake rate under normoxic conditions was measured at 155 < P,,, < 120 torr by periodically opening a set of three way valves. During a typical experimental run, the concentration of the dissolved CO, in the test chamber increased only 5 ppm while the pH decreased by 0.X units. The critical oxygen tension (PC,,), defined as the PO2 below which the VO, ceases to be independent of the ambient P,, was essentially determined as described by Ultch et al. (1980). The intersection of the median VO, at 155 < Po, < 20 torr and a regression line fitted to those points in the range where VO, declined with falling Po with VOz < 75% of median VO, was taken as P co2. Observation of Aquatic Surface Behavior (ASR) Observation were made in a cylindrical glass container (diameter = 25 cm, height = 30 cm) that was divided into eight intervals between the bottom and the water surface (Fig. 1). After the O2 concentration was lowered to a concentration lower than the Pco2 of all species involved (0.8-1.5 ppm 0,) by bubbling N,, we introduced one fish at a time into the experimental tank. Immediately, we monitored the vertical movements of the fish in the water column until it lost equilibrium. Afterwards, the tested specimen was placed in aerated water to recover. This experiment was carried out with at least four specimens/species and as a blank run the experiment was repeated with the same fishes under normoxic conditions (>90% 0, saturation).

lo magnetic

stirrer

Fig. I. Experimental set-up for the experiments to observe the presence/absence of aquatic surface respiration (ASR) in the species studied here. To mix the N, bubbled water a magnetic stirrer was used prior to the introduction of the experimental fish. The Po, as well as the water temperature were continuously monitored with a WTW OXI 91 oxygen probe. The fishes were observed from behind a red colored transparant plastic screen placed at about 1 m from the experimental tank (H = 30 cm, diameter = 25 cm).

Results VOZ under normoxia VO, of the cichlids was determined by 54 experiments conducted at 25°C at 120 < Po, < 150 torr. Differences of the “massspecific” oxygen uptake rate (VOZ as pg O2/g/hr) are apparent, however, direct comparison is impossible because of the relative important differences in body weight (W = g. wet weight) among the species and species groups considered here (Table 2). The change in VOZ with relation to body weight can usually be modeled by the equation VO,=A/WkorlogVO *=log A +(k - l).log W, where VO, is the amount of oxygen consumed in a unit of time by fish of weight W (A and k are parameters with usually k < 1) To interpret our data that are scaled for body size (VOZ as pg O,/g/hr; Fig. 2) we fitted regression lines to the log VO,/log W data of the three experimental groups (Table 3a). To test whether VO, is different for our three data sets, taking into account differences in covariate W, we performed analysis of covariante (Packard and Boardman, 1988). Our data show that the slopes and elevation of the log VO,/log W data sets do not differ significantly (Table 3b). VO, and PcoZ under hypoxia As in the literature data, the hypoxia tolerant Oreochromis niloticus maintains its V02 over a considerable PO, range of Po,. At 25°C and for specimens between 2 and 290 g body weight, the

Erik Verheyen ef al

406

V02 of this species declines only below a Po, of 26 torr (Fig. 3j). This value is in very close agreement with the Pco, obtained under similar conditions by Fernandes et al. (1984) (Pco, = 28 torr). Also Dimidiochromis compressiceps, Nimbochromis venustus, Melanochromis auratus and Tropheus moorii are capable to regulate

their VO, over a considerable range. However, most of these species show a clearly increasing VOz at Po values >Pco, before their VOt declines with decreasing ambient oxygen levels (Fig. 3a-i). The obtained Pco, values are statistically different among these cichlids (Table 2;

Table 2. Summary of our results on the VO,(routine oxygen uptake rate), the critical partial oxygen pressure (PC,,) and of the percentage of specimens/species that survived the respiration measurements under declining dissolved oxygen levels. (The VO, and PC, data shown are only those obtained for the specimens that survived the experimentally induced gradual hypoxia by at least 48 hr; nd. = not determined) Species Melanochromis auratus

Tropheus moorii

Eretmodus cyanostictus

Neolamprologus brichardi Julidochromis marlieri

Sreaiocranus tinanti

Astarotilapia burtoni

Dimidiochromis compressiceps

Weight (g) 4.8 12.9 11.2 16.4 6.6 6.1 9.1 10.9 4.9 5.6 6.4 6.4 4.0 5.5 2.4 3.6 3.1 3.9 2.2 2.3 3.4 4.0

1.1 3.0 3.2 15.2 5.1 11.0 11.0

11.1 Nimbochromis venustus

Oreochromis niloticus

1.7 3.2 3.4 3.4 4.2 2.1 9.2 9.5 16.6 17.1 22.8 23.5 28.0 29.9 30.0 39.6 80.4 101.4 144.7 195.0 203.1 228.0 229.3 250.3

Mean VO, &SD (pg %/g/hr) 345.5 * 22.2 150.0 f 45.3 159.9 * 27.1 98.0 + 24. I 323.5 + 52.1 183.9 k 55.1 159.8 + 33.1 353.9 5 65.4 311.6+52.0 367.6 5 68.9 228.6 k 32.2 225.9 k 43.7 219.8 + 51.7 293.3 + 73.2 218.9 + 32.5 522.8 + 56.9 299.0 &-73.2 447.6 + 49.6 301.3 _+ 34.6 569.4 + 79.3 166.3 + 32.1 118.4&37.1 416.2 & 24.0 279.5 _+20.9 189.6 + 61.4 67.3 + 10.6 216.6 + 48.3 92.9 + 11.9 97.5 + 29.6 210.6 + 31.4 383.9 + 76.2 263.8 $- 60.0 184.5 k 14.7 180.0 k 19.6 257.1 & 26.4 245.9 & 23.2 185.9 + 34.4 142.4 + 14.1 123.0 + 26.1 155.9 + 20.5 123.5 + 18.6 107.2 & 18.5 112.3 _+ 16.2 122.2 f 18.2 103.6 k 19.4 80.7 k 13.8 72.8 + 13.8 88.0 It 12.2 51.6 + 8.7 52.7 + 8.3 56.3 & 5.9 46.0 + 7.0 52.7 & 6.8 53.3 + 8.5

Survival (“/)

N 5 5 4

I1 8 8 7 8 7 10 9 9 5 4 5 5 4 4 4 4 6 5 4 9 7 9 6 5 7 4 5 8 5 7 6 8 13 7

11 13 II 20 15 13 15 16

10 7 15 10 16 18 19 9

24.5 28.8 29.7 21.9 50.6 39.7 36.8 61.5 38.7 51.1 33.6 30.5 -

100

80

80 50 50

27.3

n.d. 28.4 33.5 n.d. nd. 28.0 22.2 40.8 30.2 28.9 44.5 27.0 27.5 24.5 23.2 30. I nd. 40.0 40.2 34. I 31.2 25.2 20.0 29.4 30.0 35.7 31.7 28.2 28.8 31.3 28.4 35.9 26.7 34.2 28.6

50

100 100

100

100

Respiration and hypoxia in cichhd fish

407

that the hypoxia during these experiments was only lethal to species from well aerated habitats (Julidochromis marlieri, Eretmodus cyanostictus, Neolamprologus brichardi, Tropheus moorii and Steatocranus tinanti). From this group Melanochromis auratus was the only species for

which all the specimens survived the respiration measurements under declining dissolved oxygen levels (Table 2). Occurrence of ASR under hypoxia 0

1000

500

1500

w

2000

2500

3000

(8)

Fig. 2. The relationship between body weight [W(g)] and routine oxygen uptake rate (vO1 in all the studied species, supplemented with literature data (see Table 1). Results are: VO, = 405.7 W-o.3s6 with R *= 0.594; linear regression on pooled logarithmically transformed data points: R 2= 0.786. Obviously, the smaller species and specimens have higher VO, than larger species. However, as shown in Table 3 this seemingly higher VO, can be explained as the result of the allometric relationship between VO,and W.

Kruskall Wallis statistic H = 19.92; x2 0.005 [7] = 20.278; 0.005 -CP -CO.Ol), Subsequent Mann-Whitney U-tests indicate that the PcO, of Dimidiochromis compressiceps (P = 0.05), Tropheus moor-ii (0.01 < P < 0.05) and Eretmodus cyanostictus (P = 0.05) are significantly higher than for Oreochromis niloticus. In contrast, the Pco, for Steatocranus tinanti and Melanochromis auratus are not significantly higher than the PC,, of Oreochromis niloticus.

Finally,

it

is

important

to

note

that

Julidochromis marlieri and Neolamprologus brichardi are not capable to regulate their VO,

over the Po, range studied here. It is also clear Table 3. (a) Summary of the routine oxygen uptake rates VO, measured under normoxia. For the three groups there is a significant linear relationship between log VO, and log W (VO, as peg O*/g/hr, W = wet body weight (g), N = number of specimens. (b) Comparison of the three linear log VO,/ log W regressions indicates that their slopes and elevations (Y-intercepts) do not differ significantly

(a) Group

N

I

23 II 22 III 46 Overall 81

r

df

F

-0.56 -0.74 -0.88 -0.86

1,21 I ,20 144 1,79

9.60 24.07 150.37 235.56

P 0.001 < P < 0.005

P
(b) Summary ANCOVA tables: differences among adjusted means Source Adj. means Error Source Among B Sum of grp. dev. Common slope

DF 2 78 DF 2 76 -0.379

P

SS 0.153 2.03

MS 0.076 0.026

F 2.934

n.s.

SS 0.045 1.984

MS 0.023 0.026

F 0.870

n.s.

P

Under the experimental conditions described earlier, all Oreochromis niloticus, Astatotilapia burtoni, Dimidiocromis compressiceps and Nim bochromis venustus specimens moved to the water surface and performed ASR (Fig. 4ad). Oreochromis specimens remained motionless at the bottom for about 2 min before moving to the water surface. The other three species immediately moved upward and remained close to the water surface during the rest of the experiment. It is interesting to note that Dimidiochromis compressiceps specimens were capable to maintain their extremely streamlined body in a position suitable to perform ASR. Melanochromis auratus, Tropheus moorii and Neolamprologus brichardi also moved higher up into the water column, but they never assumed the typical position below the water surface to suggest that ASR was performed. The three remaining species, Eretmodus cyanostictus, Julidochromis marlieri and Steatocranus tinanti became very agitated when placed in hypoxic water. Their movements were undirected and they remained close to the bottom of the test tank. Particularly Eretmodus cyanostictus specimens appeared to be extremely vulnerable to this sudden exposure to hypoxia. To prevent mortality of the tested Eretmodus cyanostictus fishes, all the experiments with this species had to be interrupted after 4 min. They reacted less violently when subjected to additional tests under milder hypoxic conditions (1.8-2.5 ppm 0,) however, again we did not observe ASR for Eretmodus cyanostictus.

Discussion Metabolic rate, hypoxia tolerance and adaptive radiation in cichlids

The importance of respiratory adaptations in the evolution of lacustrine cichlids has mainly been inferred from anatomical and field data. Differences in their gill area suggests that even closely related species from Lake Victoria have different oxygen requirements, i.e. oxygen uptake capacities and hypoxia tolerances (Galis and Barel, 1980) and it has been suggested that bathymetric segregation may have played a role in the speciation and current depth distribution

408

Erik Verheyen

et al 400

500 400 300 200

. .

d

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.*

**...

..r. **. . ;*.: . ‘I. . .A . ..*. : *. $3:. .-.

100

t-

n vO

.

*

.

*

40

60

120

160

40

60

120

160

.a*

*.

400 300 200 100 0

40

80

120

0 0

160

PO, (torr) 0”“’ i

500~ 400. 300. zoo100.

l0 . .. :

.a ..:* * .t* 5, ..a . .I’:J.. ,.* . . .’ :... . l -• ,2.**. . , . ’ :*+ .,...‘; . : w* .a:: ‘5 . .l.S . g

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5;t.; 0

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160

,I 160

PO, (torr) Fig. 3. (a-f) The routine rate of oxygen uptake (VO?) subjected to progressive hypoxia (see text). Data shown are grouped data/species for group I; i.e. (a) Melunochromis auratus (N = 3), (b) Tropheus moorii (N = 4), (c) Erermodus cyanostictus (N = 4), (d) Neolamprologus brichardi (N = 2), (e) Julidochromis marlieri (N = 4) and (f) Steatocranus Iinanti (N = 4). (g-j) The routine rate of oxygen uptake (VO,) subjected to progressive hypoxia (see text). Data shown are grouped data/species for groups II and III; i.e. (g) Astatoiilapia burtoni (N = 2), (h) Dimidiochromis compressiceps (N = 4), (i) Nimbochromis venustus (N = 4) and (j) Oreochromis niloticus (N = 5 because only specimens with weight range 16-30 g are shown).

of some of these haplochromine cichlids (Hoogerhoud et al., 1983). Also the distribution of other lacustrine cichlids suggests that they have different oxygen requirements. In the hypolimnion of Lake Tanganyika several cichlid

species occur at 200-210 m depth, even during periods of complete anoxia (Coulter, 1967). On the other hand, a large number of species from lake Tanganyika and Malawi are only found in the rocky littoral habitats where the water is

Respiration

and hypoxia

virtually permanently saturated with dissolved oxygen. All these observations seem to suggest that the metabolic rates of these fishes may relate to their life styles and habitats. However, measured in terms of VO,, the routine metabolic rates of the three groups of cichlids studied here do not reveal significant differences that cannot be explained by an allometric effect of their body weight on VOz. We are aware that VOz and hypoxia tolerance as measured in laboratory conditions can be influenced by varying levels of stress of the test organisms (e.g. Ross and McKinney, 1988), their physiological

in cichlid

fish

409

condition (e.g. Love, 1980), food uptake (e.g. Brett and Groves, 1979), light conditions (e.g. De Silva et al., 1986), temperature (e.g. Beamish, 1964b) or water quality (e.g. Ultch et al., 1980). Thus one could argue that it is unlikely that experiments such as the one described here will detect ecologically relevant differences. However, other studies using essentially the same approach have shown that ecologically relevant differences of VOz and/or hypoxia tolerance between related fish species (Galis and Smith, 1979) and even between populations (Hiippop, 1986) can be detected under similar laboratory conditions. However, it

Aquatic Surface

a

d

C

f

Fig. 4. Cumulative data on the proportion of occasions particular depth range between the bottom of the tank during the period of observation. The black bars (left) bars (right) are the results under normoxic conditions hypoxia and 6 in blank run), (b) Astatotilapia burtoni

Respiration

i

that any of the tested fishes was observed at any (lowest bar) and at the water surface (highest bar) are experimental data under hypoxia, the striped (blank run). (a) Oreochromis niloticus (N = 5 in (N = 7 and 5), (c)

410

Erik Verheyen et al.

remains possible that VO, measured under particular conditions; e.g. as a function of swimming speed; might reveal interspecific differentiation among the cichlids studied here that cannot be detected under the experimental conditions used in our study. Like Galis and Smith (1979) we observe important differences in the capacity for the regulation of VOZ under declining Po, and hypoxia tolerance among the studied Cichlidae. Except for Julidochromis marlieri and Neolamprologus brichardi, all cichlids tested here were clearly capable to regulate their oxygen uptake rate under lowered dissolved oxygen levels. Among this latter group of “aerobic regulators” all species except Oreochromis niloticus and Eretmodus cyanostictus show a very important increase of their VO, under lowered dissolved oxygen levels. The fact that we observe increased VO, under lowered Po, is not unusual. Especially in fish species from well oxygenated habitats, the oxygen uptake rate increases when the ambient P, approaches the Pco,-value (Ott et al., 1980) and suggests that these fishes are seriously disturbed by lowered dissolved oxygen levels. However, no deaths were recorded for the majority of these species. Eretmodus cyanostictus and Steatocranus tinanti were the two only “regulator” species where some specimens died during-or after-being subjected to hypoxia whereas for both “conformer” species studied here a number of specimens died (Table 2). ASR in cichlid jishes We acknowledge that the experimental procedure used to study the presence/absence of ASR in cichlids is unrefined. However, many of the more elaborate experiments usually expose the tested species to gradually lowered dissolved oxygen levels to determine the ASR threshold (e.g. dissolved oxygen level where 10% of the time was spent performing ASR). We feel, however, that the experimental set-up implemented here is appropriate to answer whether or not a given species has the capacity to show ASR under hypoxia. Like many other fishes that are not capable of air-breathing, a number of cichlids respond to decreased dissolved oxygen levels by irrigating their gills with the oxygen-rich water from the thin layer at the water-air interface. This behavioral response is one of the components of a well-integrated strategy of many hypoxia tolerant fishes including a number of neotropical cichlids (Kramer and McClure, 1982; Kramer, 1983) and Oreochromis niloticus (unpublished results). It is interesting to note that in our study the species that show the lowest mortality rates after the respiration measurements under hypoxia, in conditions where ASR cannot contribute to

surviving hypoxia, are the same species that perform ASR. Thus the species that are physiologically better equipped to deal with low dissolved oxygen levels share this behavioral response. However, the proportion of species in our study that perform ASR is low compared to the general trend in literature suggesting that ASR is a widespread behavior among tropical waterbreathing freshwater fishes. We suggest that this discrepancy between our results and literature data may very well be explained by the fact that the species that we tested include very specialized lacustrine species instead of the more often studied neotropical riverine species that are more often exposed to low dissolved oxygen levels in their natural habitats.

Conclusions It is surprising that there are clearly important differences in capacity for the regulation of oxygen uptake under hypoxia and behavioral responses (ASR) under hypoxia for species from essentially similar habitats where dissolved oxygen levels are concerned. Since species like Melanochromis auratus, Tropheus moorii, Julidochromis marlieri, Neolamprologus brichardi and Eretmodus cyanostictus are not likely to be confronted with different oxygen levels in their natural habitats, the observed differences cannot be explained as the result of adaptations to different dissolved oxygen levels. We suggest that the observed differences concerning their capacity to regulate VOZ at PoZ < 155 tort-, their tolerance to exposure to hypoxia and the presence/absence of ASR behavior under hypoxia may be reflections of evolutionary constraints and/or different ecological characteristics of their pre-lacustrine ancestors. Indeed, the fact that Steatocranus tinanti-one of the tested riverine species-is very vulnerable to low dissolved oxygen levels (i.e. suffers high mortality rates and absence of ASR under hypoxia) indicates that some of the ancestral riverine lineages that colonized the Proto-Lake Tanganyika may have been ecological similar to this species, and evolved into lacustrine species with a demand for elevated environmental dissolved oxygen levels. Our observation that two representatives of closely related lamprologine genera Julidochromis and Neolamprologus (Poll, 1986;, Verheyen et al., 1991, Nishida, 1991) are poor aerobic regulators, show limited resistance to hypoxia and do not perform ASR, seems to support this hypothesis. Also the fact that all the tested taxa that are considered to have evolved from haplochromine ancestral stocks (Astatotilapia burtoni, Dimidiochromis compressiceps, Nimbochromis venustus, Melanochromis auratus

Respiration and hypoxia in cichlid fish

and Tropheus moorii; Meyer et al., 1990; Meyer pets. comm.) show a limited mortality and display a pronounced tendency to perform ASR under hypoxia can be interpreted in such an evolutionary context. However, it is clear that testing this hypothesis will require more detailed (eco)physiological studies on the aerobic, anaerobic metabolism and behavioral responses to lowered dissolved oxygen levels (ASR) of cichlid taxa with known ph ylogenetic interrelationships. Ac knowledgemenu-This

research was funded by F.K.F.O. programmes 2.9005.84 and 2.900490. During part of this work E.V. was supported by an I.W.O.N.L. grant. R.B. is a Senior Research Assistant of the National Fund for Screntific Research, Belgium.

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