1. The respiratory behaviour and the rate of O2 consumption and CO2 elimination has been studied in Clarias batrachus under different environmental conditions which were also designed to test its suitability for life in water and on land.

  2. The mean from water and air is about 93 cc/kg/h. It consumes more O2 from air (58·4%) than from water (41·6%). The rate of CO2 release through the air-breathing organs is very low (RQ = 0·11), much more CO2 is released through the gills and skin in water.

  3. When the fish is submerged under air-saturated water and prevented from surfacing is low (about 65 cc/kg/h). However, the fish does not struggle to breath air over a period of 6–8 h in aerated water. It exchanges about 17 % of O2 through the skin and the rest through the gills in aerated water.

  4. If the fish is maintained in still water in a closed chamber is about 61 cc/kg/h. It starts to search for air once the O2 tension in water is reduced below 100 mmHg and this searching becomes vigorous below 60 mmHg ().

  5. When exposed to air its is about 71 cc/kg/h; in air-exposed fish is about 37 cc/kg/h; hence RQ in air is only 0·52. It shows independent respiration in air although in ambient air was reduced to about 80 mmHg and rose to about 51 mmHg.

  6. When the fish is kept in deoxygenated water but allowed free access to air, is low, but RQ air is not reduced (0·51) from that of air-exposed fish. It shows dependent respiration under these conditions when aerial is reduced below 80 mmHg and raised above 50 mmHg.

  7. Clarias batrachus can live in deoxygenated water for several days if allowed free access to air, and appears to be more suited for life in poorly oxygenated water than Saccobranchus fossilis or Anabas testudineus.

The Asian species of air-breathing catfish, Clarias batrachus, lives in freshwater pools which usually have low O2 and high CO2 content and may also dry up during summer. Like several other fishes, this species has an air-breathing organ which functions in direct gas exchange with the atmospheric air. Munshi (1961) made a detailed study of the structure of the respiratory organs and also described the mechanism of ventilation. The accessory organs chiefly comprise (a) the supra-branchial chambers situated dorsally to the gill cavities and lined by the respiratory membrane, (b) the fan or gill plates borne on each arch and (c) the respiratory tree or dendritic plates borne by the second and fourth gill arches.

The African species (C. lazera) seems unable to obtain enough O2 via the gills alone for long periods even when submerged in air-saturated water, as it died after 14–17 h (Moussa, 1957). Recently it has been observed that Anabas testudineus and Saccobranchus fossilis can survive in air-saturated water, exchanging gases with water alone for 6–8 h or for longer in Saccobranchus, without showing much sign of stress or struggling (Hughes & Singh, 1970, 1971). Moreover, Saccobranchus can survive even in hypoxic water ( 50–110 mmHg), depending on aquatic respiration alone. Since Clarias batrachus and Saccobranchus fossilis occur in similar habitats, often sharing the same ponds, it is of interest to compare their respiratory behaviour and gas exchange. The experiments were also designed to find out the suitability of this animal for life in water and on land under various environmental conditions.

Live specimens of magur (Clarias batrachus) were collected in India and transported by air to England in Polythene bags containing water and charged with O2. The fishes were kept in large aquaria at 25 ± 1 °C and acclimated to laboratory conditions. They were fed at regular intervals with tropical fish food and meat. Oxygen uptake and CO2 release from water and/or air were measured by methods similar to those previously described (Hughes & Singh, 1970, 1971). The measurements in water were made using a Beckman O2 macroelectrode with a 160 physiological gas analyser. O2 and CO2 contents in air were determined using a 0·5 ml Scholander gas analyser. total of 12 fish were used and some experiments were repeated using the same individuals under similar conditions. All experiments were carried out at 25 ± 1 °C.

Clarias batrachus is an obligate air-breather, coming to the water surface at irregular intervals to gulp air. Intervals between air breaths usually vary between 2 and 20 min in relation to the O2 content of the water and of the air. When exchanging gases with normal tap water and air, the opercular frequencies are 24–32/min but their amplitude is very low. In air of low O2 content (5–10 vols%), air-breathing frequencies increase and the intervals vary from 15 sec to 5 min. When the fish is given free access to N2 but maintained in air-saturated water, the air-breathing intervals are reduced to between 20 sec and 2 min, especially 7–8 h after the beginning of an experiment. Moreover, the fish often repeats a nitrogen breath as if ‘not satisfied’.

When prevented from surfacing, Clarias can survive in air-saturated water for at least 6–8 h without showing many signs of stress or struggling. The opercular frequency and amplitude increase during such submersion. When exposed to air, Clarias breathes quietly for 5–6 h; the air-breathing intervals are irregular during this period.

in water when access to air prevented

(a) Air-saturated water

The fish were a little restless at the start of an experiment but quickly settled down following attempts to gain access to the air. The opercular frequencies increased (38–50/min) as did their amplitude. Individual variations in frequency and amplitude were also observed. The fish can live under resting conditions, breathing water alone, for 6–8 h. During the later part of some experiments the fish started struggling and rising to the air tube apparently in search of air. The mean of six fishes was 64·9 cc/kg/h (Table 1); there was no good correlation between the weight of the fish (80–157 g) and the O2 consumption/gm.

Table 1.

Summary of results obtained for the rate of oxygen consumption and carbon dioxide release in Clarias batrachus when exposed to different experimental conditions

Summary of results obtained for the rate of oxygen consumption and carbon dioxide release in Clarias batrachus when exposed to different experimental conditions
Summary of results obtained for the rate of oxygen consumption and carbon dioxide release in Clarias batrachus when exposed to different experimental conditions

(b) via skin in air-saturated water

The mean rate of O2 consumption through the skin was 10·71 cc/kg/h. As deduced from the total in air-saturated water,, via the skin constitutes about 16·6% of the total, and the remainder occurs via the gills when the fish is not allowed free access to air.

(c) in a continuous flow of hypoxic water

When Clarias is kept in hypoxic water with access to air prevented, the frequency and amplitude of gill ventilation increase considerably (Fig. 1). in nearly 50% saturated water is reduced to almost half that in fully saturated water. At a of 40 mmHg, is reduced to 16·9 cc/kg/h. The amplitude of the ventilatory movements becomes very marked and the fish begins to struggle and rises repeatedly to the air tube.

Fig. 1.

Reductions in the rate of O2 consumption (▴) with lowering of O2 tension in water. The opercular movements (▪) and the depth of gill ventilation increase in low O2 tension but the effect of hypoxia is quite apparent throughout the range (156–40 mmHg WPo2) studied. The range of the determinations at each WPo2 level are given in each case.

Fig. 1.

Reductions in the rate of O2 consumption (▴) with lowering of O2 tension in water. The opercular movements (▪) and the depth of gill ventilation increase in low O2 tension but the effect of hypoxia is quite apparent throughout the range (156–40 mmHg WPo2) studied. The range of the determinations at each WPo2 level are given in each case.

If after breathing hypoxic water ( about 40 mmHg) for about 1 h the fish chamber is flushed with air-saturated water, the opercular amplitude and frequency return to normal (32–40/min) in 5–7 min as the fish comes to rest.

(d) from still water in a closed chamber

The effect of gradual hypoxia is clearly seen under such conditions with the fish breathing water alone. About h after having made unsuccessful attempts to gain access to the air, the fish rests at the bottom of the respirometer breathing water as is gradually reduced to 70–80 mmH. Below this level the fish becomes restless and struggling increases with further decline in is reduced from 95·6cc/kg/h at 151 mmHg to 44·0 cc/kg/h at 70 mmHg.

When the fish was given free access to air after had been gradually reduce to about 32 mmHg it took several air breaths within a few minutes. After such repeated air breaths, it stopped water-breathing for some time but later continued with a normal pattern of aquatic and aerial respiration.

It is of interest that, when the fish is subjected to gradual hypoxia in a closed respirometer, is slightly raised (Fig. 2) compared to its level at the same in a continuous flow system (Fig. 1). The opercular frequency and amplitude of breathing increase under gradual hypoxia in a closed respirometer (Fig. 2).

Fig. 2.

Dependent form of respiration in water in a closed chamber when Clarias is prevented from surfacing. V˙o2 (▴) is lowered throughout a WPo2 range between 70–155 mmHg, although the frequency of opercular movements (▪) and the amplitude of breathing are increased at low O2 tensions. However, significantly different rates of O2 consumption at the same WPo2 are found in a continuous flow of water (Fig. 1) and the closed system (Fig. 2). Range of determinations in each case is also given at different O2 tensions.

Fig. 2.

Dependent form of respiration in water in a closed chamber when Clarias is prevented from surfacing. V˙o2 (▴) is lowered throughout a WPo2 range between 70–155 mmHg, although the frequency of opercular movements (▪) and the amplitude of breathing are increased at low O2 tensions. However, significantly different rates of O2 consumption at the same WPo2 are found in a continuous flow of water (Fig. 1) and the closed system (Fig. 2). Range of determinations in each case is also given at different O2 tensions.

V˙o2 and V˙co2 in air-saturated water with free access to air

The mean from air and water is 93·3 cc/kg/h; the fish consuming more O2via the air-breathing organs (58·4%) than from water (41·6%) (Table 1). However, in one fish, which became very active during the experiment, increased to about 170 cc/kg/h during a period of observation (Table 1) as compared with its mean resting of 97·14 cc/kg/h. The amount of O2 obtained from air (63·14%) was still greater an from water (36·86 %) during this active period. Opercular frequency rose to 56·60/min and the amplitude of the gill ventilatory movements was very great. Air-breathing intervals were much reduced. The rate of CO2 release to the air from the air-breathing organs was 88 cckg/h. The gas-exchange ratio for aerial respiration was only 0·11. The amount of CO2 released into the water was not determined as it was not possible to measure the small changes in CO2 content of the water circulated through the respirometer.

V˙o2 and V˙co2 of air-exposed fish

When Clarias is exposed to air, is 71·17 cc/kg/h (Table 1). Measurements were made after about h when the fish had settled in a closed air respirometer. The fish consumed a little more O2 during the first h when aerial was 134–152mmHg (Fig. 3). In the later part of the experiment it consumed a nearly constant level of oxygen at between 134 and 79 mmHg (Fig. 3). During the h of observation the in the closed respirometer gradually increased from 7 mmHg to 52·4 mmHg as a result of breathing and rebreathing the same air, but the increased CO2 levels did not appear to have any marked effect on the of this fish (Fig. 3).

Fig. 3.

The rate of O2 consumption (▵) and CO2 elimination (▿) in the air-exposed fish over a period of about 6 h in a closed respirometer. The fish consumes little more O2 in the first 112 h but later V˙o2 is levelled at about 65–70 cc/kg/h. V˙co2 shows a clear dependence on V˙o2 throughout the observations. The Po2 (○) in the closed air-space declined from about 150–80 mmHg and Pco2 (•) rose to about 51·5 mmHg, but neither of these appear to have any marked effect on V˙o2. The range of determinations at each 12 h is given for V˙o2 and V˙co2 and only mean values are plotted for Po2, and V˙co2.

Fig. 3.

The rate of O2 consumption (▵) and CO2 elimination (▿) in the air-exposed fish over a period of about 6 h in a closed respirometer. The fish consumes little more O2 in the first 112 h but later V˙o2 is levelled at about 65–70 cc/kg/h. V˙co2 shows a clear dependence on V˙o2 throughout the observations. The Po2 (○) in the closed air-space declined from about 150–80 mmHg and Pco2 (•) rose to about 51·5 mmHg, but neither of these appear to have any marked effect on V˙o2. The range of determinations at each 12 h is given for V˙o2 and V˙co2 and only mean values are plotted for Po2, and V˙co2.

The mean rate of CO2 release appears to be related to the rate of O2 consumption at a particular time of observation during 5–6 h of exposure (Fig. 3). Mean was 34·77 cc/kg/h at an RQ of 0·52 (Table 1).

Of the five fishes that were exposed to air on one or more occasions for periods of 5–6 h, none showed signs of restlessness when kept in moist conditions. Considerable changes in skin coloration were found in most individuals, particularly in the later part of an experiment. Two fish died 24 h after having been returned to water following a 5–6 h period of air exposure. There appeared to have been considerable damage to the skin of these fish which might have been due to the long periods of exposure to air. The remainder of the fish survived for at least several months when returned to water after air exposure.

V˙o2
and
V˙co2
from still air of fish in deoxygenated water

When Clarias is kept in still deoxygenated water with free access to air, in a closed respirometer, its is very much reduced (mean = 40·84 cc/kg/h) (Table 1). The air-breathing frequencies increased markedly when the of the air was below 80 mm Hg. But it was observed that Clarias can live for 4–5 days or more in nearly deoxygenated water so long as it is free to come to the water surface and breathe air. from air is low (17·24 cc/kg/h) when a fish was kept in deoxygenated water. decreased from the beginning of an experiment as is clear in Fig. 4. The fish consumes less O2 at the lower levels of and the amount of CO2 released decreases. It is interesting to find that the resulting gas-exchange ratio with air was 0·51, which is almost identical with the value obtained for fish exposed to air (Table 1).

Fig. 4.

V˙o2 (▫) and V˙co2 (▿) plot in fish living in deoxygenated water but given free access to still air. V˙co2 in this condition is sharply reduced after Po2 (○) has fallen below 80 mmHg and Pco2 (•) raised above 45 mmHg as a result of breathing and rebreathing of still air. The fish shows a dependent form of respiration in such a condition, which is markedly different when it is exposed to the air (Fig. 3).

Fig. 4.

V˙o2 (▫) and V˙co2 (▿) plot in fish living in deoxygenated water but given free access to still air. V˙co2 in this condition is sharply reduced after Po2 (○) has fallen below 80 mmHg and Pco2 (•) raised above 45 mmHg as a result of breathing and rebreathing of still air. The fish shows a dependent form of respiration in such a condition, which is markedly different when it is exposed to the air (Fig. 3).

The present study has shown how Clarias batrachus can survive some of the respiratory stresses which it might encounter in its normal environment. Survival is achieved in several ways; for example, by lowering the total metabolic rate when exposed to air or completely immersed in water (Table 2), or modifying the pattern and depth of its ventilatory movements. The relative rates of O2 uptake, and CO2 release through the gills and air-breathing organs may also change in air or water. Changes in O2 uptake and CO2 release by these routes can be related to their suitability as respiratory media. Similar adjustments in metabolic rate have been found in other air-breathing fishes such as Anabas (Hughes & Singh, 1970) and Saccobranchus (Hughes & Singh, 1971).

Table 2.

V˙o2in different experimental respiratory media of three air-breathing teleosts studied at 25 ± 1℃

V˙o2in different experimental respiratory media of three air-breathing teleosts studied at 25 ± 1℃
V˙o2in different experimental respiratory media of three air-breathing teleosts studied at 25 ± 1℃

In A. testudineus, C. batrachus and S.fossilis, the total is reduced in fish prevented from surfacing in air-saturated water. The total under water at 25 °C is 64·94 cc/ kg/h in Clarias, which is very close to the values obtained for Anabas (75 cc/kg/h), Saccobranchus (66·5 cc/kg/h) and in water-breathing fishes such as the bluegill sunfish, rainbow trout, and brown bullhead catfish (about 70 cc/kg/h) by Marvin & Heath (1968). However, the figures obtained for the rainbow trout by other authors have been lower than this (Hughes & Saunders, 1970). Lowering of the total when these air-breathing fishes are submerged may be related to the thickness of the gill epithelium and their smaller surface area, but also the fish is less active because it does not come to the surface.

Munshi & Singh (1968) reported a very high haemoglobin content in the blood of Clarias batrachus and Saccobranchus fossilis and a few other air-breathing teleosts. Some preliminary determinations of the O2 capacity of the blood of Clarias and Saccobranchus have shown that the O2 capacity of the blood in these fishes is indeed high (Clarias, about 18·0 vols% and Saccobranchus, 17·5 vols %) (unpublished observations). This similarity in an important respiratory characteristic of the blood indicates that features of the water/blood pathway are important factors reducing the rate of O2 transfer from water in the gills of Clarias relative to Saccobranchus.

From Table 3 of Saccobranchus (Hughes & Singh, 1971) it can be seen that among both air-breathing teleosts and lungfishes the amount of O2 entering from the water (via the gills and/or skin), and from air (via the air-breathing organs and/or skin), vary a great deal in different species. This variation appears to be related to the nature and development of the gills and air-breathing organs and also to the ability of the skin to participate in gaseous exchange. As mentioned earlier, about 16 % of the total occurs through the skin and 84 % by the gills in both Clarias and Saccobranchus when exchanging gases with air-saturated water alone (Table 3).

Table 3.

A comparison between certain air-breathing teleosts in respect of percentageV˙o2from water via the skin and via the gills when the fish is submerged under water and not allowed to surface

A comparison between certain air-breathing teleosts in respect of percentageV˙o2from water via the skin and via the gills when the fish is submerged under water and not allowed to surface
A comparison between certain air-breathing teleosts in respect of percentageV˙o2from water via the skin and via the gills when the fish is submerged under water and not allowed to surface

The effect of hypoxia in water is quite apparent in Clarias between tensions of 80–150 mmHg when the fish is breathing in a closed chamber (Fig. 2). The effect is also clear in a continuous flow of hypoxic water (Fig. 1). Dependent aquatic respiration is also found in Anabas (Hughes & Singh, 1970) but Saccobranchus shows an independent respiration in water (Hughes & Singh, 1971). This similarity in behaviour during hypoxia of Clarias and Anabas is shown in Fig. 5 by the graph for the three species measured under similar conditions. The more linear decline in with time in Saccobranchus is similar to that found in Electrophorus by Farber & Rahn (1970).

Fig. 5.

A comparison between three air-breathing fishes, Clarias batrachus (▾), Anabas testudineas (▪) and Saccobranchus fossilis (▴), in the process of O2 uptake from water as indicated by lowering in WPo2 after the onset of the experiment, when they are maintained in still water in a closed chamber and prevented from surfacing. Clarias and Anabas show dependent respiration and resemble each other, whereas Saccobranchus shows an independent water respiration and resembles more the O2 independent water-breathing fishes. For details see text. Only mean values have been plotted and the values for Anabas and Saccobranchus are those determined by Hughes & Singh (1970, 1971).

Fig. 5.

A comparison between three air-breathing fishes, Clarias batrachus (▾), Anabas testudineas (▪) and Saccobranchus fossilis (▴), in the process of O2 uptake from water as indicated by lowering in WPo2 after the onset of the experiment, when they are maintained in still water in a closed chamber and prevented from surfacing. Clarias and Anabas show dependent respiration and resemble each other, whereas Saccobranchus shows an independent water respiration and resembles more the O2 independent water-breathing fishes. For details see text. Only mean values have been plotted and the values for Anabas and Saccobranchus are those determined by Hughes & Singh (1970, 1971).

It is interesting to notice the slightly different effects of hypoxic water on Clarias by these two methods, i.e. still water in a closed respirometer and a continuous flow of hypoxic water (Figs. 1, 2). In rainbow trout, changes with different O2 tensions in water and has been shown to be related to the rate at which is changed (Hughes & Saunders, 1970). The same process appears to be involved in the two methods of producing water hypoxia in the present study. However, as the rate of O2 consumption at the same O2 tension appears to be higher in still water, perhaps it suggests that the fish may extract O2 more readily under these conditions. This could be advantageous for fish like Clarias and Saccobranchus and others which are found in still-water ponds of tropical countries.

Moussa (1957) found that the African species Clarias lazera could survive only 14–17 h when kept in aerated water and prevented from surfacing. During this period the fish struggled hard to gulp air and finally died. The present study shows that C. batrachus does not show any signs of stress for 6–8 h when maintained in air saturated water and prevented from surfacing. During submersion C. batrachus, like C. lazera, initially showed an increase in opercular frequency but the depth of breathing increased only slightly in C. batrachus and once the frequency had risen to between 38–50/min it showed a fairly constant rate and depth of gill ventilation. This respiratory behaviour of C. batrachus differs from C. lazera (Moussa, 1957) as the latter showed a continuously increasing rate and depth of breathing and finally died of asphyxia. Magid (personal communication) also observed that C, lazera survived for several days in air-saturated water without showing many signs of stress when prevented from surfacing.

The present observations show that C. batrachus does not usually attempt air breathing until is reduced below 100 mm Hg. Gill ventilation increases in hypoxic water and so maintains an optimal The fish may depend on aquatic respiration alone at tensions as low as 60–100 mmHg, but below this level it must surface to gulp air. Thus C. batrachus can survive for longer periods, in both air-saturated and hypoxic waters exchanging gases with water alone than was observed for C. lazera by Moussa (1957). This adaptation is probably advantageous to C. batrachus as it inhabits stagnant waters with low O2 content except in the rainy seasons when the waters may approach air-saturation.

Clarias in still deoxygenated water but given free access to still air has a very much reduced total (40·84 cc/kg/h). The fish shows little activity and rests quietly and apparently waits for an air-breath with its head directed towards the air tube. Low activity and low O2 content in the air tube (reduced by breathing and re-breathing still air) might be the reason for such a low from air under these conditions. The skin of Clarias is smooth, thin and well vascularized, so that some O2 absorption probably occurs by this route in fish exposed to air and/or placed in oxygenated water. In fact, Clarias absorbs about 16% of its total O2 cutaneously and the remainder through the gills when submerged in aerated water. In deoxygenated water O2 exchange is not possible through the body surface. Consequently, not only is very low in deoxygenated water but the effect of hypoxia is also more apparent (Fig. 4). However, in Anabas from air when it was maintained in deoxygenated water was very high (133 cc/kg/h) (Hughes & Singh, 1970). But the Anabas measurements were made using continuously flowing deoxygenated water and the fish was active during the experiments. Direct comparison of levels and effects of between Clarias and Anabas when in deoxygenated water is therefore not possible. However, under the same conditions of Clarias is slightly greater than that of Saccobranchus (Table 2).

When Clarias was exposed to air did not show much dependence on aerial between 78 and 134 mmHg. It seems likely therefore that the skin of this fish helps O2 absorption when exposed to air. A very similar response of metabolism under similar conditions was also observed in Saccobranchus (Hughes & Singh, 1971). Berg & Steen (1965) found an efficient absorption of oxygen via the skin in the air-exposed eel, Anguilla vulgaris. Moreover, it is interesting to note the linearity of plots for air-exposed Clarias and Saccobranchus, both of which indicate O2 independence whereas Anabas shows a dependence on in air (Fig. 6). This similarity between Clarias and Saccobranchus is probably related to their ability for cutaneous respiration particularly at lower levels (i.e. below about 125 mmHg).

Fig. 6.

A comparison between Clarias batrachus (▿) Saccobranchus fo ssilis (▵) and Anabas testudineus (▫) of their O2 uptake from still air as shown in terms of lowered V˙o2 after the onset of observation in a closed chamber. Both Clarias and Saccobranchus show O2 independence and follow the linear pattern while Anabas shows dependent respiration in which Pco2 is lowered as the V˙o2 in air is reduced below 125 mmHg as a result of breathing and re-breathing of the still air. For details see text.

Fig. 6.

A comparison between Clarias batrachus (▿) Saccobranchus fo ssilis (▵) and Anabas testudineus (▫) of their O2 uptake from still air as shown in terms of lowered V˙o2 after the onset of observation in a closed chamber. Both Clarias and Saccobranchus show O2 independence and follow the linear pattern while Anabas shows dependent respiration in which Pco2 is lowered as the V˙o2 in air is reduced below 125 mmHg as a result of breathing and re-breathing of the still air. For details see text.

Lenfant & Johansen (1968) observed that in Protopterus aethiopicus is independent of CO2 in air until the reaches 35 mmHg, but above this level is reduced. In Clarias it can be seen that of air-exposed fish is independent of in air as high as 51·0 mmHg (Fig. 3). Although there is some lowering in total during the first few hours of air-exposure, this could not be accounted for by the increased aerial Saccobranchus also, of air-exposed fish is independent of as high as 15–20 mmHg. This indicates that Clarias has even higher tolerance of in air than Protopterus.

Table 4 indicates that in almost all of the air-breathing fishes studied much more CO2 is released into the water than into the air. If we assume a total RQ of about 1 from water and air in Clarias and Saccobranchus, as indeed is the case in Anabas and Electrophorus, then measurements made on CO2 released into the air clearly indicate that in Clarias and Saccobranchus also much CO2 is released into the water (Table 4). However, it must be borne in mind that as the skin takes part in gas exchange in Clarias and Saccobranchus, it is not improbable that some CO2 is released into the water through the skin of these fishes.

Table 4.

A comparison of theV˙co2and RQ in water and air in some air-breathing fishes

A comparison of theV˙co2and RQ in water and air in some air-breathing fishes
A comparison of theV˙co2and RQ in water and air in some air-breathing fishes

When Clarias is exposed to air the gas-exchange ratio is low (0·52). Similar reduction in the RQ of air-exposed Anabas and Saccobranchus (Hughes & Singh, 1970,1971) has also been determined. However, it is interesting to find that RQ with air in Clarias kept in deoxygenated water is also about 0·5, whereas in Anabas and Saccobranchus it is much lower (Table 4). Our control experiments have shown that there was no significant diffusion across water and air in the respirometers designed for Anabas and Saccobranchus (Hughes & Singh, 1970, 1971). The present study indicates that diffusion of gases across the two media in such experiments under these conditions is negligible, since the RQ in fish exposed to air and to deoxygenated water is nearly the same. The use of paraffin oil as a barrier in all these three fishes was avoided, as Spurway & Haldane (1963) reported that oil interferes with breathing in Anabas.

It has been observed that in nearly deoxygenated water and given free access to atmospheric air, C. batrachus can survive for 4–5 days or more. Moussa (1957) found that when C. lazera was maintained in deoxygenated water and given free access to atmospheric air, it died after 11–16 h due to exhaustion because of its activity in taking frequent air-breaths. However, neither Clarias nor Saccobranchus died of exhaustion although showing increased air-breathing in our experiment when they had to swim upwards about 4 in. in order to obtain air.

It appears that when Clarias is in deoxygenated water it exchanges both CO2 and O2 through air-breathing organs and cuts down any exchange of gases through the skin or gill. This would of course be advantageous for the fish, since in eliminating CO2 through the skin or gill in deoxygenated water these fishes would run a danger of losing O2 from blood into the deoxygenated water. This could be achieved in Clarias probably by shunting the blood flow into the peripheral capillaries of the skin. A shunting of blood flow through a part of the respiratory organ which is not needed for gas exchange in an unsuitable environment has also been suggested by Johansen, Hanson & Lenfant (1970) and Hughes & Singh (1970). It may be concluded therefore that Clarias is probably more adapted for its life in water of very low O2 tensions than is Saccobranchus or Anabas.

We should like to thank the Nuffield Foundation for financial support. Funds for the supply of fish were kindly made available through the Smithsonian Institution, Washington, D.C., and we should especially like to thank Dr Stan Weitzman for his help.

Berg
,
T.
&
Steen
,
J. B.
(
1965
).
Physiological mechanisms for aerial respiration in the eel
.
Comp. Biochem. Physiol
.
15
,
469
84
.
Farber
,
J.
&
Rahn
,
H.
(
1970
).
Gas exchange between air and water and the ventilation pattern in the electric eel
.
Resp. Physiol
.
9
,
151
61
.
Hughes
,
G. M.
&
Saunders
,
R. L.
(
1970
).
Responses of the respiratory pumps to hypoxia in the rainbow trout (Salmo gairdneri)
.
J. exp. Biol
.
53
,
529
45
.
Hughes
,
G. M.
&
Singh
,
B. N.
(
1970
).
Respiration in air-breathing fish, the climbing perch, Anabas testudineas (Bloch). I. Oxygen uptake and carbon dioxide release into air and water
.
J. exp. Biol
.
53
,
265
80
.
Hughes
,
G. M.
&
Singh
,
B. N.
(
1971
).
Gas exchange with air and water in an air-breathing catfish, Saccobranchus (Heteropneustes) fossilis
.
J. exp. Biol
,
(in the Press)
.
Johansen
,
K.
,
Hanson
,
D.
&
Lenfant
,
C.
(
1970
).
Respiration in a primitive air-breather, Amia calva
.
Resp. Physiol
.
9
,
162
74
.
Lenfant
,
C.
&
Johansen
,
K.
(
1968
).
Respiration in the African lungfish Protopterus aethiopicus. I. Respiratory properties of blood and normal patterns of breathing and gas exchange
.
J. exp. Biol
.
49
,
437
52
.
Marvin
,
D. E.
&
Heath
,
A. G.
(
1968
).
Cardiac and respiratory responses to gradual hypoxia in three ecologically distinct species of fresh-water fish
.
Comp. Biochem. Physiol
.
27
,
349
55
.
Moussa
,
T. A.
(
1597
).
Physiology of the accessory respiratory organs of the teleost Clarias lazera (C. & V
.).
J. exp. Zool
.
136
,
419
54
.
Munshi
,
J. S. D.
(
1961
).
The accessory respiratory organs of Clarias batrachus (Linn
.).
J. Morph
.
109
,
115
39
.
Munshi
,
J. S. D.
&
Singh
,
B. N.
(
1968
).
The structure of the respiratory epithelium of the gills of certain freshwater and air-breathing fishes
.
J. Zoot. (India)
9
,
91
107
.
Spurway
,
H.
&
Haldane
,
J. B. S.
(
1963
).
The regulation of breathing in a fish, Anabas testudineus
.
In The Regulation of-Human Respiration
(ed.
D. J. C.
Cunningham
and
B. B.
Lloyd
), pp.
431
4
.
Oxford
:
Blackwell
.