Comp. BIoehem. Physlol., 1975, Vol. 50A, pp. 121 to 124. Pergamon Press. Prlnted ln Greet Brita#t
CUTANEOUS RESPIRATION IN THE CONGO EEL AMPHIUMA MEANS (AMPHIBIA: URODELA) P. J. BENTLEY* Departments ot" Ophthalmology and Pharmacology, Mount Sinai School of Medicine of The City University of New York, New York 10029, U.S.A. (Reeeioed 1 October 1973)
A b s t r a c t ~ l . At 20~ the Congo eel, Amphiunm means, can survive for at least 24 hr when submerged in aerated tap water. Oxygen is readily taken up, from the bathing water, across the skin of these amphibians. 2. The rate of oxygen uptake declines linearly as the oxygen tension in this solution decreases. 3. In air, A. tneans breathes sporadically; the rate of oxygen consumption was similar to that when they were submerged in water nearly saturated with air. 4. When pulmonary ventilation was plaarmacologically blocked (with gallamine triethiodide) the rate of metabolism was maintained by Oa uptake across the integument. 5. The ability of A. means to utilize its skin for respiration in air and water is compared to that in two other urodeles Sh'en lacertlna anti Neeturtts ntacttlosus.
INTRODUCTION AMPrmmi^NS have skin which is relatively permeable c o m p a r e d to that of most other vertebrates. This provides an avenue across which osmotic exchanges oi' water a n d salts as well as respiratory gases may occur. I n d e e d among amphibians the skin is a m a j o r p a t h w a y for the loss of carbon dioxide while a substantial uptake of oxygen also takes place (see, for instance, Krogh, 1941; Jones, 1972). Plethodontid salamanders lack lungs so that all of their r e s p i r a t o r y exchanges take place across their integument. The anabystomatid salamanders, on t h e other hand, possess lungs but it has been shown t h a t at 15~ in air, as much as 75 per cent of their oxygen u p t a k e occurs across the skin (Whitford & H u t c h i s o n , 1966). While anurans have a specialized b l o o d supply to the skin this is not present in urodeles a n d the latter are apparently not comp r o m i s e d by this. I have investigated the rates of cutaneous oxygen u p t a k e in a neotenous urodele, the congo eel, A m p h i u m a means (Urodela: Amphiumidae), during s u b m e r s i o n in its usual aquatic environment and when it is in air. This is an extension of recently described experiments on several other species of urodele and anuran amphibians (Bentley & Shield, 1973 ; Shield & Bentley, 1973). A. means lacks the prominent external gills seen in some neotenous, or "semi-larval", urodeles. It has lungs and in A m p h i u m a trMactyhtm these are * Correspondence to: Dr. P. J. Bentley, Dept. or" Ophthalmology, Mt. Sinai School of Medicine, 100 Street and Fifth Avenue, New York, N.Y. 10029. 121
used for respiration (Toews et al., 1971) though the animals only surface irregularly (on the average once every 45 rain). In the intervening periods the oxygen tension in the lungs declines to levels as low as 25 mmHg. I have found that the cutaneous uptake of oxygen in A. means kept in water saturated with air (at 20~ is adequate for their normal resting requirements; however, when the external pO~ is reduced there is a linear decline in the rate o f cutaneous oxygen uptake so that in poorly mixed or stagnant water pulmonary respiration may be advantageous. MATERIALS AND METHODS Congo eels, Amphiuma means, were obtained from a biological supplier (Mr. W. F. Prince, Silver Springs, Florida). They weighed 59-100 g. These animals were kept in a tank of tap water at 18-22~ They were starved for 10 days prior to the experiments. Oxygen uptake during submersion in tap water was measured with the aid of an oxygen electrode and monitor (YSI 53). The Congo eels were placed in a chamber (volume 1600 ml) through which water was rapidly circulated so that the mixing time was about 2 rain. The oxygen electrode was placed in contact with the fluid in a smaller chamber (8 ml) through which some of this water was circulated and where the temperature and mixing could be controlled more rigidly than in the main bath. The temperatures in the two baths corresponded to within 0.1~ The bathing media was initially saturated with air and after sealing off the system the decline in oxygen tension was measured and recorded on a chart recorder (Angus Esterllne). The system was checked for leaks after reducing the oxygen tension by bubbling the water with nitrogen.
122
P.J. B~NTI.I~Y
The oxygen uptake in Congo eels kept exposed in air was measured with the aid of a paramagnetic oxygen analyzer (Beckman E2). The animals were placed, on damp paper towels, in chambers through which air was passed at a rate of about 60 cm~/min and the change in oxygen tension of the effluent was continuouslymonitored. The oxygen analyzer was calibrated by reducing the partial pressure of oxygen in the cell and this was frequently checked with standard gases. The oxygen consumption was expressed as /zl O, at STP. The surface areas of the Congo eels were calculated using the formula: surface area cma - 12.5 g@ as used by Benedict (1934) for snakes (which are of a similar shape). This approximation was used to facilitate comparison with rates of oxygen uptake across the integument as reported in other species. RESULTS Survival o f congo eels during submersion in water Five C o n g o eels (mean weight 72 g) were submerged in a bath o f tap water at 20~ This was circulated and kept saturated with air. After 24 hr all of the animals were alive and apparently in good condition, Oxygen uptake across the integument would appear to be adequate to sustain life under these conditions, Oxygen uptake during submersion in water The oxygen uptake of six A. means (mean weight 3 g) was measured at 20~ during forced submer-
sion in tap water. When the saturation of the water was between 100 and 95 per cent the oxygen uptake was 28.0+ 1.6 cm" O d k g per hr. As the external pO~ declined, however, so did the oxygen uptake, This occurred in an essentially linear manner (Fig. 1) and there was no evidence of any regulatory mechanisms that could facilitate or maintain the normal rate of oxygen consumption. With respect to oxygen uptake across the skin A. means thus behaves as an "oxygen-conformer". Oxygen uptake in air These experiments were performed with a dual purpose: firstly, to see what the rate of oxygen consumption may be expected to be when the Congo eels have continual access to air; secondly, following a pharmacological blockade of p u l m o n a r y respiration (with gallamine triethiodide) to record the rate of oxygen uptake across the integument. In seven A. means (mean weight, 77 g) the rate or" oxygen consumption at 21~ measured over a 2-hr period (following about 2 hr that were necessary to allow the animals to adjust to conditions in the measuring chamber) was 32.65:4.0 cmS/kg p e r hr, This is similar to that seen during submersion w h e n only cutaneous respiration was possible. T h e pattern of breathing varied a great deal from a n i m a l to animal, some breathing eight or nine times a n hour, others only about twice in that time. T w o examples are shown in Fig, 2 which records a p p a r e n t
30
0
0 0 20 o
0 o
E 0
156
I 125
I 94
I 62
I 31
I 0
External ,aOz mm Hg
Fig. 1. Oxygen uptake (across the integument) of A. means when submerged in tap water at 20~ and with a declining oxygen tension. Each point represents the mean value for five animals (mean weight, 84 g).
Respiration in Amphflmta
123
200
(o)
ISO
I00
5C
Be
(b)
.P. (a.
50
C)
_1
,
3O
..
6O
I
90
Mia
Fig. 2. Respiration of two A. means at 20~ in air. Animal A weighed 92 g and had an overall rate of O~ consumption of 30 cm3/kg per hr. Animal B was 66 g and consumed 34 cm~ O~tkg per hr. changes in oxygen consumption reflecting the periodic excursions in the pO~ of the effluent air. Respiratory movements were inhibited by injecting the C o n g o eels with gallamine triethiodide (about 40 mr/animal). After 1-2 hr no respiratory movements were visible and the excursions of the oxygen tension, described above, were no longer apparent so that a steady value was seen. The rate of oxygen consumption in the same Congo eels was then 33"3 -/- I ' 4 cm~/kg per hr which is the same as when the animals were ventilating their lungs. DISCUSSION
A. m e a n s readily accumulates oxygen across its integument when it is either submerged in water or is exposed to air. Under the experimental conditions, resting animals in water saturated with air at about 20~ this was adequate to sustain the Congo eels and was similar to that consumed when the lungs could also be ventilated. A comparison
between A. means and two other aquatic urodeles, Siren laeertina (the mud eeI) and Necturus maculosus (the mudpuppy) is shown in Table 1. In water the rate of oxygen uptake across the skin surface was similar in the three species. In air the rate of cutaneous uptake across the skin of N . mactdosus was about half that in the other two urodeles. In A. means cutaneous uptake of oxygen corresponded to the total uptake of the animals when exposed to air and when they were able to ventilate their lungs. In the other species, however, this only represented about 50 per cent of their requirements under the experimental conditions. The difference reflects the lower metabolic rate of A. means rather than a more favourable rate of cutaneous uptake. In addition, the other urodeles were slightly larger so that their surface area-body weight ratio would be less favourable. This, however, could not account for the observed differences. In the experimental conditions the skin appeared to be adequate to support the respiration o f A. means.
124
P . J . BENTLEY
Table 1. Cutaneous oxygen uptake at 20--22~ (in terms of surface area) in A. means compared to two other species of urodeles /L10Jcm~ skin per hr
Species
Submerged in water
In air: pulmonary respiration inhibited*
Normal: in air (or water for N. mactdosus)
A. means (6) S. laeertina? (3) IV. maculosttst (9)
I5.0_+ 1.2 17.2+ 1"4 15.5_+ 1.1
17'6_+0.9 (7) 17"5-+0'8 (6) 10"2+__0.6 (7)
17.1 +2.0 (7) 34-0+ 1"4 (6) 25'7___2.6 (7)
* With injected gallamine triethiodide. t From Shield & Bentley (1973) and Bentley &Shield (I973). For the measurements of cutaneous oxygen uptake in water the gills of S. lacerth,a and N. maculosus were tied off. The average weights were: A. means, 80 g; S. laeertina, 144 g; and iV. maeulosus, 134 g. Results axe as mean + S.E. Number of experiments in parentheses. Whether or n o t this is always so is doubtful, for as apart from the above surface area-weight relationships, the metabolic rate can vary considerably depending on activity, temperature, seasons and so on. I n addition the oxygen tension in natural waters can be low (such as in stagnant water) which would considerably limit its rate of uptake. The skin can nevertheless make a substantial contribution to oxygen uptake in several species of urodeles (for _N. maculosas, see also Guimond & Hutchison, 1972). This could be especially important in their normal aquatic environment as it may prolong the time that they can stay submerged. SUMMARY
At 20~ the Congo eel, A. means, can survive for at least 24 hr when submerged in aerated tap water. Oxygen is readily taken up, from the bathing water, across the skin of these amphibians. The rate of oxygen uptake declines linearly as the oxygen tension in this solution decreases. In air, A. means breathes sporadically at rates ranging from two to nine times each hour. The rate of oxygen consumption under these conditions was similar to that when they were submerged in water nearly saturated with air. When pulmonary ventilation was pharmacologically blocked (with gallarrdne triethiodide) this rate of metabolism was maintained by O~ uptake across the integument. The ability of A. means to utilize its skin for respiration in air and water is compared to that in two other urodeles S. lacertina and N. maculosus.
Acknowledgement--This work was supported by National Science Foundation Grant No. GB-28543X. REFERENCES
BENEDICT F. G. (1934) Die Oberflfichenbestlmmung
verschiedener Tiergattungen. Ergebn. Physiol. exp. Path. 36, 300-346. BENTLEY P. J. & S}ImLD J. W. (1973) Respiration o f some urodele and anuran amphibia--II. In air: i'olo of the skin and lungs. Comp. Biochem. Physiol. 46A, 29-38. GOIMOND R. W. & I-IUTCHISONV. H. (1972) Puhnonary, branchial and cutaneous gas exchange in the m u d puppy, Necturus mactdosus maeulosu.~ (Rafinesque), Comp. Bioehem. PhysioL 42A, 367-392. JONES J D (1972) The Comparative Physiology o f Resph'ation, pp 43--46,54 Edward Arnold, Edinburgh. KROOH A. (1941) The Comparative Physiology r Resph'atory Mechanisms, pp. 54. University of Pennsylvania, Philadelphia. SHmLD J. W. & BENTLEY P. J. (1973) Respiration o f some urodele and anuran amphibians--L In water ; role of the skin and gills. Comp. Biochem. PhysioL 46A, 17-28. Tomws D. P., SHELTON G. & RANDALL D. J. (1971) Gas tensions in the lungs and major blood vessels o f the urodele, Amphiuma tridactylum, d. exp. BioL 55, 47-61. WHITFORD W. G. t~ HUTCmSON V. H. (1966) Cutaneous and pulmonary gas exchange in Ambystomid salamanders. Copeia 1966, 573-577.
Key Word Index Amphiuma means; respiration; amphibian respiration.
cutaneous
CUTANEOUS RESPIRATION IN THE CONGO EEL AMPHIUMA MEANS (AMPHIBIA: URODELA) P. J. BENTLEY* Departments ot" Ophthalmology and Pharmacology, Mount Sinai School of Medicine of The City University of New York, New York 10029, U.S.A. (Reeeioed 1 October 1973)
A b s t r a c t ~ l . At 20~ the Congo eel, Amphiunm means, can survive for at least 24 hr when submerged in aerated tap water. Oxygen is readily taken up, from the bathing water, across the skin of these amphibians. 2. The rate of oxygen uptake declines linearly as the oxygen tension in this solution decreases. 3. In air, A. tneans breathes sporadically; the rate of oxygen consumption was similar to that when they were submerged in water nearly saturated with air. 4. When pulmonary ventilation was plaarmacologically blocked (with gallamine triethiodide) the rate of metabolism was maintained by Oa uptake across the integument. 5. The ability of A. means to utilize its skin for respiration in air and water is compared to that in two other urodeles Sh'en lacertlna anti Neeturtts ntacttlosus.
INTRODUCTION AMPrmmi^NS have skin which is relatively permeable c o m p a r e d to that of most other vertebrates. This provides an avenue across which osmotic exchanges oi' water a n d salts as well as respiratory gases may occur. I n d e e d among amphibians the skin is a m a j o r p a t h w a y for the loss of carbon dioxide while a substantial uptake of oxygen also takes place (see, for instance, Krogh, 1941; Jones, 1972). Plethodontid salamanders lack lungs so that all of their r e s p i r a t o r y exchanges take place across their integument. The anabystomatid salamanders, on t h e other hand, possess lungs but it has been shown t h a t at 15~ in air, as much as 75 per cent of their oxygen u p t a k e occurs across the skin (Whitford & H u t c h i s o n , 1966). While anurans have a specialized b l o o d supply to the skin this is not present in urodeles a n d the latter are apparently not comp r o m i s e d by this. I have investigated the rates of cutaneous oxygen u p t a k e in a neotenous urodele, the congo eel, A m p h i u m a means (Urodela: Amphiumidae), during s u b m e r s i o n in its usual aquatic environment and when it is in air. This is an extension of recently described experiments on several other species of urodele and anuran amphibians (Bentley & Shield, 1973 ; Shield & Bentley, 1973). A. means lacks the prominent external gills seen in some neotenous, or "semi-larval", urodeles. It has lungs and in A m p h i u m a trMactyhtm these are * Correspondence to: Dr. P. J. Bentley, Dept. or" Ophthalmology, Mt. Sinai School of Medicine, 100 Street and Fifth Avenue, New York, N.Y. 10029. 121
used for respiration (Toews et al., 1971) though the animals only surface irregularly (on the average once every 45 rain). In the intervening periods the oxygen tension in the lungs declines to levels as low as 25 mmHg. I have found that the cutaneous uptake of oxygen in A. means kept in water saturated with air (at 20~ is adequate for their normal resting requirements; however, when the external pO~ is reduced there is a linear decline in the rate o f cutaneous oxygen uptake so that in poorly mixed or stagnant water pulmonary respiration may be advantageous. MATERIALS AND METHODS Congo eels, Amphiuma means, were obtained from a biological supplier (Mr. W. F. Prince, Silver Springs, Florida). They weighed 59-100 g. These animals were kept in a tank of tap water at 18-22~ They were starved for 10 days prior to the experiments. Oxygen uptake during submersion in tap water was measured with the aid of an oxygen electrode and monitor (YSI 53). The Congo eels were placed in a chamber (volume 1600 ml) through which water was rapidly circulated so that the mixing time was about 2 rain. The oxygen electrode was placed in contact with the fluid in a smaller chamber (8 ml) through which some of this water was circulated and where the temperature and mixing could be controlled more rigidly than in the main bath. The temperatures in the two baths corresponded to within 0.1~ The bathing media was initially saturated with air and after sealing off the system the decline in oxygen tension was measured and recorded on a chart recorder (Angus Esterllne). The system was checked for leaks after reducing the oxygen tension by bubbling the water with nitrogen.
122
P.J. B~NTI.I~Y
The oxygen uptake in Congo eels kept exposed in air was measured with the aid of a paramagnetic oxygen analyzer (Beckman E2). The animals were placed, on damp paper towels, in chambers through which air was passed at a rate of about 60 cm~/min and the change in oxygen tension of the effluent was continuouslymonitored. The oxygen analyzer was calibrated by reducing the partial pressure of oxygen in the cell and this was frequently checked with standard gases. The oxygen consumption was expressed as /zl O, at STP. The surface areas of the Congo eels were calculated using the formula: surface area cma - 12.5 g@ as used by Benedict (1934) for snakes (which are of a similar shape). This approximation was used to facilitate comparison with rates of oxygen uptake across the integument as reported in other species. RESULTS Survival o f congo eels during submersion in water Five C o n g o eels (mean weight 72 g) were submerged in a bath o f tap water at 20~ This was circulated and kept saturated with air. After 24 hr all of the animals were alive and apparently in good condition, Oxygen uptake across the integument would appear to be adequate to sustain life under these conditions, Oxygen uptake during submersion in water The oxygen uptake of six A. means (mean weight 3 g) was measured at 20~ during forced submer-
sion in tap water. When the saturation of the water was between 100 and 95 per cent the oxygen uptake was 28.0+ 1.6 cm" O d k g per hr. As the external pO~ declined, however, so did the oxygen uptake, This occurred in an essentially linear manner (Fig. 1) and there was no evidence of any regulatory mechanisms that could facilitate or maintain the normal rate of oxygen consumption. With respect to oxygen uptake across the skin A. means thus behaves as an "oxygen-conformer". Oxygen uptake in air These experiments were performed with a dual purpose: firstly, to see what the rate of oxygen consumption may be expected to be when the Congo eels have continual access to air; secondly, following a pharmacological blockade of p u l m o n a r y respiration (with gallamine triethiodide) to record the rate of oxygen uptake across the integument. In seven A. means (mean weight, 77 g) the rate or" oxygen consumption at 21~ measured over a 2-hr period (following about 2 hr that were necessary to allow the animals to adjust to conditions in the measuring chamber) was 32.65:4.0 cmS/kg p e r hr, This is similar to that seen during submersion w h e n only cutaneous respiration was possible. T h e pattern of breathing varied a great deal from a n i m a l to animal, some breathing eight or nine times a n hour, others only about twice in that time. T w o examples are shown in Fig, 2 which records a p p a r e n t
30
0
0 0 20 o
0 o
E 0
156
I 125
I 94
I 62
I 31
I 0
External ,aOz mm Hg
Fig. 1. Oxygen uptake (across the integument) of A. means when submerged in tap water at 20~ and with a declining oxygen tension. Each point represents the mean value for five animals (mean weight, 84 g).
Respiration in Amphflmta
123
200
(o)
ISO
I00
5C
Be
(b)
.P. (a.
50
C)
_1
,
3O
..
6O
I
90
Mia
Fig. 2. Respiration of two A. means at 20~ in air. Animal A weighed 92 g and had an overall rate of O~ consumption of 30 cm3/kg per hr. Animal B was 66 g and consumed 34 cm~ O~tkg per hr. changes in oxygen consumption reflecting the periodic excursions in the pO~ of the effluent air. Respiratory movements were inhibited by injecting the C o n g o eels with gallamine triethiodide (about 40 mr/animal). After 1-2 hr no respiratory movements were visible and the excursions of the oxygen tension, described above, were no longer apparent so that a steady value was seen. The rate of oxygen consumption in the same Congo eels was then 33"3 -/- I ' 4 cm~/kg per hr which is the same as when the animals were ventilating their lungs. DISCUSSION
A. m e a n s readily accumulates oxygen across its integument when it is either submerged in water or is exposed to air. Under the experimental conditions, resting animals in water saturated with air at about 20~ this was adequate to sustain the Congo eels and was similar to that consumed when the lungs could also be ventilated. A comparison
between A. means and two other aquatic urodeles, Siren laeertina (the mud eeI) and Necturus maculosus (the mudpuppy) is shown in Table 1. In water the rate of oxygen uptake across the skin surface was similar in the three species. In air the rate of cutaneous uptake across the skin of N . mactdosus was about half that in the other two urodeles. In A. means cutaneous uptake of oxygen corresponded to the total uptake of the animals when exposed to air and when they were able to ventilate their lungs. In the other species, however, this only represented about 50 per cent of their requirements under the experimental conditions. The difference reflects the lower metabolic rate of A. means rather than a more favourable rate of cutaneous uptake. In addition, the other urodeles were slightly larger so that their surface area-body weight ratio would be less favourable. This, however, could not account for the observed differences. In the experimental conditions the skin appeared to be adequate to support the respiration o f A. means.
124
P . J . BENTLEY
Table 1. Cutaneous oxygen uptake at 20--22~ (in terms of surface area) in A. means compared to two other species of urodeles /L10Jcm~ skin per hr
Species
Submerged in water
In air: pulmonary respiration inhibited*
Normal: in air (or water for N. mactdosus)
A. means (6) S. laeertina? (3) IV. maculosttst (9)
I5.0_+ 1.2 17.2+ 1"4 15.5_+ 1.1
17'6_+0.9 (7) 17"5-+0'8 (6) 10"2+__0.6 (7)
17.1 +2.0 (7) 34-0+ 1"4 (6) 25'7___2.6 (7)
* With injected gallamine triethiodide. t From Shield & Bentley (1973) and Bentley &Shield (I973). For the measurements of cutaneous oxygen uptake in water the gills of S. lacerth,a and N. maculosus were tied off. The average weights were: A. means, 80 g; S. laeertina, 144 g; and iV. maeulosus, 134 g. Results axe as mean + S.E. Number of experiments in parentheses. Whether or n o t this is always so is doubtful, for as apart from the above surface area-weight relationships, the metabolic rate can vary considerably depending on activity, temperature, seasons and so on. I n addition the oxygen tension in natural waters can be low (such as in stagnant water) which would considerably limit its rate of uptake. The skin can nevertheless make a substantial contribution to oxygen uptake in several species of urodeles (for _N. maculosas, see also Guimond & Hutchison, 1972). This could be especially important in their normal aquatic environment as it may prolong the time that they can stay submerged. SUMMARY
At 20~ the Congo eel, A. means, can survive for at least 24 hr when submerged in aerated tap water. Oxygen is readily taken up, from the bathing water, across the skin of these amphibians. The rate of oxygen uptake declines linearly as the oxygen tension in this solution decreases. In air, A. means breathes sporadically at rates ranging from two to nine times each hour. The rate of oxygen consumption under these conditions was similar to that when they were submerged in water nearly saturated with air. When pulmonary ventilation was pharmacologically blocked (with gallarrdne triethiodide) this rate of metabolism was maintained by O~ uptake across the integument. The ability of A. means to utilize its skin for respiration in air and water is compared to that in two other urodeles S. lacertina and N. maculosus.
Acknowledgement--This work was supported by National Science Foundation Grant No. GB-28543X. REFERENCES
BENEDICT F. G. (1934) Die Oberflfichenbestlmmung
verschiedener Tiergattungen. Ergebn. Physiol. exp. Path. 36, 300-346. BENTLEY P. J. & S}ImLD J. W. (1973) Respiration o f some urodele and anuran amphibia--II. In air: i'olo of the skin and lungs. Comp. Biochem. Physiol. 46A, 29-38. GOIMOND R. W. & I-IUTCHISONV. H. (1972) Puhnonary, branchial and cutaneous gas exchange in the m u d puppy, Necturus mactdosus maeulosu.~ (Rafinesque), Comp. Bioehem. PhysioL 42A, 367-392. JONES J D (1972) The Comparative Physiology o f Resph'ation, pp 43--46,54 Edward Arnold, Edinburgh. KROOH A. (1941) The Comparative Physiology r Resph'atory Mechanisms, pp. 54. University of Pennsylvania, Philadelphia. SHmLD J. W. & BENTLEY P. J. (1973) Respiration o f some urodele and anuran amphibians--L In water ; role of the skin and gills. Comp. Biochem. PhysioL 46A, 17-28. Tomws D. P., SHELTON G. & RANDALL D. J. (1971) Gas tensions in the lungs and major blood vessels o f the urodele, Amphiuma tridactylum, d. exp. BioL 55, 47-61. WHITFORD W. G. t~ HUTCmSON V. H. (1966) Cutaneous and pulmonary gas exchange in Ambystomid salamanders. Copeia 1966, 573-577.
Key Word Index Amphiuma means; respiration; amphibian respiration.
cutaneous
