Download Gas exchange and habitat selection in the aquatic salamanders

Oecologia (1990) 83:250-258
Oecologia
9 Springer-Verlag 1990
Gas exchange and habitat selection in the aquatic salamanders
Necturus maculosus and Cryptobranchus Mlegam'ensis
Gordon R. Ultsch and Jeffrey T. Duke
Department of Biology,The University of Alabama, Tuscatoosa, AL 35487, USA
ReceivedDecember 15, 1989 / Accepted January 12, 1990
Summary. The standard metabolic rate (SMR) and critical 02 tension (Pc) of water-breathing mudpuppies and
hellbenders were determined at 20 ~ C using open-system
respirometry. Both species are metabolic 02 regulators,
although the Pc of hellbenders (90 mmHg) is much higher than that of mudpuppies (40 mmHg). The SMR of
the two species in water saturated with air was similar
(19.5 and 20.0 gl O2/g" h for Cryptobranchus and Necturus, respectively) and not different from that of salamanders in general. Both species were able to survive for
at least 5-11 days in severely hypoxic water (910 mmHg) by breathing air, indicating that the lungs
are functional accessory respiratory structures.
We conclude that hellbenders are restricted to relatively cool and flowing waters because of their limited
gas exchange capabilities, particularly with regard to
their limited aerobic scope for activity and slow recovery
from exercise. Necturus maculosus is much more tolerant
of hypoxia, but it is not known if they can inhabit areas
were hypoxia is combined with hypercarbia.
Key words: Cryptobranchus alleganiensis - Necturus maculosus - Oxygen consumption - Metabolic rate - Critical oxygen tension
The giant salamanders of North America include the
genera Siren, Amphiuma, Cryptobranchus (hellbenders),
and Necturus (i.e.N. maculosus, most other species being
smaller stream-dwelling forms). All exhibit cutaneous
gas exchange, all have lungs, and Necturus and Siren
have external gills. The group is of particular interest
because of the differing degree of development of each
of these modes of gas exchange and the resultant implications for their respiratory physiology, acid-base balance, energetics, and habitat selection. In particular,
none of the species has all three modes of gas exchange
highly developed. Amphiuma, with no gills, and Siren,
Offprint requests to: G.R. Ultsch
with poorly developed gills, both have highly developed
lungs.
Adult Siren lacertina and Amphiuma means are obligate air-breathers at temperatures that routinely occur
in the summer (e.g. 25 ~ C) within their habitats (Guimond and Hutchison 1976; Ultsch 1973a, 1976a). In
contrast, mudpuppies and hellbenders can tolerate continuous (at least two weeks) submergence in aerated
water at temperatures up to 25 ~ C (Guimond and Hutchison 1972, 1973, 1976). Necturus maculosus is much the
smallest of the group (200-300 g), its relatively small
size and gas-permeable integument increasing the efficacy of cutaneous gas exchange; in addition, it has highly arborized external gills that can be actively ventilated.
Cryptobranchus alleganiensis is much larger (over 1 kg)
and does not have gills, but has a flattened body form
and highly vascularized skin folds that in the aggregate
function as a gill. A significant respiratory role for the
lungs is often dismissed for both species (Guimond and
Hutchison 1976; Lenfant and Johansen 1967; Shield and
Bentley 1973) because they can survive prolonged submergence, because the lungs are simple sacs in comparison to the highly septate lungs of Amphiuma and Siren,
and especially because the pulmonary contribution to
resting gas exchange is small (at 25 ~ C, < 8 % for 02
and < 3 % for CO2, Guimond and Hutchison 1972,
1973).
The limited aerial respiration of Cryptobranchus,
paired with its lack of gills, may be responsible for its
habitat being restricted to permanent streams and small
rivers in the drainage systems of the Appalachian and
Ozark mountains. The 02 tensions in these habitats are
normally near air saturation and maximal temperatures
are usually <25 ~ C (Beffa 1976; Brown 1985; Nickerson
and Mays 1973). In the case of Necturus, the gills have
been presumed to allow it to occupy hypoxic waters
in spite of its poor lungs, which may explain its much
wider range (Conant 1975). However, although some
species of Necturus are reputed to occupy habitats that
would be expected to become occasionally hypoxic (Harris 1959), we are not aware of any field data concerning
the PO2 of the microhabitat of Neeturus.
251
I n spite o f the p o o r v a s c u l a r i z a t i o n a n d lack o f septation in the lungs o f b o t h species, the lungs are relatively
large ( G u i m o n d a n d H u t c h i s o n 1976), a n d they are ventilated regularly (albeit it at low frequencies) at higher
t e m p e r a t u r e s , as i n d i c a t e d b y o u r o b s e r v a t i o n s a n d the
presence o f a m e a s u r a b l e p u l m o n a r y O2 u p t a k e (Boutilier et al. 1980; G u i m o n d a n d H u t c h i s o n 1976; Miller
a n d H u t c h i s o n 1979). This suggests t h a t they are serving
some r e s p i r a t o r y f u n c t i o n . Miller a n d H u t c h i s o n (1979),
for example, f o u n d t h a t while a l m o s t all O2 u p t a k e in
Necturus maculosus was a q u a t i c at 5 a n d 15 ~ C, that
d u r i n g periods o f s p o n t a n e o u s aerobic activity over 50%
of the 0 2 u p t a k e was p u l m o n a r y . I n a d d i t i o n , m o s t o f
the studies o f gas exchange in b o t h species have been
d o n e u s i n g aerated water, so the responses to h y p o x i a
are n o t well k n o w n . I n studies o f a q u a t i c h y p o x i a in
Necturus (Miller a n d H u t c h i s o n 1979; Shield a n d Bentley 1973), conflicting results have been o b t a i n e d regarding their ability to regulate the rate o f O2 c o n s u m p t i o n .
Here we r e p o r t that b o t h of these s a l a m a n d e r s c a n
survive at least 5-11 days i n nearly anoxic water at 20 ~ C
by utilizing p u l m o n a r y respiration, s h o w i n g t h a t the
lungs serve a n i m p o r t a n t r e s p i r a t o r y f u n c t i o n d u r i n g periods o f low water PO2. We also d e m o n s t r a t e t h a t b o t h
species are m e t a b o l i c O2 regulators w h e n respiring
aquatically, a l t h o u g h the critical 0 2 t e n s i o n o f Cryptobranchus is over twice t h a t o f Necturus.
Material and methods
Animals. Hellbenders (Cryptobranehus alleganiensis alleganiensis)
were collected from Cypress Creek in northwestern Alabama and
from the Hiwasee River in northeastern Georgia. Both sites are
part of the Tennessee River drainage and are near the southern
limits of the range of hellbenders (Conant 1975). Mudpuppies (Neeturus maculosus maculosus) were purchased from Kons Scientific
Co., and were collected in Wisconsin. The salamanders were kept
in flowing well water that was adjusted to circa pH 7 by passage
through marble chips; they were maintained at 20 + 2~ C with a
12L-12D photoperiod centered on 1400 h. The animals were fed
mealworms, earthworms, and recently killed minnows, except that
no animal was fed within a week of use in an experiment.
Oxygen consumption measurements. Oxygen consumption (VO2)
of submerged animals was measured continuously using a flowthrough respirometer similar to that described by Ultsch et al.
(1981). Flow rate was set so the difference in PO 2 between incurrent
and excurrent water was 10-20 mmHg in the range of metabolic
O2 regulation. Corrections were made for electrode drift and for
changes in barometric pressure during the experiment; in practice,
the latter correction is usually so small that it could be ignored.
When exit PO2 is constant (i.e. the system is in a steady state),
the ~/O2 can be readily calculated according to the Fick principle.
It is also possible to calculate the VO2 in a non-steady state, such
as occurs when the input PO2 is changed and the chamber water
is in a washout phase. The derivation of appropriate formulae
have been discussed by Ultsch and Anderson (1988) and Steffensen
(1989). Here we calculated the equilibrium exit PO2 (PO2eq) that
would be reached in an eventual steady state from the PO2's of
an interval during the preceding washout phase according to Eq. 1 :
POze q - P02(2)--PO2(D q-PO2(1)
1 -- e - v(a t/v)
(Eq. 1)
where POz(1) is the exit PO2 at the beginning, and PO2(=) at the
end, of the time interval A t; V is the flow rate through the respir-
ometer, and V is the water volume of the chamber and associated
tubing and mixing pump. POzeq can then be substituted for the
exit PO2 in the Fick equation to calculate VO2, and the PO2 at
which this VO2 occurred can be approximated as the average of
PO2(1) and PO2~2). An inherent assumption is that the "~Oz is constant throughout the interval A t. This assumption can be checked
by comparing the calculated VO2 during the non-steady state with
that found during the ensuing steady state. Over the range of ambient PO2 where VO2 is regulated, we rejected any values that were
more than 10% above the steady state value achieved after washout, on the assumption that the animal was active during the interval and that the calculated VOz therefore did not represent a measure of the standard metabolic rate (SMR), which was the rate
in which we were interested. However, once the ambient PO2 during the progressive hypoxia approach we used fell below the critical
O2 tension (Pc, see below), then VO2 does decrease throughout
the fall in POz associated with the time interval A t, and the data
points can not be rejected by such a criterion. In practice, this
did not appear to present a problem, and all data below the Pc
were used.
Determination of Pc- Given VO2 as a function of ambient PO2,
a variety of methods have been used to determine Po, ranging
from fits of data by eye to a number of statistical approaches.
We used the method of Yeager and Ultsch (1989), which basically
involves assuming that the data can be satisfactorily fit by two
regression lines whose point of intersection is an approximation
of the Pc- The method utilizes a program that calculates the two
best-fit regression lines and their intersection. It is imperative that
these lines and their intersection be plotted on a graph of the
data to verify visually that the intersection is a good approximation
of the Pc; if it is not, then an alternative "midpoint approximation"
is used (see Yeager and Ultsch 1989, for a discussion of both approximations of Pc and a BASIC program for analysis of data).
Here the Pc approximation derived from the intersection of the
two best-fit regression lines was satisfactory in 15 of 21 cases.
Ecological metabolic 02 regulation and conformity. In order to provide a reasonable range of PO2 over which to calculate regression
lines above the Pc, we measured VO2 at a PO2 as high as near
200 mmHg. We also calculated Pc using only the {zO2 data for
POz'S <156 mmHg (air-saturation) in order to describe the relation between the two in the likely ecological range of PO2. For
comparisons of SMR, we use the VO2 shown at 156 mmHg.
Tolerance of severe water hypoxia. The efficacy of the lungs as
gas exchangers when the water PO2 was critically low was assessed
by lowering the PO2 while allowing the animals access to air. One
to three salamanders were placed in the respiration chamber and
the input PO2 was lowered to <2 mmHg by using Nz as the gas
source in the water equilibration system. The chamber was slightly
inclined with two 1.2 cm holes in the lid left open to the atmosphere. This maneuver created an air pocket of 300400 ml connected to the atmosphere by the holes. Most animals quickly
learned to position their heads below the open surface. Water POz
usually remained near 9-10 mmHg (range 4.6-17.2), a level well
below the Pc of both species, and one that would be lethal to
submerged animals in < 24 h. Survival was noted daily.
In a separate set of experiments, surfacing frequency in recirculating hypoxic water was noted periodically for 3 hellbenders observed over 0.5 h intervals for a total of 5 h each, and for 3 mudpuppies observed over 0.5 and 1.0 h intervals for a total of 10 h
each. As we did not want to inhibit surfacing by the animals,
we did not take any measures to prevent diffusion of 02 from
the air into the water. The entire water surface was open to the
atmosphere, resulting in an equilibrium water P O 2 that ranged
from 22 to 30 mmHg, which nevertheless represents a relatively
severe hypoxia (about 16% of air-saturation). To eliminate influences of body size, animals of similar body weight were used.
Statistics. Differences between variables for the two species were
tested for by a t-test. The significance of the slopes of individual
252
TaMe 1. Summary of oxygen uptake data for submerged Cryptobranchus alleganiensis and Necturus maculosus acclimated to and tested at 20~ C. Pc, critical 02 tension; "~O2(sat), standard rate of 02 consumption
in air-saturated water (PO2= 156 mmHg); m (conformity)=A{rO2/AP02 in the zone of metabolic 02
conformity; m (regulation)= AVCOz/AP02in the zone of metabolic O2 regulation. Data are 2_+SE (range)
Weight
(g)
n
Pc
(mmHg)
g o 2 (sat)
m
(conformity)
m (regulation)
(gl O2/g'h)
Cryptobranchus alleganiensis
395 _ 83
(69--796)
11
124 _+8.6
(72-170)
I0
Necturus rnaculosus
90.0 _+7.5
(57.5--136.5)
19.51 +_1.37
(12.99--26.73)
~0.197 + 0.022
(0.094-0.320)
b0.007_+0.006
(--0.0240.015)
c39.7 _ 3.1
(27.1-62.5)
19.96 _ 0.99
(14.70-25.92)
a.c0.429 + 0.051
(0.199-0.673)
b0.008 + 0.005
(--0.012-0.037)
significantly greater than zero
b not significantly different from zero
significantly different from Cryptobranchus
a
regression lines used to determine Pc's was determined from a Student's t-table using the t-statistics generated by the data analysis
program (Yeager and Ultsch 1989). The mean slope of ~'Oz on
POz regressions in the ranges of regulation and conformity was
tested for being significantly different from zero by calculating
the confidence intervals. All significance levels were set at 95%.
Results
fact that it could regulate g o 2 a t PO2'sabove air-saturation. By this definition, 2 of 11 Cryptobranchus were ecological 02 conformers (e.g. Fig. 3 B), and 2 more were
very close with r2=0.88 (e.g. Fig. 2C), a reflection of
the high Pc of hellbenders. No Necturus were ecological
O2 conformers, as all had a Pc well below the PO2 of
air-saturated water (Table 1), and were therefore able
to regulate aquatic O2 uptake over a fairly wide range
of naturally occurring 02 tensions.
Standard metabolic rates (SMR) and critical 02 tensions
(Pc). There was no difference in the SMR's of the 2 species (Table 1), even when an adjustment was made for
the larger size of the hellbenders (see below). The Pc
of the hellbenders was much higher than that of the
mudpuppies. Because of these 2 results, the rate at which
an animal could increase its Oz consumption with an
increase in PO/(AVO2/APO2) in the zone of metabolic
O2 conformity (e.g. at PO2's below Pc) for Necturus was
over twice that of Cryptobranchus (Table 1). When the
ambient PO2 was increased beyond the Pc, the mean
AVOz/APO2 was not significantly different from zero
for either species, indicating metabolic 02 regulation.
However, when the individual data sets are considered,
AVOz/AP02 above Pc was significantly different from
zero for 3 of 10 Necturus and 4 of 11 Cryptobranchus
(examples include Figs. 1 B, C and 3 A-C), being positive
in all cases (perhaps due to slight increases in physically
dissolved O2 in the blood), but always much less than
in the zone of metabolic 02 regulation (Figs. 1-3).
As mentioned in the methods section, the Pc could
be satisfactorily approximated in 15 of 21 cases by fitting
2 regression lines to the data and taking their intersection as the Pc; examples are given in Figs. 1 C - F for
Necturus and Fig. 3 for Cryptobranchus. In the remaining 6 cases, we used an approximation described by
Yeager and Ultsch (1989) to determine Po (e.g. Figs. 1 A B, and 2).
Tolerance of severe hypoxia. Six hellbenders survived severe water hypoxia (9-10 mmHg, see above) in the respiration chambers for the entire 5 11 day observation period when there was an air pocket from which to breathe
(Table 2). They were removed when it was evident that
severe water hypoxia was not an immediate threat; how
long they could have survived was not determined. They
spent most of their time near the air pocket, did not
struggle, often floated, and recovered without incident.
Similar results were obtained for mudpuppies, which occasionally moved their gills back and forth rapidly, but
also often held them retracted against the body. Mudpuppies did not float. When Necturus were returned to
aerated water, the gills were maximally protruded and
perfused, and were waved vigorously.
In aquaria open to the air with a moderately severe
water hypoxia (22-30 mmHg), hellbenders air-breathed
about 6 x more than mudpuppies of a similar size (Table 3). I-Iellbenders spent much more time at the surface,
often floating for up to 10 rain, and occasionally swam
vigorously at the surface as if attempting to escape.
Mudpuppies were generally inactive, and were not seen
to float.
Ecological 0
for animals that have no access to air, and therefore
could have been somewhat depressed relative to those
of the same animals breathing both air and water (Ultsch
1976b), which means that the rates can not be unequivocally compared to those found by other workers. How-
2
conformation and regulation. If the elimi-
nation of ~rO2 data at POz's > 156 m m H g resulted in
a single regression line fitting the data with a coefficient
of determination (r 2) _>0.90, then we accepted that the
animal was an ecological 02 conformer, in spite of the
Discussion
Rates of 0 2 cosumption. The VO2's reported here are
253
25
a
A
I,
|
it.
20
!
9
.>o
IO
/-
.7"
/"
'
SMR =20.79
Pc =29.5
m =0.4246
C
.
9
#
o
o ~ ~o
20
/
E
~Y
W=170
Pc =62.5
SMR=17.27
m- 0.1993
2bo
,
25
..
D
,%
/.
y.."
20
W=92
SMR=2EII
Pc =58.7
m= 0.4551
9
.
9
,_
~
t
/:
I0
,
i~ 9
..;,.
15
,/
0
~J"
280
9 tS
/
15
/
st
9. ~ , "
)'~
~------~i~
"2 20
~
../
I0
. . . .
~
,.. :......_~M~.'. -'". - 7 ~ "
15
W= 126
~o
9
B
5
'
25
20
fs
,u 15
O
~
,~
,r
~149
!
..~..
S
SMR=20,53
m=0.5702
W=125
Pc = 40.0
5/
//
IOO
,it .
./
~8o
200
,a
9"
".,
:
t
,0
Oo ~
t
~
20o
.;
F
9
S
~
9;
/-'-'--
"
9/ 1 1
"
't
~
S
I0
4'o
'
go
W=128
SMR=17.53
Pc = 52.2
rn =0.5602
'
,~o
,;o
/
2bo
Po 2 (mmHg)
Fig. 1. Examples of the standard rate of 02 consumption (SMR)
of submerged Necturus maculosusat 20~ C as a function of ambient
oxygen tension. W (weight, g); Pc (critical 02 tension, mmHg);
SMR calculated as rate in air-saturated water (S, 156 mmHg); m,
rate of increase in ~'O~ with increasing PO2 in the zone of metabolic
02 conformity. Po determined as the intersection of two best-fit
regression lines (C F), and occasionally as an apparent discontinuity between the two best-fit regression lines (A B, see text)
ever, as the aerial components of the "s 2 of bimodally
respiring Cryptobranchus (Guimond and Hutchison
1973) and Necturus (Guimond and Hutchison 1972) are
small, metabolic rates reported by these authors can be
qualitatively compared to those found by us. Using
Qlo'S from these authors, and assuming that s
is a
function of W ~176 among salamanders (Gatten et al.
1990), our data can be compared to that of Guimond
and Hutchison (1976). We found the VO2's for our submerged hellbenders and mudpuppies were 27% lower
and 5% higher, respectively, than those of the bimodally
respiring slamanders studied by Guimond and Hutchison (1976). Neither of these differences is great, considering the differences in techniques and the approximations used in the calculations.
Among amphibians, salamanders have a lower rate
4'o
'
80
W=l15
SMR= 14.70
Pc4o.5
~o3,43
120
26o
POe (mmHg)
of 0 2 consumption than anurans (Feder 1976) by about
a third (Gatten et al. 1990). However, among salamanders, neither this study nor those of Guimond and
Hutchison (1972, 1973, 1976) found the VO2 o f m u d p u p pies or hellbenders to be especially low. Whether the
27% reduction of VO2 below the rough 'predicted' value
described above is real is equivocal. Such a depression
can be viewed as adaptive for an animal with a relatively
limited gas exchange capability, if it were real. But if
our data for Cryptobranchus is scaled to the same body
size as for our Necturus using a regression equation generated from our Cryptobranchus data CQO2/W~~46.36 W -~
pl O2/g'h and g, weight range 69-796 g),
then a submerged hellbender of 124 g is predicted to
have a VO2 7% higher than a mudpuppy, which is to
say that there is no difference in the SMR of similarly
sized animals of the the two species. Because of this
observation and the above comments, it does not appear
that hellbenders have an SMR that is below that of
mudpuppies, and perhaps not even below that of salamanders in general, although the intraspecific data on
large species is sparse. Therefore the lack of gills and
the relatively infrequent use of lungs under resting conditions do not seem to inhibit the SMR of hellbenders,
cutaneous respiration being sufficient to maintain a normal S M R because o f the skin folds, dorsoventral flatten-
254
A
B
25
25
,
9.:'_ _ _
0
i
m,
F
20
t
S
%/i
15
9
9/
o
5
X
i
i
i
W=69
SMR=21.OI
Pc =51.5
m=0.3204
i
i
i
i
i
i
/
C
,
,
~176
t
S
/
i
200
~ ,;
9
I I
/
5
I00
25
r
I0
9
o
,%
i
20
."."
"
05
9
i
W=104
SMR=25.16
Pc =74.0
m =0.2975
~
i
0
i
i
i
i
2OO
I00
D
25
''.,,.
%
20
~.
20
J
15
~.....-y" : "
c,i I0
9
i
LO
4
8
..././..~
15
Y
I0
W=408
SMR=20,14
Pc =136.5
m=00938
i0
i
i
120
i
.,d'.";
5
i
160
i
i
200
i
SMR=21.72
Pc : 87.0
m =0.2128
2C
20
'-'---"--'--'--'.'C'-::-" " - ~ " "
9149
t
J=
.//
0
.,/
.2
~
i
L
W=132
SMR= 16.82
Pc = 61.9
M =0.297
i
i
~
i
0
i
I00
~)
2 0
,it
'
0
W=557
SMR=2&72
PC=95.9
M=0.1655
,Go
'
,/
:.v_.~-~. . ~
..
/
cJ
.,. " . . ~ r - - - - - ' - - - - ~
,z.~
'
;
. .."
9
;
'
.z.r
~176
4'0
'
W=698
SMR=I4.08
Pc= 68.4
M=0.1745
8'0
,'
9
9
,
...,'
S
~
/.
,
t
9
/.
o
s
..~-.J':
'
120
'
'
I~ 0
'
200
'
P02 (mmHg)
Fig. 2. SMR as a function of ambient POz (as in Fig. 1), for Cryptobranchus alleganiensis, where the two best-fit regression lines
showed an apparent discontinuity
'
/
/,/~,~
4'o
9
~149149
9
"
9
"
9
zbo
"
'
SMR = 12.99
Pc = 82.9
M = O. 1580
'
i20
....
9
l
s
W = 767
8'o
.',~
9
..
/..
9
/..
3
200
D
10
5
. .,-<',
15
C
v
i
S
IC
i
"2
,~o
i
B
25
"-~
,~o
i
P02 (mmHg)
P02 (mmHg)
A
9 .. 9149149
f
s
W=420
~o
w
.o
0
.:.~ "
'
160
''
200
P% (mmHg)
Fig. 3. As in Fig. 2, for examples of Cryptobranchus alleganiensis
where Pc was determined as the intersection of the two best-fit
regression lines
255
Table 2. Tolerance of Necturus maculosus and Cryptobranchus alleganiensis to hypoxic water (9-10 mmHg) at 20~ C with access to
air. All animals were removed alive (and subsequentlyrecovered)
at the end of an experiment. Data are ~_+SE (range)
Species
n
Weight
(g)
Minimal survival
(days)
Necturus
maculosus
Cryptobranchus
alleganiensis
5
88_+11
(60-123)
89 _+15
(44-152)
6.4 -+1.2
(5-11)
6.5 _+0.9
(5 11)
6
3. Surfacingfrequencyof similarly-sizedindividualsof Cryptobranchus alleganiensis and Necturus maculosus in hypoxic water
(PO2= 22-30 mmHg) at 20~ C. Data are i _ SE (range)
Table
Species
Number of
observations
Weight(g)
(n = 3)
Surfacing
frequency
(surfacings/h)
Cryptobranchus
alleganiensis
Necturus
maculosus
30
83_+12
(67-107)
85 _+17
(60-118)
47.13_+4.48
(4-88)
* 7.79 _+0.75
(2-16)
33
* Significantlydifferentfrom Cryptobranchus
ing, and the fact that the capillaries reach into the epidermis (Noble 1925).
Critical 0 2 tensions and regulation of 02 consumption.
We found Necturus maeulosus to be capable of regulation
of VO2 over a range of PO2 extending down to approximately 30% of normoxic values, in contrast to Shield
and Bentley (1973), who found ~rO2 to increase linearly
with P O 2 up to 150 mmHg at 20 ~ C and therefore concluded that mudpuppies were metabolic O2 conformers.
As these investigators reported metabolic rates at
150 mmHg about twice those we and Guimond and
Hutchison (1972) found, the conflicting conclusions may
be the result of activity in their animals. Miller and
Hutchison (1979), using mudpuppies at 5 and 15~ C also
found that that Necturus could regulate ~'O2 down to
at least 40 mmHg.
The Pc of several fish species, as determined in a
similar system at 20 ~ C, is near 20-30 mmHg (Ott et al.
1980; Ultsch et al. 1981). Although there was trend in
the salamanders toward an increase in Pc with body size,
it was not significant for either species (P = 0.08 for Necturus and 0.15 for Cryptobranchus). At 40 mmHg, the
Po of Necturus is somewhat higher, but considering the
much more efficacious respiratory system of fishes, the
ability of mudpuppies to tolerate hypoxia is rather impressive. The role of the external gills is evident, as the
mean P~ for 4 hellbenders that were of similar size (69132 g) to the mudpuppies was 75 mmHg. In addition,
the gills take up twice as much 02 as does the skin
at 5-25 ~ C, and can account for up to 61% of total
VO2 in animals breathing both air and water at 15 and
25 ~ C (Guimond and Hutchison 1972).
Responses and adaptations to aquatic hypoxia. Necturus
is unarguably more adapted to hypoxia than Cryptobranchus, but mudpuppies may not be more tolerant
of hypoxia than most other water-breathing animals
such as fishes, as evidenced by the higher Pc of mudpuppies. This means that they can not be assumed to be
capable of inhabiting environments that become severely
hypoxic unless they can gain access to the surface. In
portions of the range where ice cover may occur for
extended periods during overwintering, this would exclude this species from shallow ponds subject to anoxic
winterkill, and in fact the most typical habitat for mudpuppies in northern areas is large lakes.
In more southern areas, there are several physiological adaptations that could allow mudpuppies to inhabit
relatively hypoxic waters, although we stress that environmental measurements of the degree of hypoxia in
habitats of mudpuppies are lacking. Mudpuppies have
the highest blood 02 affinity yet found among salamanders (Pso = 10.6 mmHg at 20~ C, Weber et al. 1985). The
low Pso is not only important because of its obvious
value in enabling the animal to remove 02 from hypoxic
water, but also because hypoxic water is often hypercarbic (Ultsch 1973b, 1976b; Wakeman and Ultsch
1975). Necturus blood has a large Bohr shift (Weber
et al. 1975), which could inhibit Oz loading from hypoxic-hypercarbic waters if the degree of hypoxia was
large. The extremely low Pso can be viewed as adaptive
to this Bohr shift, allowing a mudpuppy in hypoxic
water to maintain an adequate saturation of cutaneous
and gill efferent blood with a minimum of surfacing
in spite of any hypercarbia, which could save energy
and reduce the risk of predation. The survival for days
when in water of very low PO2 (Table 2) when the animals have access to air indicates that the lungs can be
of crucial importance during periods of extreme hypoxia.
Finally, the high value for AVO2/APO 2 at 02 tensions
below the Pc would enable mudpuppies to pay back an
02 debt or recycle lactate to glucose (Miller and Hutchison 1979) with a much smaller increase in ambient P O 2
than would be the case for hellbenders.
While the adaptations of mudpuppies to hypoxia
suggest that they could inhabit hypoxic waters, it is clear
that hellbenders are not well-adapted to hypoxia and
that this may be why they are limited to habitats that
are normally well-oxygenated. However, this does not
mean that Cryptobranchus has no defenses against a
moderate hypoxia. A mean Pc of 90 mmHg (Table 1)
means that hellbenders can maintain their SMR at a
40% reduction from normoxic PO2, which in turn implies that they must have some control over 02 transport
across the integument. One response is to rock the body
from side to side in hypoxia, and the frequency of this
rocking has been shown to increase as PO2 falls (Beffa
1976; Harlan and Wilkinson 1981). Presumably this behavior enhances diffusion by reducing the water boundary layer (Burggren and Feder 1986) as well as by renewing the water near the skin in a manner similar to the
waving of the gills of Necturus during hypoxia. Rocking
in hypoxic or hypercarbic water raises blood PO2 by
4-7 mmHg (Boutilier and Toews 1981b; Harlan and
256
Table 4. Transcutaneous02 condutance (GO2) of Cryptobranchus
alleganiensis as a function of temperature, calculatedfrom cutaneous routine "VO2and water to blood PO2 gradient
Temp
(o C)
aCutaneous bTranscutaneous Cutaneous
~'O2
PO2 gradient
GO2
(I.tl 02/g" h)
(mmHg)
(gl 02/
g. h- mmHg)
Qlo
5
15
25
8.15
18.57
28.52
2.29
1.54
(158-17)=141
(157-17)= 140
(155-15)=140
0.0578
0.1326
0.2037
" fromGuimondand Hutchison(1973)
b fromMoalliet al. (1981)
Wilkinson 1981). Since the animals are usually operating
on the steep portions of their rather sigmoidal 02 dissociation curves (Boutilier and Toews 1981 a; McCutcheon
and Hall 1937), these relatively small increases in the
PO2 of the blood can be significant in terms of maintaining 02 saturation levels (Boutilier and Toews 198] b).
There also appears to some control in hellbenders
over both O2 demand and the cutaneous conductance
of 02 (GO2). As temperature is increased from 15 to
25~ C, Cryptobranchus has the lowest Qlo of the giant
salamanders (1.48 vs. 1.96 for Amphiuma means, 2.19
for Siren lacertina, and 2.38 for Necturus maculosus, calculated from Guimond and Hutchison 1976). Cutaneous
conductance has been argued to be a relatively noncontrollable parameter because the movement of gas across
the skin is primarily diffusion-limited (see review of
Feder and Burggren 1985). In Cryptobranchus this was
found to be the case for CO2, where increases in aquatic
VCO2 were due to increases in the transcutaneous CO2
gradient associated with increases in acclimation temperature (Moalli et al. 1981), with the result that GCO2
was constant over a temperature range of 5-25 ~ However, if the same approach used by these authors to calculate GCO2 is used to calculate GO2, then GO2 is
found to increase substantially with temperature with
no increase in the transcutaneous 02 gradient (Table 4).
This could be the result of cutaneous capillary recruitment that increases the effective surface area for O2 diffusion (Burggren and Moalli 1984; Feder and Burggren
1985), an effect that may not be shown for CO2 because
of the much higher diffusivity of this gas (i.e. perfusing
additional cutaneous capillaries may not change the effective surface area for diffusion of CO2). Presumably
the same response could be used as PO2 falls at a given
temperature, thereby partially explaining the ability of
hellbenders to regulate VOz over a naturally occurring
range of 60 mmHg.
Circulatory considerations and the role of the lungs. As
mentioned, the value of the lungs of both species as
gas exchange organs has been questioned, and they have
often been stated to serve mainly a hydrostatic role. The
latter role seems unlikely, particularly for Cryptobranchus, which typically is a bottom dweller that inhabits
fairly fast-flowing water. Animals in such habitats tend
to lose their buoyancy devices (e.g. darters, a group
of fishes typically found in streams, have no gas bladder;
a number of stream-dwelling salamanders have lost their
lungs). The lungs of both mudpuppies and hellbenders
are relatively large (Guimond and Hutchison 1976), and
pulmonary respiration greatly extends survival at low
water PO2 (Table 2). The lungs of both species should
therefore be viewed as accessory respiratory structures
that can be of importance during bouts of hypoxia that
might occur, albeit rarely, during periods of drought
and high temperatures. In addition, lungs may be important in the recovery from exercise, because of the limited
aerobic scope of activity of both species (Boutilier et al.
1980; Miller and Hutchison 1979).
Since the lungs apparently can serve a respiratory
function, this raises the possibility of adaptive shifts in
perfusion of the various gas exchangers. Necturus maculosus has a fenestrated and incomplete intraatrial septum, no spiral valve in the conus arteriosus, a divided
truncus arteriosus, and an incomplete intraventricular
septum (Baker 1949; Putnam and Dunn 1978). Because
the pulmonary artery arises from the radix aorta, there
would seem to be no pathway for getting blood directly
from the heart to the lungs without it first passing over
the gills. This is a circulatory pattern apparently unique
to Necturus (Romer and Parsons 1986), and one which
would not be beneficial while the animal is breathing
air when in severely hypoxic water, as 02 would be lost
to the water via the gills. However, there are several
possible solutions to this problem. Mudpuppy gills can
be actively protruded or retracted, and when they are
retracted the arborization of the filaments is not evident,
which suggests that perfusion is also limited. Both Baker
(1949) and Darnell (1949) have described vessels that
might function as gill bypasses, and Baker (1949) suggested that lung bypass was also possible. Therefore it
is likely that the mudpuppy is able to selectively shunt
blood to the appropriate gas exchanger depending upon
the PO2 of the water.
Cryptobranchus has a cardiac anatomy that, like Necturus, suggests that it is primarily a water breather (Putnam and Parkerson 1985). There are no gills, but Baker
(1949) found extensive branching of the pulmonary artery to the stomach, raising the possibility of gastric
respiration. We have mentioned that this animal frequently floats (tail down) at the surface in hypoxic water,
but we could not tell if the frequent inspirations were
resulting in any swallowing of air.
Ecological conclusions. The hypothesis that Cryptobranchus is found only in larger streams and small rivers
with a substantial and permanent flow rate because this
salamander has a limited ability to take up Oz is supported by our results. This limited ability is evident in
the very high Pc and in the extended time required to
recover from lactacidosis caused by exercise (Boutilier
et al. 1980). Also important is that the mountain-stream
habitat characteristically has a nil PCO2. The buffer capacity of hellbender blood is not great (Boutilier and
Toews 1981 a), although it is not especially low for ectotherms. More importantly, the lungs are probably not
257
efficient enough to reduce the water to blood PCOz gradient during hypercarbia-induced hypercapnia, although
this point is not certain since the experiments that have
been done with hellbenders in hypercarbic water did not
permit them access to CO2-free air (Boutilier and Toews
1981 b). During recovery from hypercapnia, be it generated from exercise or exposure to hypercarbic water,
lung ventilations increase markedly (Boutilier et al.
1980; Boutilier and Toews 1981 b), which suggests that
pulmonary respiration plays some role in COz elimination under extreme conditions. It is unlikely, however,
that the lungs are as effective in this role as in other
giant salamanders that typically inhabit hypercarbic
waters (Heisler et al. 1982). In summary, hellbenders
may be viewed as animals that are living at the limits
of their gas exchange capabilities, which restricts them
to relatively cool and flowing waters with high levels
of dissolved Oz and nil levels of CO2. Under such conditions they can maintain rates of gas exchange comparable to other salamanders, but are restricted to a life style
of minimal activity.
In contrast, Necturus is much more tolerant of hypoxia because of its highly developed external gills,
smaller size, and very low blood P5o- Whether it is also
tolerant of the hypercarbia often found in hypoxic
waters is uncertain. We have found that this species can
tolerate 4% CO2 (unpublished data), but Stiffier et al.
(1983) found 3% to be lethal; in addition the latter authors found that Necturus did not compensate blood
pH after 24 h of a hypercarbia-induced (2% CO~) respiratory acidosis, while larval Ambystoma tigrinum compensate 26% when exposed to 3% CO2 in water. Whether Necturus is thus limited to areas where hypoxia is
not accompanied by hypercarbia, if in fact it inhabits
hypoxic areas at all, remains to be investigated.
Acknowledgements. Robert Guthrie collected most of the hellbenders. Joesph Arceneaux, Keith Blackmon, David Carroll, Bulent Cinkilic, Scott Cochran, Phillip Craft, John Fischer, Charles
Gehrdes, Kevin Johnson, Leonard Jones, Douglas Lewis, Abbott
Martinson, Arnold Mayberry, Edward Purdue, Rosalind Moore,
Deborah Stephens, Michael Sullivan, Shaun West, and Christopher
Wright provided technical help. The figures were prepared by Daryl Harrison. The manuscript was prepared while GRU was on
sabbatical at the Department of Zoology at the University of Florida, and we are thankful to that department for support and the
use of its facilities, and particularly to John Anderson for supplying
space in an already crowded environment. This research was partially supported by the Research Grants Committee of The University of Alabama.
References
Baker CL (1949) The comparative anatomy of the aortic arches
of the urodeles and their relation to respiration and degree
of metamorphosis. J Term Acad Sci 24:12-40
Beffa DA (1976) Responses of Cryptobranchus alleganiensis alleganiensis to different oxygen concentrations, temperatures and
photoperiod. M.S. Thesis, Southwest Missouri State University,
Springfield, Missouri, p 37
Boutilier RG, McDonald DG, Toews DP (1980) The effects of
enforced activity on ventilation, circulation and blood acid-base
balance in the aquatic gill-less urodele, Cryptobranchus allega-
niensis; a comparison with the semi-terrestrial anuran, Bufo
marinus. J Exp Biol 84:289 302
Boutilier RG, Toews DP (1981 a) Respiratory properties of blood
in a strictly aquatic and predominantly skin-breathing urodele,
Cryptobranehus alleganiensis. Respir Physiol 46:161-176
Boutilier RG, Toews DP (1981 b) Respiratory, circulatory and acidbase adjustments to hypercapnia in a strictly aquatic and predominantly skin-breathing urodele, Cryptobranehus alleganiensis. Respir Physiol 46:177-192
Brown NR (1985) Changes in LDH activity in Cryptobranchus
due to imposed environmental stress. M.S. Thesis, Southwest
Missouri State University, Springfield, Missouri, p 33
Burggren WW, Feder ME (1986) Effect of experimental ventilation
of the skin on cutaneous gas exchange in the bullfrog. J Exp
Biol 121:445-449
Burggren WW, Moalli R (1984) 'Active' regulation of cutaneous
gas exchange by capillary recruitment in amphibians: experimental evidence and a revised model for skin respiration. Respir
Physiol 55 : 379-392
Conant R (1975) A field guide to reptiles and amphibians of eastern
and central North America. Houghton Mifflin Co., Boston
Darnell RM (1949) The aortic arches and associated arteries of
caudate Amphibia. Copeia 1949:18-31
Feder ME (1976) Lunglessness, body size, and metabolic rate in
salamanders. Physiol Zool 49 : 398-406
Feder ME, Burggren WW (1985) Cutaneous gas exchange in vertebrates: design, patterns, control and implications. Biol Rev
60 : 1-45
Gatten RE, Miller K, Full R (1990) Energetics of amphibians at
rest and during locomotion. In: Feder ME, Burggren WW (eds)
Environmental Physiology of the Amphibia. University of Chicago Press, Chicago, Illinois. (In press)
Guimond RW, Hutchison VH (1972) Pulmonary, branchial and
cutaneous gas exchange in the mudpuppy, Necturus maculosus
maculosus (Rafinesque). Comp Biochem Physiol 42A: 36%392
Guimond RW, Hutchison VH (1973) Aquatic respiration: an unusual strategy in the hellbender Cryptobranchus alleganiensis
alleganiensis (Daudin). Science 182:1263-1265
Guimond RW, Hutchison VH (1976) Gas exchange of the giant
salamanders of North America. In: Hughes GM (ed) Respiration of Amphibious Vertebrates, pp 313-338. Academic Press,
New York
Harlan RA, Wilkinson RF (1981) The effects of progressive hypoxia and rocking activity on blood oxygen tension for hellbenders,
Cryptobranchus alleganiensis. J Herpetol 15:383-387
Harris JP (1959) The natural history of Necturus: I. Habitats and
habits. Field Laborat 27:11-20
Heisler N, Forcht G, Ultsch GR, Anderson JF (1982) Acid-base
regulation in response to environmental hypercapnia in two
aquatic salamanders, Siren lacertina and Amphiuma means. Respir Physiol 49:141-158
Lenfant C, Johansen K (1967) Respiratory adaptations in selected
amphibians. Respir Physiol 2: 247-260
McCutcheon FH, Hall F G (1937) Hemoglobin in the amphibia.
J Cell Comp Physiol 9:191-197
Miller K, Hutchison VH (1979) Activity metabolism in the mudpuppy, Necturus maculosus. Physiol Zool 52:22-37
Moalli R, Meyers RS, Ultseh GR, Jackson DC (1981) Acid-base
balance and temperature in a predominantly skin-breathing salamander Cryptobranchus alleganiensis. Respir Physiol 43:1-11
Nickerson MA, Mays CE (1973) A study of the Ozark hellbender
Cryptobranchus alleganiensis bishopi. Ecology 54:1164-1165
Noble GK (1925) The integumentary, pulmonary, and cardiac
modifications correlated with increased cutaneous respiration
in the amphibia: a solution of the 'hairy frog' problem. J Morphol Physiol 40: 341-416
Ott ME, Heisler N, Ultsch GR (1980) A re-evaluation of the relationship between temperature and the critical oxygen tension
in freshwater fishes. Comp Biochem Physiol 67A:337-340
Putnam JL, Dunn JF (1978) Septation in the ventricle of the heart
of Necturus maeulosus. Herpetologica 34: 292-297
258
Putnam JL, Parkerson JB (1985) Anatomy of the heart of the
amphibia II. Cryptobranchus alleganiensis. Herpetologica
41 : 287-298
Romer AS, Parsons TS (1986) The vertebrate body. Saunders College Publishing, New York
Shield JW, Bentley PJ (1973) Respiration of some urodele and
anuran amphibia - I. In water, role of the skin and gills. Comp
Biochem Physiol 46A, 17-28
Steffenson JF (1989) Some errors in respirometry of aquatic
breathers: how to avoid and correct for them. Fish Physiol
Biochem 64: 49-59
Stiffler DF, Tufts BL, Toews DP (1983) Acid-base and ionic balance in Ambystoma tigrinum and Necturus rnaculosus during
hypercapnia. Am J Physiol 245:R689-R694
Ultsch GR (1973a) A theoretical and experimental investigation
of the relationships between metabolic rate, body size, and oxygen exchange capacity. Respir Physiol 18 : 143-160
Ultsch GR (1973 b) The effects of water hyacinths (Eichhornia crassipes) on the microenvironment of aquatic communities. Archiv
Hydrobiol 72: 460-473
Ultsch GR (1976 a) Eeo-physiological studies studies of some metabolic and respiratory adaptations of sirenid salamanders. In:
Hughes GM (ed) Respiration of Amphibious Vertebrates,
pp 287-312. Academic Press, New York
Ultsch GR (1976 b) Respiratory surface area as a factor controlling
the standard rate of Oz consumption of aquatic salamanders.
Respir Physiol 26:357-369
Uttsch GR, Anderson JF (1988) Gas exchange during hypoxia
and hypercarbia of terrestrial turtles : a comparison of a fossorial species (Gopherus polyphemus) with a sympatric nonfossorial
species (Terrapene carolina). Physiol Zool 61:142-152
Ultsch GR, Jackson DC, Moalli R (1981) Metabolic oxygen conformity among lower vertebrates: the toadfish revisited. J
Comp Physiol 142: 439-443
Wakeman JM, Ultsch GR (1975) The effects of dissolved Oz and
COz on metabolism and gas exchange partitioning in aquatic
salamanders. Physiol Zool 48:348-359
Weber RE, Wells RMG, Rossetti JE (1985) Adaptations to neoteny
in the salamander, Necturus maculosus. Blood respiratory properties and interactive effects of pH, temperature, and ATP on
hemoglobin oxygenation. Comp Biochem Physiol 80A: 495-501
Yeager DP, Ultsch GR (1989) Physiological regulation and conformation: A BASIC program for the determination of critical
points. Physiol Zool 62:888-907