9. AIR BREATHING IN FISHES
397
60
L
c
aJ
I n
0
L 40
f
a
: 20
L
._
a
0
50
100
I50
200
250
Oxygen tension in water ( m m H g )
04
I
0
5 0
100
I50
200
250
(B)
Oxygen tension in water ( m m Hq)
Fig. 16B.
gill surface area is reduced. Thus there is a gradual reduction of the
diffusion link between the external water and the blood as the fish shifts
emphasis from water to air breathing. Deoxygenation of the water
elicits no change in the breathing pattern of these fishes. In a sense
they have become relieved of the environmental deoxygenation stress
which originally provided the selection pressure favoring the air-breathing habit. Blood gas measurements have substantiated that no change in
internal O2 tension occurs in response to deoxygenation of the water, nor
do the fishes exhibit any restlessness suggesting the presence of external
chemosensation of O? that could guide the fish in avoidance or attraction
to the degree of external oxygenation (Johansen and Lenfant, 1968).
397
60
L
c
aJ
I n
0
L 40
f
a
: 20
L
._
a
0
50
100
I50
200
250
Oxygen tension in water ( m m H g )
04
I
0
5 0
100
I50
200
250
(B)
Oxygen tension in water ( m m Hq)
Fig. 16B.
gill surface area is reduced. Thus there is a gradual reduction of the
diffusion link between the external water and the blood as the fish shifts
emphasis from water to air breathing. Deoxygenation of the water
elicits no change in the breathing pattern of these fishes. In a sense
they have become relieved of the environmental deoxygenation stress
which originally provided the selection pressure favoring the air-breathing habit. Blood gas measurements have substantiated that no change in
internal O2 tension occurs in response to deoxygenation of the water, nor
do the fishes exhibit any restlessness suggesting the presence of external
chemosensation of O? that could guide the fish in avoidance or attraction
to the degree of external oxygenation (Johansen and Lenfant, 1968).