9. AIR BREATHING IN FISHES
385
employing accessory or supplemental air breathing. Note, also, the
marked effect of temperature on the rate of 0, depletion from the air
bladder in Amiu.
A comparison of the slopes for 0, depletion and CO, accumulation
reveals the former to be much steeper, suggesting a low gas exchange
ratio. The slopes for CO, accumulation also show a more abrupt change
to a gentler slope late in the interval between surfacings for air, a fact
suggesting that the low gas exchange ratio becomes even lower, later in
the interval between air breaths. In the case of Protopterus and Electrophorus, this change is particularly apparent (Figs. 9 and 12). Thus
aquatic CO? elimination varies and may increase with time, possibly
owing to a vasodilatory effect of increasing CO,, in gills and skin
vessels.
The slopes given for 0, depletion in Fig. 8 represent average values,
and variations likely reflect changes in the blood perfusion through the
exchange organ. The average slopes seem related with the breathing
habits of the fishes. Thus, in Electrophorus, intervals between air breaths
rarely exceeded 2 min, while in Lepidosiren and Protopterus the intervals
varied but were usually between 3 and 6 min, whereas, Symbranchus
and Amiu both showed infrequent surfacings often exceeding 2530 min
between air breaths.
Few studies have assessed the total volume and tidal volume of
air-breathing organs in fishes. Juvenile specimens of Protopterus ( weight
57-78 g) are reported to show average lung volumes of 1.7 ml (or 25.3
ml/ kg ) measured by radiological techniques. Tidal volumes measured
by spirometry averaged 1.45 ml (or 19.8 ml/kg) and, hence, represent
a fraction of total lung volume as large as 80% (Jesse et al., 1967).
C. Internal Gas Transport in Air-Breathing Fishes
Gas transport between the respiratory organs and the metabolizing
tissues depends on the metabolic rate, the pattern and rate of blood
circulation, and on the respiratory properties of blood. Very few airbreathing fishes have been adequately studied by frequent sampling of
blood and gas to allow an evaluation of internal gas transport. Table IV
summarizes available information on gas composition in blood and aerial
exchange organs.
The Australian lungfish, Neoceratodus, is special among the lungfishes by being predominantly a water breather. During rest, in aerated
water, the gills are responsible for the entire 0, absorption which suffices
to maintain the arterial blood 95% saturated with 0,. Comparisons of
gas tensions in samples from the pulmonary artery, pulmonary vein, and
385
employing accessory or supplemental air breathing. Note, also, the
marked effect of temperature on the rate of 0, depletion from the air
bladder in Amiu.
A comparison of the slopes for 0, depletion and CO, accumulation
reveals the former to be much steeper, suggesting a low gas exchange
ratio. The slopes for CO, accumulation also show a more abrupt change
to a gentler slope late in the interval between surfacings for air, a fact
suggesting that the low gas exchange ratio becomes even lower, later in
the interval between air breaths. In the case of Protopterus and Electrophorus, this change is particularly apparent (Figs. 9 and 12). Thus
aquatic CO? elimination varies and may increase with time, possibly
owing to a vasodilatory effect of increasing CO,, in gills and skin
vessels.
The slopes given for 0, depletion in Fig. 8 represent average values,
and variations likely reflect changes in the blood perfusion through the
exchange organ. The average slopes seem related with the breathing
habits of the fishes. Thus, in Electrophorus, intervals between air breaths
rarely exceeded 2 min, while in Lepidosiren and Protopterus the intervals
varied but were usually between 3 and 6 min, whereas, Symbranchus
and Amiu both showed infrequent surfacings often exceeding 2530 min
between air breaths.
Few studies have assessed the total volume and tidal volume of
air-breathing organs in fishes. Juvenile specimens of Protopterus ( weight
57-78 g) are reported to show average lung volumes of 1.7 ml (or 25.3
ml/ kg ) measured by radiological techniques. Tidal volumes measured
by spirometry averaged 1.45 ml (or 19.8 ml/kg) and, hence, represent
a fraction of total lung volume as large as 80% (Jesse et al., 1967).
C. Internal Gas Transport in Air-Breathing Fishes
Gas transport between the respiratory organs and the metabolizing
tissues depends on the metabolic rate, the pattern and rate of blood
circulation, and on the respiratory properties of blood. Very few airbreathing fishes have been adequately studied by frequent sampling of
blood and gas to allow an evaluation of internal gas transport. Table IV
summarizes available information on gas composition in blood and aerial
exchange organs.
The Australian lungfish, Neoceratodus, is special among the lungfishes by being predominantly a water breather. During rest, in aerated
water, the gills are responsible for the entire 0, absorption which suffices
to maintain the arterial blood 95% saturated with 0,. Comparisons of
gas tensions in samples from the pulmonary artery, pulmonary vein, and