9. AIR BREATHING IN FISHES
375
vascular beds. In the electric eel (type B), the entire gain in oxygenation resulting from perfusing the mouth is shunted back to the systemic
veins before blood is redistributed to the systemic and respiratory
arteries. This deficiency prevails to various degrees in all types except
the very few fishes able to use their gills alternately for aerial and aquatic
breathing [e.g., Symbrunchw or the few fishes employing air breathing
with specialized air chambers extending from the opercular cavities
such as Clarius (type C)].
Only in fishes employing the air bladder for aerial gas exchange have
specialized vessels developed as connections between the primary branchial circulation and the gas exchange organ (types E and F). These
vessels take origin from the sixth aortic arch and are homologue with
the pulmonary arteries of higher vertebrates. In addition to the dipnoans,
Polypterus and Amiu possess these vessels. The dipnoans are unique
among fishes by also having a specialized efferent circulation from the
air bladder connected directly with the left side of the heart. Gymnurchus
niloticus, a teleost using the air bladder for accessory respiration, has an
arterial supply to the bladder from the dorsal aorta but a venous drainage connecting directly with the atrium. No septation of the heart is
evident. However, the heart in dipnoans is in part structurally adapted
for accommodation of two bloodstreams by showing partial atrial and
ventricular septa, structures which clearly forecast a further develop
ment of the heart and circulation toward conditions in terrestrial vertebrates (Johansen and Hanson, 1968; Johansen and Hol, 1968).
The mechanics of air breathing in lungfishes has been carefully
studied (Bishop and Foxon, 1968; McMahon, 1969). The mechanics of
inhalation and exhalation of air is derived from a series of basically
aquatic breathing cycles typical of fishes modified to serve a specific step
in the air-breathing cycle. McMahon (1969) emphasizes the role of the
buccal force pump as the main ventilatory mechanism in purely aquatic
as well as transitional bimodal breathers among fishes and amphibians.
111. PHYSIOLOGICAL ADAPTATIONS IN
AIR-BREATHING FISHES
A. Respiratory Properties of Blood
Studies of respiratory properties of blood from air-breathing fishes
have disclosed adaptive features which correlate with environmental
heart allows a high degree of selective perfusion in the respiratory and systemic
circuits. Arrangement seen in lungfishes and in Psotopterus and Lepidosisen in
particular.
375
vascular beds. In the electric eel (type B), the entire gain in oxygenation resulting from perfusing the mouth is shunted back to the systemic
veins before blood is redistributed to the systemic and respiratory
arteries. This deficiency prevails to various degrees in all types except
the very few fishes able to use their gills alternately for aerial and aquatic
breathing [e.g., Symbrunchw or the few fishes employing air breathing
with specialized air chambers extending from the opercular cavities
such as Clarius (type C)].
Only in fishes employing the air bladder for aerial gas exchange have
specialized vessels developed as connections between the primary branchial circulation and the gas exchange organ (types E and F). These
vessels take origin from the sixth aortic arch and are homologue with
the pulmonary arteries of higher vertebrates. In addition to the dipnoans,
Polypterus and Amiu possess these vessels. The dipnoans are unique
among fishes by also having a specialized efferent circulation from the
air bladder connected directly with the left side of the heart. Gymnurchus
niloticus, a teleost using the air bladder for accessory respiration, has an
arterial supply to the bladder from the dorsal aorta but a venous drainage connecting directly with the atrium. No septation of the heart is
evident. However, the heart in dipnoans is in part structurally adapted
for accommodation of two bloodstreams by showing partial atrial and
ventricular septa, structures which clearly forecast a further develop
ment of the heart and circulation toward conditions in terrestrial vertebrates (Johansen and Hanson, 1968; Johansen and Hol, 1968).
The mechanics of air breathing in lungfishes has been carefully
studied (Bishop and Foxon, 1968; McMahon, 1969). The mechanics of
inhalation and exhalation of air is derived from a series of basically
aquatic breathing cycles typical of fishes modified to serve a specific step
in the air-breathing cycle. McMahon (1969) emphasizes the role of the
buccal force pump as the main ventilatory mechanism in purely aquatic
as well as transitional bimodal breathers among fishes and amphibians.
111. PHYSIOLOGICAL ADAPTATIONS IN
AIR-BREATHING FISHES
A. Respiratory Properties of Blood
Studies of respiratory properties of blood from air-breathing fishes
have disclosed adaptive features which correlate with environmental
heart allows a high degree of selective perfusion in the respiratory and systemic
circuits. Arrangement seen in lungfishes and in Psotopterus and Lepidosisen in
particular.