9. AIR BREATHING IN FISHES
377
factors and the relative importance of air and water breathing (Willmer,
1934; Fish, 1956; Swan and Hall, 1966; Lenfant and Johansen, 1968).
Table I1 summarizes available information in representative species.
A large variability is apparent in hemoglobin concentration and 0,
capacity, with no adaptive trend evident in air-breathing fishes as a
group. However, a comparison of tropical fishes studied by Willmer
(1934) disclosed a definite tendency for 0, capacities to be higher in
the air-breathing forms. This difference probably reflects the nature of
the fishes’ habitats, particularly the degree of 0, deficiency rather than
the habit of air breathing per se. The fast flowing rivers and creeks
where the purely aquatic breathers Myelus setiger and Hoplias malabaricus live are well aerated, whereas the habitat of the air-breathing
fishes Hoplosternum and Electrophorus can be severely 0, deficient.
In evaluating this type of adaptive adjustment, one must recognize
the relative importance of air and aquatic breathing in the total gas
exchange of these fishes. If water breathing is the dominate mode and
air breathing is only accessory or supplemental, adaptive adjustment
of the blood to meet the conditions in the water may be expected. An
early documentation by Krogh and Leitch (1919) showed that aquatic
species tended to show higher 0, capacities when living in 0, deficient
water. The obligate air breathers, among fishes, have an ample reservoir
of atmospheric oxygen available. However, these fishes may be compared
with the diving animals among higher vertebrates which, by means of
high oxygen capacities, increase their 0, stores and consequently extend
the time between surfacings. Sluggish or active habits also may influence
hemoglobin levels in fishes (Root, 1931). Still another type of selection
pressure for development of high 0, capacities in air-breathing fishes
has been suggested and exemplified in the electric eel, Electrophorms
(Johansen et al., 1968), and applies to most other air-breathing fishes.
This relates to the shunting of respiratory efferent blood into systemic
veins which results in systemic arterial perfusion with mixed blood
(Fig. 4). Thus arterial blood never gets fully saturated, a condition
which obviously reduces the efficiency of the gas transport system. The
situation in lungfishes differs considerably in that recirculation of the
well-oxygenated blood from the lung is largely prevented by a separate
pulmonary vascular circuit and a partial division of the heart and its
outflow channels.
The general shape of 02-Hb dissociation curves conform to those
of other fishes, Oxygen affinity generally decreases in vertebrates with
increasing dependence on aerial gas exchange ( McCutcheon and Hall,
1937; Lenfant and Johansen, 1967). Another generalization relates an
increased 0, affinity to the ability of a species to survive in an 0, poor
377
factors and the relative importance of air and water breathing (Willmer,
1934; Fish, 1956; Swan and Hall, 1966; Lenfant and Johansen, 1968).
Table I1 summarizes available information in representative species.
A large variability is apparent in hemoglobin concentration and 0,
capacity, with no adaptive trend evident in air-breathing fishes as a
group. However, a comparison of tropical fishes studied by Willmer
(1934) disclosed a definite tendency for 0, capacities to be higher in
the air-breathing forms. This difference probably reflects the nature of
the fishes’ habitats, particularly the degree of 0, deficiency rather than
the habit of air breathing per se. The fast flowing rivers and creeks
where the purely aquatic breathers Myelus setiger and Hoplias malabaricus live are well aerated, whereas the habitat of the air-breathing
fishes Hoplosternum and Electrophorus can be severely 0, deficient.
In evaluating this type of adaptive adjustment, one must recognize
the relative importance of air and aquatic breathing in the total gas
exchange of these fishes. If water breathing is the dominate mode and
air breathing is only accessory or supplemental, adaptive adjustment
of the blood to meet the conditions in the water may be expected. An
early documentation by Krogh and Leitch (1919) showed that aquatic
species tended to show higher 0, capacities when living in 0, deficient
water. The obligate air breathers, among fishes, have an ample reservoir
of atmospheric oxygen available. However, these fishes may be compared
with the diving animals among higher vertebrates which, by means of
high oxygen capacities, increase their 0, stores and consequently extend
the time between surfacings. Sluggish or active habits also may influence
hemoglobin levels in fishes (Root, 1931). Still another type of selection
pressure for development of high 0, capacities in air-breathing fishes
has been suggested and exemplified in the electric eel, Electrophorms
(Johansen et al., 1968), and applies to most other air-breathing fishes.
This relates to the shunting of respiratory efferent blood into systemic
veins which results in systemic arterial perfusion with mixed blood
(Fig. 4). Thus arterial blood never gets fully saturated, a condition
which obviously reduces the efficiency of the gas transport system. The
situation in lungfishes differs considerably in that recirculation of the
well-oxygenated blood from the lung is largely prevented by a separate
pulmonary vascular circuit and a partial division of the heart and its
outflow channels.
The general shape of 02-Hb dissociation curves conform to those
of other fishes, Oxygen affinity generally decreases in vertebrates with
increasing dependence on aerial gas exchange ( McCutcheon and Hall,
1937; Lenfant and Johansen, 1967). Another generalization relates an
increased 0, affinity to the ability of a species to survive in an 0, poor