362
K JELL JOHANSEN
drostean, and holostean fishes are dominated by air-breathing forms.
Reports of air-breathing habits among elasmobranch fishes have not
been adequately confirmed ( George, 1953).
A smaller number of air-breathing fishes live in temperate regions,
and in exceptional cases such as the bowfin, Amiu calva, they occupy
waters seasonally frozen over.
A shortage of dissolved oxygen constitutes the primary environmental
condition which has stimulated the development of air-breathing in
fishes ( Barrell, 1916; Carter and Beadle, 1931a), although such adaptations may also have been correlated with behavior or activity patterns.
A few forms, inhabiting oxygen-deficient tropical swamps, have acquired
the faculty of sustained air breathing when entirely removed from water
during droughts. Such periods are passsed in an estivating condition. Air
breathing has also evolved among fishes living in torrential mountain
streams in Asia. Here, the air-breathing habit is an adaptation to drought
only since these streams carry well-oxygenated water during the wet
season. These fishes either estivate or migrate over land to neighboring
streams (Das, 1927, 1940; Saxena, 1963). Many tropical swamps and
stagnant tributaries to larger rivers have very acid and C0,-rich water.
Adverse conditions for aquatic breathing with gills also result when
water gets excessively turbid with suspended material which may cover
the gill surfaces.
The principal causes of oxygen depletion in tropical freshwaters are
related to the abundance of organic matter which provides a substrate
for rapid oxygen consumption by microorganisms. The prevailing high
temperatures of the tropics accelerate oxygen depletion by increasing the
O2 requirements of the microfauna as well as the other inhabitants of
the swamps. Diurnal temperature variations are small, and thus little
vertical water movement occurs to promote reentry of atmospheric 0,
and the swamps become stagnant. Oxygen release from photosynthetic
activity is very slight because of the low intensity of light below the
dense foliage of the prolific vegetation in regions of tropical swamps and
rain forests. The same foliage cover prevents wind disturbance from
effectively stirring and thus aerating the water.
Naturally tropical waters show a vertical gradient in 0, concentration owing to a more well-oxygenated surface layer. However, as close
as 1 cm to the surface, 0, concentrations can be as low as 2% of saturation value. Several inhabitants of tropical swamps show behavioral
adaptations for utilization of the more 0,-rich surface layer by breathing at or near the surface. Often 0, availability is so limiting that all
species not adapted for direct air breathing must utilize the surface
water as a source for oxygen (Carter and Beadle, 1931a,b).
K JELL JOHANSEN
drostean, and holostean fishes are dominated by air-breathing forms.
Reports of air-breathing habits among elasmobranch fishes have not
been adequately confirmed ( George, 1953).
A smaller number of air-breathing fishes live in temperate regions,
and in exceptional cases such as the bowfin, Amiu calva, they occupy
waters seasonally frozen over.
A shortage of dissolved oxygen constitutes the primary environmental
condition which has stimulated the development of air-breathing in
fishes ( Barrell, 1916; Carter and Beadle, 1931a), although such adaptations may also have been correlated with behavior or activity patterns.
A few forms, inhabiting oxygen-deficient tropical swamps, have acquired
the faculty of sustained air breathing when entirely removed from water
during droughts. Such periods are passsed in an estivating condition. Air
breathing has also evolved among fishes living in torrential mountain
streams in Asia. Here, the air-breathing habit is an adaptation to drought
only since these streams carry well-oxygenated water during the wet
season. These fishes either estivate or migrate over land to neighboring
streams (Das, 1927, 1940; Saxena, 1963). Many tropical swamps and
stagnant tributaries to larger rivers have very acid and C0,-rich water.
Adverse conditions for aquatic breathing with gills also result when
water gets excessively turbid with suspended material which may cover
the gill surfaces.
The principal causes of oxygen depletion in tropical freshwaters are
related to the abundance of organic matter which provides a substrate
for rapid oxygen consumption by microorganisms. The prevailing high
temperatures of the tropics accelerate oxygen depletion by increasing the
O2 requirements of the microfauna as well as the other inhabitants of
the swamps. Diurnal temperature variations are small, and thus little
vertical water movement occurs to promote reentry of atmospheric 0,
and the swamps become stagnant. Oxygen release from photosynthetic
activity is very slight because of the low intensity of light below the
dense foliage of the prolific vegetation in regions of tropical swamps and
rain forests. The same foliage cover prevents wind disturbance from
effectively stirring and thus aerating the water.
Naturally tropical waters show a vertical gradient in 0, concentration owing to a more well-oxygenated surface layer. However, as close
as 1 cm to the surface, 0, concentrations can be as low as 2% of saturation value. Several inhabitants of tropical swamps show behavioral
adaptations for utilization of the more 0,-rich surface layer by breathing at or near the surface. Often 0, availability is so limiting that all
species not adapted for direct air breathing must utilize the surface
water as a source for oxygen (Carter and Beadle, 1931a,b).