9. AIR BREATHING IN FISHES
381
B. Gas Exchange in Air-Breathing Fishes
All air-breathing fishes possess some capacity for aquatic breathing.
Thus gas exchange is bimodal and depends upon exchange diffusion
with the water via gills or skin and with air rhythmically taken into, and
expelled, from, the air-breathing organ. The fish may employ both means
of gas exchange simultaneously or may resort to one method at a time
depending upon external circumstances. A very few fishes undertake
voluntary migrations over land in moist or wet grass. Aquatic respiration
must then be altogether halted except for some possible exchange via
the skin. Estivating air-breathing fishes represent a special case. They
must rely almost exclusively on aerial gas exchange although retention
of a moist microenvironment in their cocoons or burrows in the substratum is essential for survival. This requisite may be as important for
fluid balance and osmotic control as for continued cutaneous gas exchange. The literature is deficient of any attempt to study gas exchange
in estivating fishes.
Air-breathing fishes experience two vastly different environments. A
low solubility of O2 in water provides an oxygen availability, in
aerated water, about one-thirtieth of that in air in a tropical environment. Hence, aquatic breathers must ventilate much larger volumes than
air breathers at similar 0, uptakes. The greater work of breathing in
water is further aggravated by the huge density difference between the
two media. The high solubility of CO, in water along with the high ventilation of aquatic breathers brings about internal CO, tensions which
may be one-thirtieth of those usually found in terrestrial vertebrates.
Air breathing is advantageous in terms of 0, availability and high
diffusion rates, but it is countered by the hazards of desiccation and by
the different requirements for mechanical support under the influence of
net gravitational forces in air. Exclusive air breathers also have to meet
the need for increased efficiency of internal buffering systems because
of elevated internal CO, tensions. Air-breathing fishes largely avoid
these problems by remaining essentially aquatic, only exploiting the atmosphere as a source of oxygen.
All air-breathing organs in fishes show a low gas exchange ratio indicating that their function is mainly 0, absorption. Gills and skin conversely play their most important role in C 0 2 elimination and show
accordingly gas exchange ratios exceeding unity.
Table I11 offers a compilation of the relative role of aquatic and
aerial gas exchange in the few fishes that have been adequately studied.
Among obligate air breathers such as Lepidosiren and Protopterus,
aquatic breathing is of little significance in O2 absorption and contributes
381
B. Gas Exchange in Air-Breathing Fishes
All air-breathing fishes possess some capacity for aquatic breathing.
Thus gas exchange is bimodal and depends upon exchange diffusion
with the water via gills or skin and with air rhythmically taken into, and
expelled, from, the air-breathing organ. The fish may employ both means
of gas exchange simultaneously or may resort to one method at a time
depending upon external circumstances. A very few fishes undertake
voluntary migrations over land in moist or wet grass. Aquatic respiration
must then be altogether halted except for some possible exchange via
the skin. Estivating air-breathing fishes represent a special case. They
must rely almost exclusively on aerial gas exchange although retention
of a moist microenvironment in their cocoons or burrows in the substratum is essential for survival. This requisite may be as important for
fluid balance and osmotic control as for continued cutaneous gas exchange. The literature is deficient of any attempt to study gas exchange
in estivating fishes.
Air-breathing fishes experience two vastly different environments. A
low solubility of O2 in water provides an oxygen availability, in
aerated water, about one-thirtieth of that in air in a tropical environment. Hence, aquatic breathers must ventilate much larger volumes than
air breathers at similar 0, uptakes. The greater work of breathing in
water is further aggravated by the huge density difference between the
two media. The high solubility of CO, in water along with the high ventilation of aquatic breathers brings about internal CO, tensions which
may be one-thirtieth of those usually found in terrestrial vertebrates.
Air breathing is advantageous in terms of 0, availability and high
diffusion rates, but it is countered by the hazards of desiccation and by
the different requirements for mechanical support under the influence of
net gravitational forces in air. Exclusive air breathers also have to meet
the need for increased efficiency of internal buffering systems because
of elevated internal CO, tensions. Air-breathing fishes largely avoid
these problems by remaining essentially aquatic, only exploiting the atmosphere as a source of oxygen.
All air-breathing organs in fishes show a low gas exchange ratio indicating that their function is mainly 0, absorption. Gills and skin conversely play their most important role in C 0 2 elimination and show
accordingly gas exchange ratios exceeding unity.
Table I11 offers a compilation of the relative role of aquatic and
aerial gas exchange in the few fishes that have been adequately studied.
Among obligate air breathers such as Lepidosiren and Protopterus,
aquatic breathing is of little significance in O2 absorption and contributes