9. AIR BREATHING IN FISHES
393
1966). Thus the resting rate of water breathing provides little reserve for
increased metabolic activity. The energy expenditure of water ventilation
is high, and a definite advantage attends the use of less costly alternative
ways of gas exchange during periods of increased demand. Water
breathing during low activity, however, permits the fish to exploit its
aquatic environment without the need for frequent ascents to the surface
for air.
It is generally held that structural adaptations for air breathing in
fishes evolved in association with environmental oxygen lack. Yet fishes
such as the Australian lungfish, the holostean fishes Amia and Lepisostew,
and many teleosts breathe air even though their habitats are not oxygen
deficient. Thus, internal oxygen deficiencies resulting from activity may
have provided a selection pressure for development of air breathing independent of the oxygen availability in the water. Certainly, the exceptional air-breathing habits of marine pelagic fishes such as the tarpon,
Megalops (Schlaifer and Breder, 1940), support this contention. Recent
work comparing the relative importance of water breathing and air
breathing in Amiu at different temperatures has emphasized the importance of air breathing for support of higher metabolic activity (Johansen et aZ., 1970). The situation resembles the prompt stimulation of
breathing in exercising mammals without an apparent initiating role of
arterial chemoreceptors.
A few fishes, such as the electric eel, have come so far in the dominance of air breathing that the muscles used for water ventilation and
the gills themselves have atrophied to a point where mechanical manifestation of branchial breathing has ceased altogether. However, aquatic
gas exchange continues mainly as CO, elimination through the skin or
the atrophied gills. This important part of overall gas exchange is mainly
regulated by changes of blood perfusion at the interfaces between water
and blood in the skin or gills.
A marked change in the respiratory behavior of air-breathing fishes
likely attend their ontogenetic development. Some species such as the
African lungfish are decidedly aquatic breathers during larval and
juvenile stages, but air breathing takes on a dominating importance in
the adult fish.
F. Breathing Responses to Changes in External Gas Composition
1. CHANGES OF ENVIRONMENTAL OXYGENATION
Exposure to hypoxic water evokes different breathing responses
among air-breathing fishes. The response patterns express an apparent
393
1966). Thus the resting rate of water breathing provides little reserve for
increased metabolic activity. The energy expenditure of water ventilation
is high, and a definite advantage attends the use of less costly alternative
ways of gas exchange during periods of increased demand. Water
breathing during low activity, however, permits the fish to exploit its
aquatic environment without the need for frequent ascents to the surface
for air.
It is generally held that structural adaptations for air breathing in
fishes evolved in association with environmental oxygen lack. Yet fishes
such as the Australian lungfish, the holostean fishes Amia and Lepisostew,
and many teleosts breathe air even though their habitats are not oxygen
deficient. Thus, internal oxygen deficiencies resulting from activity may
have provided a selection pressure for development of air breathing independent of the oxygen availability in the water. Certainly, the exceptional air-breathing habits of marine pelagic fishes such as the tarpon,
Megalops (Schlaifer and Breder, 1940), support this contention. Recent
work comparing the relative importance of water breathing and air
breathing in Amiu at different temperatures has emphasized the importance of air breathing for support of higher metabolic activity (Johansen et aZ., 1970). The situation resembles the prompt stimulation of
breathing in exercising mammals without an apparent initiating role of
arterial chemoreceptors.
A few fishes, such as the electric eel, have come so far in the dominance of air breathing that the muscles used for water ventilation and
the gills themselves have atrophied to a point where mechanical manifestation of branchial breathing has ceased altogether. However, aquatic
gas exchange continues mainly as CO, elimination through the skin or
the atrophied gills. This important part of overall gas exchange is mainly
regulated by changes of blood perfusion at the interfaces between water
and blood in the skin or gills.
A marked change in the respiratory behavior of air-breathing fishes
likely attend their ontogenetic development. Some species such as the
African lungfish are decidedly aquatic breathers during larval and
juvenile stages, but air breathing takes on a dominating importance in
the adult fish.
F. Breathing Responses to Changes in External Gas Composition
1. CHANGES OF ENVIRONMENTAL OXYGENATION
Exposure to hypoxic water evokes different breathing responses
among air-breathing fishes. The response patterns express an apparent