402
K JELL JOHANSEN
its gills (Johansen, 1966). The fish shows inhibition of branchial breathing in C0,-rich water which persists as CO, leaks into the bloodstream.
However, when the fish is transferred to C0,-free water, a high frequency of forceful branchial movements is established. Hence, the fish
appears to possess a sensory mechanism that can differentiate between
the external and internal C 0 2 level. Similarly, in experiments with
Protopterus, bubbling of 5% CO, in the water stopped branchial breathing before there were appreciable changes in CO, tension of the arterial
blood.
The stimulation of air breathing by C 0 2 may be a direct effect, or it
may be secondary to the reduced oxygen uptake caused by the depression
of branchial breathing. The response of Protopterus and Electrophorus,
in which branchial breathing contributes little or nothing to the oxygen
uptake, suggests a direct effect. Also, inhalation of C0,-enriched gas
stimulates air breathing in these fish while internal oxygen levels remain
the same or increase. However, in predominantly water-breathing fishes
such as Neoceratodus or Amia, the influence of CO, on air breathing
may be both direct and indirect.
The variability found in the few studies on C 0 2 response in airbreathing fishes was also apparent in the important work of Willmer
( 1934). Figure 15 summarizes his work on the factors controlling
breathing in the yarrow, Erythrinus, a teleost which uses its air bladder
for accessory breathing. Willmer contended that a certain CO, concentration in the blood is necessary to drive the branchial breathing.
At very low 0, contents of the water, branchial movements ceased,
possibly to prevent loss to the water of 0, gained by air breathing. The
opercula also remain actively closed at higher external CO, concentrations, allegedly to prevent internal acidosis which would have resulted
if gill breathing persisted. The figure also explains the conditions under
which respiratory needs are satisfied by aquatic respiration alone and
similarly the factors in the environment which cause the fish to employ
bimodal breathing.
G. Breathing Responses to Mechanical Stimuli
A possible role of external mechanosensitive receptors in control of
breathing rhythms is suggested by recent experiments showing a marked
acceleration of branchial breathing rate in Lepidosiren and Protopterus
when the water was stirred or agitated (Johansen and Lenfant, 1967,
1968). Such a role of mechanoreceptors in fish takes on special signifi-
K JELL JOHANSEN
its gills (Johansen, 1966). The fish shows inhibition of branchial breathing in C0,-rich water which persists as CO, leaks into the bloodstream.
However, when the fish is transferred to C0,-free water, a high frequency of forceful branchial movements is established. Hence, the fish
appears to possess a sensory mechanism that can differentiate between
the external and internal C 0 2 level. Similarly, in experiments with
Protopterus, bubbling of 5% CO, in the water stopped branchial breathing before there were appreciable changes in CO, tension of the arterial
blood.
The stimulation of air breathing by C 0 2 may be a direct effect, or it
may be secondary to the reduced oxygen uptake caused by the depression
of branchial breathing. The response of Protopterus and Electrophorus,
in which branchial breathing contributes little or nothing to the oxygen
uptake, suggests a direct effect. Also, inhalation of C0,-enriched gas
stimulates air breathing in these fish while internal oxygen levels remain
the same or increase. However, in predominantly water-breathing fishes
such as Neoceratodus or Amia, the influence of CO, on air breathing
may be both direct and indirect.
The variability found in the few studies on C 0 2 response in airbreathing fishes was also apparent in the important work of Willmer
( 1934). Figure 15 summarizes his work on the factors controlling
breathing in the yarrow, Erythrinus, a teleost which uses its air bladder
for accessory breathing. Willmer contended that a certain CO, concentration in the blood is necessary to drive the branchial breathing.
At very low 0, contents of the water, branchial movements ceased,
possibly to prevent loss to the water of 0, gained by air breathing. The
opercula also remain actively closed at higher external CO, concentrations, allegedly to prevent internal acidosis which would have resulted
if gill breathing persisted. The figure also explains the conditions under
which respiratory needs are satisfied by aquatic respiration alone and
similarly the factors in the environment which cause the fish to employ
bimodal breathing.
G. Breathing Responses to Mechanical Stimuli
A possible role of external mechanosensitive receptors in control of
breathing rhythms is suggested by recent experiments showing a marked
acceleration of branchial breathing rate in Lepidosiren and Protopterus
when the water was stirred or agitated (Johansen and Lenfant, 1967,
1968). Such a role of mechanoreceptors in fish takes on special signifi-