9. AIR BREATHING IN FISHES
399
Fig. 17. Continuous tracings of blood velocity in the ventral aorta ( V A ) of
Electrophorus. Each air breath shows as a distinct larger excursion ( B ) . ( a ) shows
the very prompt change in the rate of air breathing following the first breath of air
after hypoxic breathing. ( b ) Inhalation of hyperoxic air immediately prolongs the
interval between air breaths. From Johansen et al. (1968).
breathing is governed by changes of oxygen tension in the air-breathing
organ. The situation corresponds to conditions in higher vertebrates
where oxygen inhalation is effective in removing the tonic activity of
chemoreceptor cells stimulated by the normally prevailing levels of Po,.
Hyperoxic breathing also reduces the rate of air breathing in Protopterus
and other air-breathing fishes. Species depending largely on aquatic
breathing show a depression of branchial breathing in hyperoxic water
or following inhalation of hyperoxic air.
2. CHANGES OF ENVIRONMENTAL C 0 2 TENSIONS
Internal CO, tensions are very low among purely water-breathing
fishes living in well-aerated water (Lenfant and Johansen, 1966; Piiper
and Schumann, 1967). A negative feedback control of breathing based on
the level of metabolically produced CO, appears to be of little consequence in controlling breathing in such fishes. However, in stagnant
tropical swamps, the CO, levels can build up high enough. to adversely
399
Fig. 17. Continuous tracings of blood velocity in the ventral aorta ( V A ) of
Electrophorus. Each air breath shows as a distinct larger excursion ( B ) . ( a ) shows
the very prompt change in the rate of air breathing following the first breath of air
after hypoxic breathing. ( b ) Inhalation of hyperoxic air immediately prolongs the
interval between air breaths. From Johansen et al. (1968).
breathing is governed by changes of oxygen tension in the air-breathing
organ. The situation corresponds to conditions in higher vertebrates
where oxygen inhalation is effective in removing the tonic activity of
chemoreceptor cells stimulated by the normally prevailing levels of Po,.
Hyperoxic breathing also reduces the rate of air breathing in Protopterus
and other air-breathing fishes. Species depending largely on aquatic
breathing show a depression of branchial breathing in hyperoxic water
or following inhalation of hyperoxic air.
2. CHANGES OF ENVIRONMENTAL C 0 2 TENSIONS
Internal CO, tensions are very low among purely water-breathing
fishes living in well-aerated water (Lenfant and Johansen, 1966; Piiper
and Schumann, 1967). A negative feedback control of breathing based on
the level of metabolically produced CO, appears to be of little consequence in controlling breathing in such fishes. However, in stagnant
tropical swamps, the CO, levels can build up high enough. to adversely