8. THE REGULATION OF BREATHING
329
the animal’s oxygen consumption (respiratory dependence). Activity is
usually included in the concept of respiratory dependence thus increasing its ecological significance by giving some insight into the equilibrium
between environment and active fish. Determinations can be made over
the whole range of activities shown by the animal, but for comparative
purposes the maximally active and the standard (resting) level are most
useful (Fig. 16). Fry (1947, 1957) pointed out that a decline in oxygen
consumption of a maximally active fish would indicate a similar decline
in activity if the animal were to remain in oxygen balance. This would
be a progressive effect from the “incipient limiting level” at which reduced oxygen concentration just produced a change to the “level of no
excess activity” where oxygen consumption equaled the standard rate.
In the case of a resting animal, independence should be maintained
at a steady level over a wider range of oxygen concentrations, and at
lower levels the consumption might be expected to go up as the work of
ventilation increased. Fry (1947) suggested that the cost of ventilation
could continue to rise until the standard oxygen consumption equaled
the reduced active rate.
It has been widely accepted that limitations in the gas exchange
system are largely responsible for the appearance of respiratory dependence. There are, however, a number of other factors which will complicate a possible relationship of this type. Rates of oxygen consumption,
both at the active and standard levels, are different for different species
of fish. Suggestions have been made that the species having the highest
rates are also those which show dependence in the highest concentrations
of oxygen (Fry, 1957; Basu, 1959) so that the onset of dependence is
related to the ability of the fish to be active. Determinations of active
oxygen consumption made for any particular species by different authors
(see, for example, Job, 1955; Basu, 1959; Saunders, 1962; Beamish,
1964~) show considerable variation, and it is difficult to substantiate
this suggestion, reasonable though it seems. The standardization of
activity levels in the experimental animals constitutes a major difficulty
in these determinations. Another difficulty is that fish such as the dragonet, Callionymus, and the toadfish, Opsaunus, which are inactive,
bottom-living forms, show dependence at oxygen values only just below
air saturation (Hall, 1929; Hughes and Umezawa, 1968b).
Anaerobiosis and acclimation to different oxygen tensions may also
affect respiratory dependence curves. Beamish ( 1964c) demonstrated
that at very low levels of oxygen a fish’s standard rate of oxygen consumption increased as predicted, but never to the active level (Fig. 16).
Furthermore, the increase was usually less in animals acclimated to
the low tensions concerned than in those acclimated to air saturation
329
the animal’s oxygen consumption (respiratory dependence). Activity is
usually included in the concept of respiratory dependence thus increasing its ecological significance by giving some insight into the equilibrium
between environment and active fish. Determinations can be made over
the whole range of activities shown by the animal, but for comparative
purposes the maximally active and the standard (resting) level are most
useful (Fig. 16). Fry (1947, 1957) pointed out that a decline in oxygen
consumption of a maximally active fish would indicate a similar decline
in activity if the animal were to remain in oxygen balance. This would
be a progressive effect from the “incipient limiting level” at which reduced oxygen concentration just produced a change to the “level of no
excess activity” where oxygen consumption equaled the standard rate.
In the case of a resting animal, independence should be maintained
at a steady level over a wider range of oxygen concentrations, and at
lower levels the consumption might be expected to go up as the work of
ventilation increased. Fry (1947) suggested that the cost of ventilation
could continue to rise until the standard oxygen consumption equaled
the reduced active rate.
It has been widely accepted that limitations in the gas exchange
system are largely responsible for the appearance of respiratory dependence. There are, however, a number of other factors which will complicate a possible relationship of this type. Rates of oxygen consumption,
both at the active and standard levels, are different for different species
of fish. Suggestions have been made that the species having the highest
rates are also those which show dependence in the highest concentrations
of oxygen (Fry, 1957; Basu, 1959) so that the onset of dependence is
related to the ability of the fish to be active. Determinations of active
oxygen consumption made for any particular species by different authors
(see, for example, Job, 1955; Basu, 1959; Saunders, 1962; Beamish,
1964~) show considerable variation, and it is difficult to substantiate
this suggestion, reasonable though it seems. The standardization of
activity levels in the experimental animals constitutes a major difficulty
in these determinations. Another difficulty is that fish such as the dragonet, Callionymus, and the toadfish, Opsaunus, which are inactive,
bottom-living forms, show dependence at oxygen values only just below
air saturation (Hall, 1929; Hughes and Umezawa, 1968b).
Anaerobiosis and acclimation to different oxygen tensions may also
affect respiratory dependence curves. Beamish ( 1964c) demonstrated
that at very low levels of oxygen a fish’s standard rate of oxygen consumption increased as predicted, but never to the active level (Fig. 16).
Furthermore, the increase was usually less in animals acclimated to
the low tensions concerned than in those acclimated to air saturation