260
D. J. RANDALL
tory surfaces may be to prevent excessive diffusion of water whilst
allowing adequate diffusion of gases” (page 108, “The Development of
the Lung” de Reuck and Porter, eds., 1967). Nothing is known about the
rates of diffusion of water, ions, or gases through the mucus produced
by the skin and gills of fishes.
2. WATER FLOW OVER THE GILLS
A unidirectional flow of water over the gills is produced by the
action of muscular pumps (see chapter by Shelton, this volume). The
pioneering work of van Dam (1938) indicated that between 60 and 80%
(percent utilization) of the oxygen in the water passing over the gills
is utilized by the fish. The more recent studies of Saunders (1961, 1962),
Garey ( 1967), Holeton and Randall ( 1967b), Piiper and Schumann
(1967), and Stevens and Randall (1967b) have shown that percent
utilization is variable between fish and in the same fish under different
conditions.
The cost of ventilating the gills with such a dense medium is generally
considered to be high (Hughes and Shelton, 1962) and may be as much
as 30% of the total oxygen uptake (Schumann and Piiper, 1966). Garey
(1967), however, has pointed out that the high density of water is offset
by a low resistance to flow through the gills and has calculated that the
cost of breathing in carp is only 2% of the total oxygen uptake, a value
similar to that for man (see also chapter by Shelton, this volume).
There are probably differences in the cost of breathing among fish.
Fish, like carp and tench, in still water, must pump water over their
gills. These fish probably have the highest cost of ventilation, and so
they tend to have low ventilation volumes (ventilation volume is the
volume of water passing over the gills per unit of time) and high percent
utilization of oxygen in the water (Garey, 1967; Baumgarten and Randall,
1967). Fish swimming rapidly or fish that can maintain position in fast
flowing water need only open their mouths to ventilate their gills.
Gill ventilation in this case is related to swimming speed, water flow
rate, resistance to water flow through the gills, and the gape of the mouth
and operculum (Muir and Kendall, 1968). At high swimming speeds
the problem may be to reduce water flow over the gills rather than maintain an adequate rate of ventilation. In these fish the cost of ventilating
the gills is probably small and related to the increased resistance to
forward motion produced by passing water over the gills. Salmonids
can maintain position in fast flowing water using only frictional forces
between the fish and the bottom. As water flow is increased the amplitude
of breathing decreases; residual breathing movements are always present,
D. J. RANDALL
tory surfaces may be to prevent excessive diffusion of water whilst
allowing adequate diffusion of gases” (page 108, “The Development of
the Lung” de Reuck and Porter, eds., 1967). Nothing is known about the
rates of diffusion of water, ions, or gases through the mucus produced
by the skin and gills of fishes.
2. WATER FLOW OVER THE GILLS
A unidirectional flow of water over the gills is produced by the
action of muscular pumps (see chapter by Shelton, this volume). The
pioneering work of van Dam (1938) indicated that between 60 and 80%
(percent utilization) of the oxygen in the water passing over the gills
is utilized by the fish. The more recent studies of Saunders (1961, 1962),
Garey ( 1967), Holeton and Randall ( 1967b), Piiper and Schumann
(1967), and Stevens and Randall (1967b) have shown that percent
utilization is variable between fish and in the same fish under different
conditions.
The cost of ventilating the gills with such a dense medium is generally
considered to be high (Hughes and Shelton, 1962) and may be as much
as 30% of the total oxygen uptake (Schumann and Piiper, 1966). Garey
(1967), however, has pointed out that the high density of water is offset
by a low resistance to flow through the gills and has calculated that the
cost of breathing in carp is only 2% of the total oxygen uptake, a value
similar to that for man (see also chapter by Shelton, this volume).
There are probably differences in the cost of breathing among fish.
Fish, like carp and tench, in still water, must pump water over their
gills. These fish probably have the highest cost of ventilation, and so
they tend to have low ventilation volumes (ventilation volume is the
volume of water passing over the gills per unit of time) and high percent
utilization of oxygen in the water (Garey, 1967; Baumgarten and Randall,
1967). Fish swimming rapidly or fish that can maintain position in fast
flowing water need only open their mouths to ventilate their gills.
Gill ventilation in this case is related to swimming speed, water flow
rate, resistance to water flow through the gills, and the gape of the mouth
and operculum (Muir and Kendall, 1968). At high swimming speeds
the problem may be to reduce water flow over the gills rather than maintain an adequate rate of ventilation. In these fish the cost of ventilating
the gills is probably small and related to the increased resistance to
forward motion produced by passing water over the gills. Salmonids
can maintain position in fast flowing water using only frictional forces
between the fish and the bottom. As water flow is increased the amplitude
of breathing decreases; residual breathing movements are always present,