7. GAS EXMANGE IN FISH
277
Krogh (1904) showed that eels have a cutaneous oxygen uptake in air
that is up to 60% of the total oxygen uptake in water at the same temperature. His eels were prevented from ventilating their gills. Eels normally
pump air over their gills when out of water. Berg and Steen (1965) have
shown that the oxygen uptake in air is reduced to between 40 and 63%
of that in water and from 31 to 47% of the oxygen enters the blood via
the gills when the fish is in air. At higher temperatures there is a larger
reduction in oxygen uptake when the animal is in air compared with
the oxygen uptake in water and a smaller percentage of the total oxygen
uptake enters the body via the gills. Berg and Steen observed that the
longitudinal fin becomes more red when the eel is in air indicating a
vasodilation of skin capillaries. In water, 85-!N% of the oxygen uptake
occurs across the gills. The eel has a well-developed cutaneous circulation
and often comes out on land, to steal peas, according to Day (1880)
who also reports a case of an eel “which lived upwards of 31 years in a
well and was then choked by a frog that was larger than it could
swallow” (Day, 1880, p. 243).
Jakubowski (1963) has carried out an extensive study of the structure
and vascularization of the skin of a variety of teleosts. He reports values
of between 0.5 and 1.5 cmg/g body weight for the surface area of blood
vessels in the skin of a variety of teleosts, as well as ratios for the surface
area of gill to skin capillaries, ranging from 3 : l in the pond loach to
1O:l in the carp and flounder. Although the area of capillaries in the
skin is large, it is important to note that the capillaries are covered by
an epidermis, ranging in thickness from 31-38 p in the flounder to 263
and 338 p in the eel and pond loach, respectively. The differences in
thickness largely result from differences in size of mucous and goblet
cells within the epidermis. The number of mucous cells and the
thickness of the skin are probably controlled by prolactin (Ball, 1969).
By comparison the gill epithelium is usually between 1 and 5 p in
thickness.
Jakubowski could not find any relationship between the extent of
the skin vascularization, thickness of the epidermis, and the amount
of oxygen uptake occurring through the skin. Gas exchange across the
skin will not only depend on the extent of skin vascularization but also
on the gas gradients, diffusion distances, and the blood flow and number
of capillaries open in the skin. Nothing is known about many of the
parameters which affect gas exchange across the skin.
There appears to be considerable variability in the extent to which
fish respire through their skin. In many fish with thick, poorly vascularized skins, cutaneous respiration is probably not important. Obvious
exceptions to this are larval fishes in which the gills have not yet de-
277
Krogh (1904) showed that eels have a cutaneous oxygen uptake in air
that is up to 60% of the total oxygen uptake in water at the same temperature. His eels were prevented from ventilating their gills. Eels normally
pump air over their gills when out of water. Berg and Steen (1965) have
shown that the oxygen uptake in air is reduced to between 40 and 63%
of that in water and from 31 to 47% of the oxygen enters the blood via
the gills when the fish is in air. At higher temperatures there is a larger
reduction in oxygen uptake when the animal is in air compared with
the oxygen uptake in water and a smaller percentage of the total oxygen
uptake enters the body via the gills. Berg and Steen observed that the
longitudinal fin becomes more red when the eel is in air indicating a
vasodilation of skin capillaries. In water, 85-!N% of the oxygen uptake
occurs across the gills. The eel has a well-developed cutaneous circulation
and often comes out on land, to steal peas, according to Day (1880)
who also reports a case of an eel “which lived upwards of 31 years in a
well and was then choked by a frog that was larger than it could
swallow” (Day, 1880, p. 243).
Jakubowski (1963) has carried out an extensive study of the structure
and vascularization of the skin of a variety of teleosts. He reports values
of between 0.5 and 1.5 cmg/g body weight for the surface area of blood
vessels in the skin of a variety of teleosts, as well as ratios for the surface
area of gill to skin capillaries, ranging from 3 : l in the pond loach to
1O:l in the carp and flounder. Although the area of capillaries in the
skin is large, it is important to note that the capillaries are covered by
an epidermis, ranging in thickness from 31-38 p in the flounder to 263
and 338 p in the eel and pond loach, respectively. The differences in
thickness largely result from differences in size of mucous and goblet
cells within the epidermis. The number of mucous cells and the
thickness of the skin are probably controlled by prolactin (Ball, 1969).
By comparison the gill epithelium is usually between 1 and 5 p in
thickness.
Jakubowski could not find any relationship between the extent of
the skin vascularization, thickness of the epidermis, and the amount
of oxygen uptake occurring through the skin. Gas exchange across the
skin will not only depend on the extent of skin vascularization but also
on the gas gradients, diffusion distances, and the blood flow and number
of capillaries open in the skin. Nothing is known about many of the
parameters which affect gas exchange across the skin.
There appears to be considerable variability in the extent to which
fish respire through their skin. In many fish with thick, poorly vascularized skins, cutaneous respiration is probably not important. Obvious
exceptions to this are larval fishes in which the gills have not yet de-