The extent of the exchange diffusion of ions across the gills of the flounder,
and so the magnitude of the 'instantaneous' response of the gills to transfer to fresh
water, can be adjusted; it is less in fish previously adapted to low salinities than
to sea-water. Such an adaptation takes several days to develop fully or to reverse.
MOTAIS et al., 1966) have suggested that a 'carrier' mediates the exchange diffusion
and the processes of such 'long term' adaptation involve changes in the amounts
of such a 'carrier', or alternatively the synthesis of a substance which inhibits its
action.
The exchange diffusion of sodium and chloride across the gills of fish can be
considered as an adaptation to euryhalinity (MOTAIS et al., 1966). In sea-water the
outflux of sodium normally exceeds the influx, the difference being the result of
the activity of an active transport mechanism for these ions. However, this latter
process is accompanied by an unavoidable 'leakiness' of the gills to passive diffusion
of these ions, the magnitude of which is 5 to 10 times greater than the active transport. In most stenohaline fish this passive diffusion, in either direction, proceeds
in a random manner. However in other fish this may be an 'organized leakiness'
that results in a direct exchange of one ion moving in one direction, for another
moving in the opposite way. When present, the advantage of this to fish in marine
environments is not clear though it is possible that such a linkage, by its increased
efficiency, may conserve some energy that would otherwise be used for active transport of these ions. When such marine fish are transferred to fresh water, however,
they fare better than fish lacking such a linkage, as they possess a mechanism that
immediately, and automatically, reduces the sodium loss from the gills. This, followed by the 'delayed' reduction in gill permeability, seals off the gills to diffusional
salt losses and allows the fish to live in fresh water. Teleost fish are thought to have
originated in sea-water and the appearance of such a system for controlling the
permeability of the gills may well have arisen, and been utilized, as a pre-adaptation
for migration into fresh water.
The lability of these processes of ion transfer across the gills indicates possibilities for their control and possibly the intervention of the endocrines. Treatment
of the flounder with an inhibitor of RNA depolymerase, actinomycin D, results
in a reduction of the active extrusion of sodium from the gills of fish in sea-water,
so that they only survive a short time (MAETZ et al., 1969a). If fish, so treated, are
placed in fresh water they show no evidence or exchange diffusion, so that there
is a rapid and fatal loss of sodium from the gills as seen normally in Serranus scriba.
In contrast, when flounders in fresh water are treated with actinomycin D they live
for at least a week, the process of active sodium uptake, in contrast to active sodium
extrusion, being unaffected by the antibiotic. These results suggest (see MAETZ,
1970) that a nuclear controlled synthesis of proteins is involved in the regulation
of such processes. It will be recalled that MOTAIS (1970) has shown that the synthesis of a Na-K activated ATPase present in the gills of eels in sea-water can be
prevented by actinomycin D, while the enzyme present in freshwater fish is unaffected.
fJ) Water. Like the movements of salts, the net transfers of water in euryhaline fishes
in fresh water and sea-water are opposite in direction. When in the sea such teleost
fish lose water by osmosis, while in fresh water they gain water. Comparisons of
237
and so the magnitude of the 'instantaneous' response of the gills to transfer to fresh
water, can be adjusted; it is less in fish previously adapted to low salinities than
to sea-water. Such an adaptation takes several days to develop fully or to reverse.
MOTAIS et al., 1966) have suggested that a 'carrier' mediates the exchange diffusion
and the processes of such 'long term' adaptation involve changes in the amounts
of such a 'carrier', or alternatively the synthesis of a substance which inhibits its
action.
The exchange diffusion of sodium and chloride across the gills of fish can be
considered as an adaptation to euryhalinity (MOTAIS et al., 1966). In sea-water the
outflux of sodium normally exceeds the influx, the difference being the result of
the activity of an active transport mechanism for these ions. However, this latter
process is accompanied by an unavoidable 'leakiness' of the gills to passive diffusion
of these ions, the magnitude of which is 5 to 10 times greater than the active transport. In most stenohaline fish this passive diffusion, in either direction, proceeds
in a random manner. However in other fish this may be an 'organized leakiness'
that results in a direct exchange of one ion moving in one direction, for another
moving in the opposite way. When present, the advantage of this to fish in marine
environments is not clear though it is possible that such a linkage, by its increased
efficiency, may conserve some energy that would otherwise be used for active transport of these ions. When such marine fish are transferred to fresh water, however,
they fare better than fish lacking such a linkage, as they possess a mechanism that
immediately, and automatically, reduces the sodium loss from the gills. This, followed by the 'delayed' reduction in gill permeability, seals off the gills to diffusional
salt losses and allows the fish to live in fresh water. Teleost fish are thought to have
originated in sea-water and the appearance of such a system for controlling the
permeability of the gills may well have arisen, and been utilized, as a pre-adaptation
for migration into fresh water.
The lability of these processes of ion transfer across the gills indicates possibilities for their control and possibly the intervention of the endocrines. Treatment
of the flounder with an inhibitor of RNA depolymerase, actinomycin D, results
in a reduction of the active extrusion of sodium from the gills of fish in sea-water,
so that they only survive a short time (MAETZ et al., 1969a). If fish, so treated, are
placed in fresh water they show no evidence or exchange diffusion, so that there
is a rapid and fatal loss of sodium from the gills as seen normally in Serranus scriba.
In contrast, when flounders in fresh water are treated with actinomycin D they live
for at least a week, the process of active sodium uptake, in contrast to active sodium
extrusion, being unaffected by the antibiotic. These results suggest (see MAETZ,
1970) that a nuclear controlled synthesis of proteins is involved in the regulation
of such processes. It will be recalled that MOTAIS (1970) has shown that the synthesis of a Na-K activated ATPase present in the gills of eels in sea-water can be
prevented by actinomycin D, while the enzyme present in freshwater fish is unaffected.
fJ) Water. Like the movements of salts, the net transfers of water in euryhaline fishes
in fresh water and sea-water are opposite in direction. When in the sea such teleost
fish lose water by osmosis, while in fresh water they gain water. Comparisons of
237