take up and extrude sodium from their gills. When the y are transferred from seawater to fresh water they respond differently, for while the rate of sodium loss
from the gills immediately decreases by about 90% in the flounder, it only declines
by 40% in Serranus. This stenohaline fish continues to lose salt at this rate through
its gills and dies from salt depletion in about 3 hours. The euryhaline flounder,
on the other hand, can rapidly 'shut down' its branchial permeability to sodium
and chloride to a level which the stenohaline species cannot do.
In the flounder, the rapid exchange of sodium across the gills is directly dependent on the external salt concentration and not on the osmotic concentration of the
medium. If the salt concentration in the external solution is decreased, but its osmotic concentration is maintained by the addition of mannitol, the decline in
sodium exchange is the same as in flounders placed in the ordinary dilute salt solution (without mannitol). In contrast, the sodium exchange in Serranus scriba in
such sodium plus mannitol solutions is similar to that in ordinary sea-water, the
40% decline that is observed in fresh water being directly related to the change
in the osmotic concentration of the solution. When flounder are transferred from
sea-water to various dilutions of this fluid, the rate of sodium extrusion and accumulation across the gills is closely related to the decline of the extern al sodium concentration, proportionately until, in fresh water the loss is negligibl e (Fig. 7.7). The
uptake and output of sodium and chloride are linked to the concentration of sodium
(and chloride) in the external media. This is reminiscent of the process of 'exchange
diffusion ', which USSlNG (1947) suggested accompanies the rapid exchanges of
sodium observed in frog muscle; one ion moving int o the fish exchanges with one
that is leaving. This process is not directly dependent on the expenditure of energy.
Such 'exchange diffusion' accounts for 85% of the sodium transferred across the
gills of the sea-water adapted flounder but only 3% in Serran us. It is notable that
the relative magnitude of this process decreases in flounders adapted to solutions
with a lower sodium chloride concentration, until in freshwater adapted fish it cannot be seen (see Fig. 7.7b) .
An additional 'delayed regulation' of branchial permeability to sodium and
chloride is obs erved to commence about 30 min after the flounder is transferred
from sea-water to fresh ' water. The onset of thi s secondary pha se can be seen in
Fig . 7.8 which shows the changes which occur after eels and killifish are transferred
from sea-water to fresh water. This reduction in the permeability of th e gills almost
completely restricts their permeability to the outflux of sodium. The nature of this
change is at present unknown. MOTAlS and his collaborators examined four more
euryhaline teleosts and six stenohaline ones. A significant exchange diffusion effect
was seen in the stenohaline marine fish, Scorpaena porcus, while a euryh aline fish,
Fundulus heteroclitus, apparently lacked such an effect. The latter species, nevertheless , still reduced the permeability of its gills in fresh water but this was related
to the decreased osmotic pressure (see also POTTS and EVANS, 1967). The int ertidal
and partly euryhaline stachaeid blenny, Xiphister atropurpureus, also exhibits exchange diffusion (EVANS, 1967b). The pre sence or absence of branchial ion exchange diffusion is not strictly confined to, or present in, all the euryhaline fishes
examined. The secondary 'delayed' phas e, whereby gill permeability is gradually
restricted in fresh water, was however, seen in all euryhaline fish, but never in a
stenohaline one.
236
from the gills immediately decreases by about 90% in the flounder, it only declines
by 40% in Serranus. This stenohaline fish continues to lose salt at this rate through
its gills and dies from salt depletion in about 3 hours. The euryhaline flounder,
on the other hand, can rapidly 'shut down' its branchial permeability to sodium
and chloride to a level which the stenohaline species cannot do.
In the flounder, the rapid exchange of sodium across the gills is directly dependent on the external salt concentration and not on the osmotic concentration of the
medium. If the salt concentration in the external solution is decreased, but its osmotic concentration is maintained by the addition of mannitol, the decline in
sodium exchange is the same as in flounders placed in the ordinary dilute salt solution (without mannitol). In contrast, the sodium exchange in Serranus scriba in
such sodium plus mannitol solutions is similar to that in ordinary sea-water, the
40% decline that is observed in fresh water being directly related to the change
in the osmotic concentration of the solution. When flounder are transferred from
sea-water to various dilutions of this fluid, the rate of sodium extrusion and accumulation across the gills is closely related to the decline of the extern al sodium concentration, proportionately until, in fresh water the loss is negligibl e (Fig. 7.7). The
uptake and output of sodium and chloride are linked to the concentration of sodium
(and chloride) in the external media. This is reminiscent of the process of 'exchange
diffusion ', which USSlNG (1947) suggested accompanies the rapid exchanges of
sodium observed in frog muscle; one ion moving int o the fish exchanges with one
that is leaving. This process is not directly dependent on the expenditure of energy.
Such 'exchange diffusion' accounts for 85% of the sodium transferred across the
gills of the sea-water adapted flounder but only 3% in Serran us. It is notable that
the relative magnitude of this process decreases in flounders adapted to solutions
with a lower sodium chloride concentration, until in freshwater adapted fish it cannot be seen (see Fig. 7.7b) .
An additional 'delayed regulation' of branchial permeability to sodium and
chloride is obs erved to commence about 30 min after the flounder is transferred
from sea-water to fresh ' water. The onset of thi s secondary pha se can be seen in
Fig . 7.8 which shows the changes which occur after eels and killifish are transferred
from sea-water to fresh water. This reduction in the permeability of th e gills almost
completely restricts their permeability to the outflux of sodium. The nature of this
change is at present unknown. MOTAlS and his collaborators examined four more
euryhaline teleosts and six stenohaline ones. A significant exchange diffusion effect
was seen in the stenohaline marine fish, Scorpaena porcus, while a euryh aline fish,
Fundulus heteroclitus, apparently lacked such an effect. The latter species, nevertheless , still reduced the permeability of its gills in fresh water but this was related
to the decreased osmotic pressure (see also POTTS and EVANS, 1967). The int ertidal
and partly euryhaline stachaeid blenny, Xiphister atropurpureus, also exhibits exchange diffusion (EVANS, 1967b). The pre sence or absence of branchial ion exchange diffusion is not strictly confined to, or present in, all the euryhaline fishes
examined. The secondary 'delayed' phas e, whereby gill permeability is gradually
restricted in fresh water, was however, seen in all euryhaline fish, but never in a
stenohaline one.
236