the permeability of such fish in different environmental solutions have recently been
undertaken with the aid of tritiated water. POITS et al. (1967) found that in Tilapia
mossambica the turnover of body water amounted in each hour to 115% of the
total present in the body when the fish are in fresh water, and this declined to 84%
in sea-water. The eel and the flounder in fresh water exchange an amount equivalent, respectively, to 42% and 31% of their total body water in an hour (MOTAIS
et al., 1969). When these fish are transfered to sea-water this fraction decreases
to 29% in the eel and 20% in the flounder. It was further found that nearly all
of this water exchange primarily takes place through the gill. It is replaced by drinking when the fish are in sea-water while it is excreted in the urine when they are
in fresh water. The osmotic gradients between the fish and its bathing fluids are
greater when they are in sea-water than in fresh water, but, despite this, the transfer
of water is less in the former medium. The permeability of the gills is thus apparently least when the fish are in sea-water. This, as discussed earlier, could be due
to a rectifying property of the gills whereby net transfers of water take place less
readily outwards than inwards. Alternatively there may be an adjustable mechanism that can reduce the branchial permeability to water when the fish are in the
sea. LAHLOU and GIORDAN (1970) have shown that hypophysectomized goldfish
have a reduced permeability to water and that both prolactin and cortisol can restore this to normal. This may reflect an endocrine mechanism for controlling the
permeability of fish to water.
The water that is accumulated by teleost fish in fresh water is excreted by the
kidney, while the losses in sea-water are replaced by drinking. When euryhaline
fish move between fresh water and sea-water these processes are adjusted according
to their physiological needs. In the flounder (LAHLOU, 1967) and eel (CHESTER
JONES el al., 1969a) the flow of urine slowly declines when they are transferred
from fresh water to sea-water and this is mainly due to a reduction in the GFR.
The blood pressure of the eel also drops in sea-water but the immediate reason for
the change in kidney function is not yet known. The rate of drinking changes in
media of different osmotic concentrations. Tilapia mossambica in fresh water only
drink fluid equivalent to 0.26% of their body weight each hour, but this increases
to 1.11% when they are in sea-water (POITS et al., 1967). Similarly eels in fresh
water drink fluid equivalent to 0.14% of their body weight in an hour, while in
sea-water they consume 0.33 % in this time (MAETZ and SKADHAUGE, 1968). The
efficiency of the mechanism for the absorption of the sodium (and consequently
water) from the gut of eels adapted to sea-water is greater than those in fresh water
(see page 213) and this may be related to the activity of adrenocorticosteroids. Factors that control the rate of drinking in fish are unknown, perhaps they just feel
thirsty.
b) Role of the Endocrines
The movement of fish between fresh water and sea water is accompanied by a number of endocrine changes but the relationship of these to the altered osmoregulatory
requirements is not always clear. When salmon and trout migrate up rivers in order
to breed, the levels of 17-hydroxycorticosteroids in their plasma increases (see for
instance ROBERTSON et al., 1961). Such increases in the circulating corticosteroids
238
undertaken with the aid of tritiated water. POITS et al. (1967) found that in Tilapia
mossambica the turnover of body water amounted in each hour to 115% of the
total present in the body when the fish are in fresh water, and this declined to 84%
in sea-water. The eel and the flounder in fresh water exchange an amount equivalent, respectively, to 42% and 31% of their total body water in an hour (MOTAIS
et al., 1969). When these fish are transfered to sea-water this fraction decreases
to 29% in the eel and 20% in the flounder. It was further found that nearly all
of this water exchange primarily takes place through the gill. It is replaced by drinking when the fish are in sea-water while it is excreted in the urine when they are
in fresh water. The osmotic gradients between the fish and its bathing fluids are
greater when they are in sea-water than in fresh water, but, despite this, the transfer
of water is less in the former medium. The permeability of the gills is thus apparently least when the fish are in sea-water. This, as discussed earlier, could be due
to a rectifying property of the gills whereby net transfers of water take place less
readily outwards than inwards. Alternatively there may be an adjustable mechanism that can reduce the branchial permeability to water when the fish are in the
sea. LAHLOU and GIORDAN (1970) have shown that hypophysectomized goldfish
have a reduced permeability to water and that both prolactin and cortisol can restore this to normal. This may reflect an endocrine mechanism for controlling the
permeability of fish to water.
The water that is accumulated by teleost fish in fresh water is excreted by the
kidney, while the losses in sea-water are replaced by drinking. When euryhaline
fish move between fresh water and sea-water these processes are adjusted according
to their physiological needs. In the flounder (LAHLOU, 1967) and eel (CHESTER
JONES el al., 1969a) the flow of urine slowly declines when they are transferred
from fresh water to sea-water and this is mainly due to a reduction in the GFR.
The blood pressure of the eel also drops in sea-water but the immediate reason for
the change in kidney function is not yet known. The rate of drinking changes in
media of different osmotic concentrations. Tilapia mossambica in fresh water only
drink fluid equivalent to 0.26% of their body weight each hour, but this increases
to 1.11% when they are in sea-water (POITS et al., 1967). Similarly eels in fresh
water drink fluid equivalent to 0.14% of their body weight in an hour, while in
sea-water they consume 0.33 % in this time (MAETZ and SKADHAUGE, 1968). The
efficiency of the mechanism for the absorption of the sodium (and consequently
water) from the gut of eels adapted to sea-water is greater than those in fresh water
(see page 213) and this may be related to the activity of adrenocorticosteroids. Factors that control the rate of drinking in fish are unknown, perhaps they just feel
thirsty.
b) Role of the Endocrines
The movement of fish between fresh water and sea water is accompanied by a number of endocrine changes but the relationship of these to the altered osmoregulatory
requirements is not always clear. When salmon and trout migrate up rivers in order
to breed, the levels of 17-hydroxycorticosteroids in their plasma increases (see for
instance ROBERTSON et al., 1961). Such increases in the circulating corticosteroids
238