migrations from the sea could involve the endocrine system, but I should honestly
state at the outset that there is no information about this. Sharks and rays regularly
move into brackish estuarine waters but the limiting dilution usually seems to be
equivalent to about 50% sea-water (see PRICE, 1967). Attempts to adapt marine
sharks and rays to lower concentrations in laboratory aquaria have not been successful (SMITH, 1936; PRICE and CREASER, 1967). PRICE (1967) measured the plasma
concentrations of skates, Raja eglanteria , caught in different parts of Delaware and
Chesapeake bays where the salinity varies from 1.8% to 3.3%. He found little difference in the concentration of electrolytes in the blood of these skates and none
which could be correlated with the salinity of the water where the fish were caught.
In the laboratory (PRICE, 1967) adaptation to such salinities was found to take place
slowly over a period of two days, so that such fish could conceivably make rapid
excursions into less saline water and return without suffering any marked changes
in the concentrations of their body fluids.
When chondrichthyeans like the sawfish, Pristis microdon, live for prolonged
periods in fresh water, the concentration of their body fluids is reduced by about
50% due to loss of both urea and salt (SMITH, 1936). Such fish produce a far more
copious urine than they do in sea-water (Table 7.7) and this is hypoosmotic to
their plasma and contains little salt . A rapid transfer from sea-water to fresh water
would be expected to produce dramatic changes in hydration as these beasts are
highly hyperosmotic to fresh water and would be expected to accumulate it at a
far higher rate than adapted fish . PRICE (1967) has suggested that in such circumstances the increased accumulation of water would lead to an elevated GFR that
exceeds the capacity of the renal tubules to absorb 'certain significant metabolities'
(and sodium?) leading to the death of the fish.
SMITH (1936) suggested that the movements of sharks and rays between fresh
and salt water may be a gradual process during which excretion or accumulation
of urea and salts could slowly take place. This may be self regulating, thus when
the fish move from the sea into more dilute solutions they may accumulate water
which results in an elevated GFR and produces an increased loss of urea. Additional
leakage of urea and salts across the gills would also be expected.
Apart from the problem of hyperhydration in fresh water it is also possible
that sharks and rays suffer an excessive loss of sodium chloride, as observed in stenohaline marine teleosts that are placed in such media. This could occur in the urine
as well as across the gill surfaces. The rectal gland regresses in freshwater sharks
and presumably its secretions follow suit (OGURI, 1964). It should be simple to
measure the overall sodium balance on such occasions. When chondrichthyeans
live in fresh water they may be expected to face the additional problem of maintaining a positive salt balance, which in teleosts is assisted by an active uptake of
sodium chloride from the bathing media. This process has not been demonstrated
in chondrichthyeans but as they are carnivorous they probably obtain adequate
salt from their food.
251
state at the outset that there is no information about this. Sharks and rays regularly
move into brackish estuarine waters but the limiting dilution usually seems to be
equivalent to about 50% sea-water (see PRICE, 1967). Attempts to adapt marine
sharks and rays to lower concentrations in laboratory aquaria have not been successful (SMITH, 1936; PRICE and CREASER, 1967). PRICE (1967) measured the plasma
concentrations of skates, Raja eglanteria , caught in different parts of Delaware and
Chesapeake bays where the salinity varies from 1.8% to 3.3%. He found little difference in the concentration of electrolytes in the blood of these skates and none
which could be correlated with the salinity of the water where the fish were caught.
In the laboratory (PRICE, 1967) adaptation to such salinities was found to take place
slowly over a period of two days, so that such fish could conceivably make rapid
excursions into less saline water and return without suffering any marked changes
in the concentrations of their body fluids.
When chondrichthyeans like the sawfish, Pristis microdon, live for prolonged
periods in fresh water, the concentration of their body fluids is reduced by about
50% due to loss of both urea and salt (SMITH, 1936). Such fish produce a far more
copious urine than they do in sea-water (Table 7.7) and this is hypoosmotic to
their plasma and contains little salt . A rapid transfer from sea-water to fresh water
would be expected to produce dramatic changes in hydration as these beasts are
highly hyperosmotic to fresh water and would be expected to accumulate it at a
far higher rate than adapted fish . PRICE (1967) has suggested that in such circumstances the increased accumulation of water would lead to an elevated GFR that
exceeds the capacity of the renal tubules to absorb 'certain significant metabolities'
(and sodium?) leading to the death of the fish.
SMITH (1936) suggested that the movements of sharks and rays between fresh
and salt water may be a gradual process during which excretion or accumulation
of urea and salts could slowly take place. This may be self regulating, thus when
the fish move from the sea into more dilute solutions they may accumulate water
which results in an elevated GFR and produces an increased loss of urea. Additional
leakage of urea and salts across the gills would also be expected.
Apart from the problem of hyperhydration in fresh water it is also possible
that sharks and rays suffer an excessive loss of sodium chloride, as observed in stenohaline marine teleosts that are placed in such media. This could occur in the urine
as well as across the gill surfaces. The rectal gland regresses in freshwater sharks
and presumably its secretions follow suit (OGURI, 1964). It should be simple to
measure the overall sodium balance on such occasions. When chondrichthyeans
live in fresh water they may be expected to face the additional problem of maintaining a positive salt balance, which in teleosts is assisted by an active uptake of
sodium chloride from the bathing media. This process has not been demonstrated
in chondrichthyeans but as they are carnivorous they probably obtain adequate
salt from their food.
251