fasting eel kept in fresh water is absorbed through the gut (MAETZ and SKADHAUGE,
1968). However, fish that are feeding actively will be expected to gain more salt ,
due to its absorption from the food . The magnitude of this varies with the nature
of the diet. In fresh water an herbivorous or detrital diet contains less sodium chloride than a carnivorous one. JORGENSEN and ROSENKILDE (1956b) calculated that
the expected accumulation of chloride by a goldfish from its normal diet would
be less than 25 % the quantity that is normally absorbed by the gills. Carnivorous
freshwater fish, like the pike, probably gain adequate salt from their diet.
4. Nitrogen Metabolism
In fishes the metabolic breakdown of amino acids may lead to the formation of
either ammonia or, by way of the ornithine cycle, to urea. The particular product
formed is closely related to the fishes' pattern of osmoregulation. Some additional
urea may be formed as the result of the breakdown of uric acid which is the endproduct of purine metabolism. This requires the presence of certain enzymes,
notably uricase. While this process has been shown to occur in fishes, including
the African lungfish (BROWN et al., 1966), it is probably not a major source of urea
(FORSTER and GOLDSTEIN, 1966) in ureotelic fishes.
When adequate water is available ammonia can be readily lost by passage across
the gills. In the trout, Salmo gairdneri, in fresh water, 60% of the fishes ' total nitrogen excretion takes place as ammonia, and less than 2 % of this is in the urine
(FROMM, 1963) . The balance is excreted across the gills, which ma y also be its principal site of formation (GOLDSTEIN and FOSTER, 1961). The loss of ammonia (as
NH 4 +) across the gills of freshwater fish ma y be coupled with the active uptake
of sodium (MAETZ and GARCIA ROMEU, 1964).
Some fishes can alter the relative quantities of ammonia and urea that the y form.
The African lungfish is enzymatically equipped to make either ammonia or urea,
but when it is in fresh water the former predominates (H. SMITH, 1930a). However,
if water is restricted, as when these fish aestivate, they only form urea. The Australian lungfish, Neoceratodus [orsteri, is normally ammoniotelic, though it also
possesses the ornithine-urea cycle enzymes, but these are only present at low levels
of activity (GOLDSTEIN, JANSSENS, and FORSTER, 1967).
Urea is relatively non-toxic and can be accumulated at high concentrations in
the bodies of many vertebrates. The chondrichthyean fish are ureotelic and hav e
utilized retention of this metabolite to maintain the osmotic concentration of their
body fluids hyperosmotic to the surrounding sea-water (see H . SMITH, 1936). Such
a strategem has been used on other occasions by distinctly different groups of vertebrates, including the marine crab-eating fro g and the coelacanth, Latimeria
(BROWN and BROWN, 1967 ; PICKFORD and GRANT, 1967). The likel y phyletic histo ry and distribution of the ornithine-urea cycle in vertebrates is shown in Fig.
7.4.
Trimethylamine oxide is another nitrogen compound that is found in the body
fluids of many fishes, especially in marine species. Its origin is uncertain and there
is little evidence to indicate that it is formed metabolically in the Chondrichthyes
(G OLDSTEIN, HARTMAN, and FORSTER, 1967). It has been suggested that it is ac227
1968). However, fish that are feeding actively will be expected to gain more salt ,
due to its absorption from the food . The magnitude of this varies with the nature
of the diet. In fresh water an herbivorous or detrital diet contains less sodium chloride than a carnivorous one. JORGENSEN and ROSENKILDE (1956b) calculated that
the expected accumulation of chloride by a goldfish from its normal diet would
be less than 25 % the quantity that is normally absorbed by the gills. Carnivorous
freshwater fish, like the pike, probably gain adequate salt from their diet.
4. Nitrogen Metabolism
In fishes the metabolic breakdown of amino acids may lead to the formation of
either ammonia or, by way of the ornithine cycle, to urea. The particular product
formed is closely related to the fishes' pattern of osmoregulation. Some additional
urea may be formed as the result of the breakdown of uric acid which is the endproduct of purine metabolism. This requires the presence of certain enzymes,
notably uricase. While this process has been shown to occur in fishes, including
the African lungfish (BROWN et al., 1966), it is probably not a major source of urea
(FORSTER and GOLDSTEIN, 1966) in ureotelic fishes.
When adequate water is available ammonia can be readily lost by passage across
the gills. In the trout, Salmo gairdneri, in fresh water, 60% of the fishes ' total nitrogen excretion takes place as ammonia, and less than 2 % of this is in the urine
(FROMM, 1963) . The balance is excreted across the gills, which ma y also be its principal site of formation (GOLDSTEIN and FOSTER, 1961). The loss of ammonia (as
NH 4 +) across the gills of freshwater fish ma y be coupled with the active uptake
of sodium (MAETZ and GARCIA ROMEU, 1964).
Some fishes can alter the relative quantities of ammonia and urea that the y form.
The African lungfish is enzymatically equipped to make either ammonia or urea,
but when it is in fresh water the former predominates (H. SMITH, 1930a). However,
if water is restricted, as when these fish aestivate, they only form urea. The Australian lungfish, Neoceratodus [orsteri, is normally ammoniotelic, though it also
possesses the ornithine-urea cycle enzymes, but these are only present at low levels
of activity (GOLDSTEIN, JANSSENS, and FORSTER, 1967).
Urea is relatively non-toxic and can be accumulated at high concentrations in
the bodies of many vertebrates. The chondrichthyean fish are ureotelic and hav e
utilized retention of this metabolite to maintain the osmotic concentration of their
body fluids hyperosmotic to the surrounding sea-water (see H . SMITH, 1936). Such
a strategem has been used on other occasions by distinctly different groups of vertebrates, including the marine crab-eating fro g and the coelacanth, Latimeria
(BROWN and BROWN, 1967 ; PICKFORD and GRANT, 1967). The likel y phyletic histo ry and distribution of the ornithine-urea cycle in vertebrates is shown in Fig.
7.4.
Trimethylamine oxide is another nitrogen compound that is found in the body
fluids of many fishes, especially in marine species. Its origin is uncertain and there
is little evidence to indicate that it is formed metabolically in the Chondrichthyes
(G OLDSTEIN, HARTMAN, and FORSTER, 1967). It has been suggested that it is ac227