Although plasma ammonia levels were two-fold higher in salmonids with the same
nitrogen intake, the quantitative excretion was the same as that in sea bass and sea
bream. In contrast, TAN excretion in turbot was about half the average TAN excretion
in the other fish whilst the plasma ammonia concentrations were of the same magnitude.
A possible hypothesis to explain this difference could be the presence of differences in
cellular permeability between the different fish groups. In salmonids, ammonia diffusion
occurs mainly through ionocyte cells of the gill (G. Boeuf, personal communication,
1995) while in marine fish, where it has not been precisely described, it may occur
through the ionocytes and respiratory cells. Moreover, marine teleosts have higher ionic
permeabilities than freshwater fish. In seawater, a higher pH (8.2 instead of 6.0 in
freshwater) in the presence of the high Naf level is considered to facilitate ammonia
excretion through specific carrier-mediated NH:/Na+ exchange (Wilson and Taylor,
1992; Wright et al., 1993) for strictly marine fish, better adapted to the marine
environment. Therefore, salmonids would need higher plasma levels to excrete the same
amount of TAN in the water.
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