Although the values of the O:N ratio obtained in the present study indicated that older
postlarvae from all treatments used a mixture of lipid and protein as energy substrate, the
proportion of different metabolic substrates used as energy source varied with diet. In
postlarvae fed on MCD, the O:N ratios (38 to 51) were related to equal catabolism of fat
and protein (Mayzaud and Conover, 1988), whereas in the other treatments, higher O:N
ratios (near 100 or higher) indicated greater influence of lipid catabolism.
The increase in ammonia excretion in postlarvae fed on MCD that produced low O:N
ratios could be, as is mentioned above a result of an increase of amino acid deamination
produced by an unbalanced diet. Mayzaud and Conover (1988) showed that if the
protein in the diet is unbalanced, the complete supply of amino acids cannot be used for
synthesis, so amino acids will be deaminated and the nitrogen excreted without using the
amino acids for tissue growth, what is in agreement with lower growth rate found in
postlarvae fed on MCD.