The sequence of catabolic substrate oxidation and enthalpy balance of developing embryos and yolk-sac larvae of turbot (Scophthalmus maximus L)

A model of catabolic metabolism during the endogenous developmental phase of turbot (Scophthalmus maximus L.), a species whose pelagic eggs contain a single oil globule, is presented. Yolk-dependent routine rates of oxygen consumption and ammonia excretion and flux of ammonium ions were related stoichiometrically to quantitative changes in the contents of free glucose, glycogen, lactate, free amino acids, proteins, and lipid classes with their associated fatty acids, in order to determine the race and sequence of catabolic substrate oxidation chat occurs with development. These data were further related to the changes in caloric contents of the eggs and larvae in order to derive an enthalpy balance equation fur the period of yolk dependence. The stoichiometric relation of the oxygen consumption and ammonia production with the quantitative changes in substrates indicated that, following the first 18-19 hr of glycogen dependence, free amino acids (84%) together with a small amount of phosphatidyl choline (9%) and later wax esters (5%) comprised the metabolic fuels of embryonic development. Following hatch (day 4.4 post fertilisation), wax esters (33%) and triacylglycerols (25%) were initially catabolised with the remaining free amino acids (10%). Upon exhaustion of the free amino acids on day 6 post fertilisation, body proteins (32%) were recruited and catabolised together with wax esters and triacylglycerols. Thus, the: catabolic metabolism of endogenously feeding turbot embryos and larvae were equally fuelled by amino acids (50%, with similar amounts being supplied from both the free and protein bound pools) and lipids (50%, mainly of neutral origin), while carbohydrates (predominantly glycogen) were only quantitatively important during the early cellular division stages. We argue that this pattern of catabolic substrate oxidation is also generally applicable to other marine fishes which spawn eggs containing oil globules. For an enthalpy balance equation of the form P = C + R + E, a turbot larva at 15 degrees C, utilised 60% of yolk and oil globule enthalpy (C) for growth (P), 37% was dissipated due re, metabolism (R) while only 3% was lose via excretion of nitrogenous end products (E).