THE SEQUENCE OF CATABOLIC SUBSTRATE OXIDATION AND ENTHALPY BALANCE OFDEVELOPING EMBRYOS AND YOLK-SAC LARVAE OF TURBOT (SCOPHTHALMUS-MAXIMUS L)

A model of catabolic metabolism during the endogenous developmental ph ase of turbot (Scophthalmus maximus L.), a species whose pelagic eggs contain a single oil globule, is presented. Yolk-dependent routine rat es of oxygen consumption and ammonia excretion and flux of ammonium io ns were related stoichiometrically to quantitative changes in the cont ents of free glucose, glycogen, lactate, free amino acids, proteins, a nd lipid classes with their associated fatty acids, in order to determ ine the race and sequence of catabolic substrate oxidation chat occurs with development. These data were further related to the changes in c aloric 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 qu antitative changes in substrates indicated that, following the first 1 8-19 hr of glycogen dependence, free amino acids (84%) together with a small amount of phosphatidyl choline (9%) and later wax esters (5%) c omprised 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 (1 0%). Upon exhaustion of the free amino acids on day 6 post fertilisati on, body proteins (32%) were recruited and catabolised together with w ax esters and triacylglycerols. Thus, the: catabolic metabolism of end ogenously feeding turbot embryos and larvae were equally fuelled by am ino 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 thi s pattern of catabolic substrate oxidation is also generally applicabl e to other marine fishes which spawn eggs containing oil globules. For an enthalpy balance equation of the form P = C + R + E, a turbot larv a at 15 degrees C, utilised 60% of yolk and oil globule enthalpy (C) f or growth (P), 37% was dissipated due re, metabolism (R) while only 3% was lose via excretion of nitrogenous end products (E).