Respiration, nitrogen and energy metabolism of developing yolk-sac larvae of Atlantic halibut (Hippoglossus hippoglossus L.)

Abstract

The present investigation quantifies the partitioning of yolk enthalpy (C) into production of biomass (P), dissipation due to metabolism (R) and losses associated with excretion of nitrogenous end products (E) during the endogenous nutrition of developing yolk-sac larvae of Atlantic halibut ( Hippoglossus hippoglossus L.). Further, the sequence of catabolic substrate oxidation and the mass-specific metabolic rate in relation to development have been established in order to estimate the nutritional and enthalpy requirements of first-feeding larvae. The larvae grew exponentially ( G = 6% · day −1 , 1%·°day −1 ) and maintained positive energy balance until day 33 post-hatch (200°day). This point coincided with maximum tissue mass, protein content, standard length, myotome height and respiratory exertion of the larvae. During this phase, the larvae conserved 59–61% of yolk enthalpy for biomass deposition, and 35% was dissipated in metabolism, with only 4–6% being lost via nitrogenous excretion, Thereafter, the larvae experienced negative energy balance, and the excess metabolism was supported by proteolytic and lipolytic recruitment of somatic tissue. This resulted in an increased (14%) excretory component. For the first 2 weeks following hatch, amino acids from the rapidly declining free pool constituted the sole metabolic fuel. Free amino acids were also simultaneously polymerized into protein. Thereafter, protein-bound amino acids became the preferred substrate, joined initially by polar lipids (mainly phosphatidyl choline) and then by neutral lipids (mainly triacylglycerol). During the period of positive energy balance, free amino acids constituted 31%, proteinic amino acids 44%, polar lipids 21% and neutral lipids 3% of the substrates used to fuel metabolism. At the time of maximum tissue mass, the yolk-sac larvae showed a significantly higher rate of oxygen consumption in light compared with in darkness. Within the size range of 200–800 μg · ind −1 , the rate of oxygen consumption scaled isometrically with dry body mass throughout development, such that the mass-specific aerobic metabolic rate at 7°C was maintained at 72 ± 13 μmol · g −1 · hr −1 in light, and 62 ± 9 μmol · g −1 · hr −1 in darkness. Using the aerobic metabolic rate of the light-adapted larvae, we estimate that first-feeding larvae of Atlantic halibut (at 7°C) demand an exogenous enthalpy at a rate of 110 J · g −1 · hr −1 , (2.6 kJ · g −1 · day −1 ) in order to sustain an instantaneous growth rate of 6% · day −1 .