H. J. Fyhn

The supply of amino acids during early feeding stages of marine fish larvae: a review of recent findings

In marine fish larvae, the sum of protein deposition, turnover and catabolism necessary for their rapid growth dictates a high amino acid (AA) requirement. Once the yolk is exhausted, the digestive tract becomes the vital organ that ensures a steady supply of dietary AA to the growing larval tissues. In this paper, we discuss the demand and availability of AA (free and polymerised pools) in relation to larval digestive capacity. The sources of AA from compound and live diets are described, and the early regulatory roles of cholecystokinin (CCK) and a retrograde peristaltic activity are highlighted.

Energy metabolism during development of eggs and larvae of gilthead sea bream (Sparus aurata)

Developing eggs and larvae of laboratory-reared gilthead sea bream (Sparus aurata) maintained in filtered seawater (40 ppt) at 18°C, were measured for oxygen uptake, ammonia excretion, contents of free amino acids (FAA), protein, fatty acids (FA) accumulated ammonia, and volumes of yolk-sac and oil globule. Absorption of the yolk coincided with the consumption of FAA and was complete ca. 100 h post-fertilisation. Amino acids from protein were mobilised for energy in the last part of the yolk-sac stage. Absorption of the oil globule occurred primarily after hatching following yolk absorption, and correlated with catabolism of the FA neutral lipids. Overall, FAA appear to be a significant energy substrate during the egg stage (60 to 70%) while FA from neutral lipids derived from the oil globule are the main metabolic fuel after hatching (80 to 90%).

Physiological energetics of developing embryos and yolk-sac larvae of Atlantic cod (Gadus morhua). I. Respiration and nitrogen metabolism

This paper provides the basis for a general model of catabolic metabolism for developing embryos and yolk-sac larvae of Atlantic cod (Gadus morhua L.). Yolk-dependent routine rates of oxygen consumption, ammonia excretion, and accumulation of ammonium ions were related to quantitative changes in contents of glucose, glycogen, lactate, free amino acids, proteins and lipid classes (lipid classes published separately) in order to determine the rate and sequence of catabolic substrate oxidation that occurs with development at 6.0°C, 34.5‰ S. The stoichiometric relation of the oxygen consumption and total ammonia production to substrate utilisation indicated that during the first 2 to 3 d of development, glycogen was the sole substrate of oxidative metabolism. After formation of the syncytium, free amino acids (75%) together with polar lipids (13%, mainly phosphatidyl choline) and neutral lipids (9%, mainly triacylglycerol) comprised the metabolic fuels of embryonic development. Following hatch (Day 16 post fertilisation), the fuels were free amino acids (32%), polar lipids (20%, mainly phosphatidyl choline), neutral lipids (17%, mainly triacylglycerol) and proteins (31%). Thus, the catabolic metabolism of endogenously feeding Atlantic cod larvae was predominantly fuelled by amino acids (67%) and lipids (32%), while glycogen only accounted for 1% of the total enthalpy dissipated. It is proposed that the above sequence of catabolic substrate oxidation is also generally applicable to other cold-water fishes which spawn eggs that do not contain oil glubules.

The sequence of catabolic substrate oxidation and enthalpy balance of developing embryos and yolksac larvae of turbot (Scophthalmus maximus L.)

Abstract 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 stoichio-metrically 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 rate and sequence of catabolic substrate oxidation that 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 for 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°C, utilised 60% of yolk and oil globule enthalpy (C) for growth (P), 37% was dissipated due to metabolism (R) while only 3% was lost via excretion of nitrogenous end products (E).

Physiological energetics of developing embryos and yolk-sac larvae of Atlantic cod (Gadus morhua). II. Lipid metabolism and enthalpy balance

This paper presents quantitative data for the changes in the contents of total lipids, lipid classes and their associated fatty acids, together with the changes in caloric contents of developing eggs and yolk-sac larvae of Atlantic cod (Gadus morhua L.). During development between Day 0 to 28 post fertilisation, 32% of catabolic metabolism was fuelled by lipids. On a mass-specific basis, polar lipids (mainly phosphatidyl choline) contributed 60%, and neutral lipids (mainly triacyl-glycerol) contributed 40% to this catabolic component, with each class supplying similar amounts of fatty acids as fuel. The fatty acids supplied by phosphatidyl choline were catabolised non-selectively (i.e. in proportion to their presence in the egg), with about half of them being polyunsaturated. However, of the fatty acids esterified in triacylglycerol, the larvae showed an apparent oxidation preference for monoenes over polyunsaturates or saturates. Routine rates of oxygen consumption and ammonia production were related to the caloric contents of the eggs and larvae in order to derive an enthalpy balance equation (of the form P=C+R+E) for an Atlantic cod larva during its period of endogenous nutrition. For the interval of Day 0 to 25 post fertilisation (the period of yolk dependence), integration of the physiological and caloric data revealed that Atlantic cod larvae conserved 53% of yolk enthalpy (C) for growth (P), 42% was dissipated due to metabolism (R) while only 5% was lost via excretion (E).

Temperature dependency of early growth of turbot (Scophthalmus maximus L.) and its implications for developmental progress

Abstract Growth in developing turbot larvae can be expressed independently of temperature when using effective day-degrees (D°eff) as an index of development. The calculation of D°eff is based on the principle of thermal summation whereby the rate of development is linearly related to the ambient temperature above a species-specific threshold temperature (T0) at which development is theoretically arrested. The T0 for turbot embryos is 5.3°C. The increase in wet body mass of developing turbot larvae at 14, 18, and 22°C can be aligned into a single exponential relationship regardless of incubation temperature when using D°eff as the index of development. Previous attempts to relate physiological processes in poikilothermic animals, including fish, to incubation temperature are discussed.

Urea and ammonia excretion by embryos and larvae of the African catfish Clarias gariepinus (Burchell 1822)

Yolk-sac larvae and starved larvae of Clarias gariepinus (Burchell 1822), reared at 28°C, were predominantly ammonotelic, but urea excretion contributed about 19±7% to the total nitrogen excretion. Exogenously feeding larvae of C. gariepinus were mainly ammonotelic until 180-205 h post fertilization, but from then on the relative urea excretion stabilized at 44±13%. The contents of total free amino acids (FAA) and ammonia in C. gariepinus peaked around complete yolk absorption. During the first period after hatching 64% of the FAA and 60% of the total ammonia were located in the yolk compartment. The body compartment contained the highest amount of FAA and total ammonia at the end of the yolk-sac period (65 and 77%, respectively). The amount of nitrogen originating from catabolism of amino acids was balanced within 6% by the excreted sum of ammonia and urea. The high degree of ureotelism in C. gariepinus larvae may constitute an adaptive mechanism to a habitat of temporal water shortage.