Hans Jørgen Fyhn

First feeding of marine fish larvae: Are free amino acids the source of energy?

Abstract Mass mortality of marine fish larvae in the ocean or in culture correlates with final yolk absorption when the larvae convert to exogenous feeding. Low planktonic abundance of appropriate prey organisms, or lack of suitable feed, is assumed to be responsible for the mortality but criteria to ascertain the suitability of the feed are not available. From studies of developing eggs and larvae of halibut and cod, it is proposed that free amino acids (FAA) are an important energy source during embryonic development of marine fishes. An exogenous supply of FAA seems necessary when the reserves of the larva are depleted. Which specific FAA are required, and for how long, may vary between species. This novel idea may be the clue to evaluate the suitability of a given feed or prey organism for marine fish larvae at first feeding.

Metabolic aspects of free amino acids in developing marine fish eggs and larvae

Abstract Investigations during the last decade have shown that free amino acids (FAA) abound in marine pelagic fish eggs. The concentration in the egg (about 150 mM) is well above what is typically found in adult teleostean tissues. FAA in fish eggs were initially studied in order to evaluate their role as osmolytes. More recently, however, FAA have been implicated as a fuel in the energy metabolism of developing marine fish eggs and larvae. Further aggregation of data verifies that pelagic eggs of marine teleosts generally have a high content of FAA. The large pool of FAA is almost exclusively contained within the yolk‐sac compartment. The relative composition of the FAA pool shows little interspecific variation. The FAA pool is depleted during development and reaches low levels at first feeding. The observed decline in the FAA content during development is not due to a loss of FAA to the ambient water, but rather results from endogenous metabolic turnover. FAA seem to be utilized to varying extents for ...

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.

Free amino acids as energy substrate in developing eggs and larvae of the cod Gadus morhua

The content of free amino acids (FAA) in the cod (Gadus morhua L.) egg is about 200 nmol at spawning, decreasing by about 100 nmol/egg during the egg stage and about 75 nmol/larva during the yolksac larval stage. Together, alanine, leucine, serine, isoleucine, lysine, and valine account for about 75% of the decrease. Ammonium accumulates gradually during the egg stage and is quickly excreted after hatching. The body protein content is maintained during the egg and yolksac larval stages. The measured oxygen uptake of the cod embryo during the egg and yolksac larval stages accounts for about 85% of the oxygen necessary to catabolize the FAA disappearing during this period. Ammonia excretion of the cod embryo, as taken from literature data, is similar to the expected ammonia production from catabolism of the FAA. Our data suggest that FAA are a major substrate for aerobic energy production in cod eggs and yolksac larvae. The implication of this finding for the production of a favourable first-feed for cod and other cultivated marine fish larvae, and for the selection of high quality eggs of marine fishes, is stressed.

The importance of free amino acids to the energy metabolism of eggs and larvae of turbot (Scophthalmus maximus)

A study was undertaken to establish the role of free amino acids (FAA) in aerobic energy dissipation in embryos of turbot (Scophthalmus maximus) which contain an oil globule in the egg. Laboratory-reared developing eggs and larvae (15°C, 34‰ salinity) were measured for oxygen uptake, ammonia excretion, contents of FAA, protein, and ammonium, and volumes of yolksac and oil globule. Newly spawned eggs from different batches contained 55 to 90 nmol egg−1 of FAA. Resorption of FAA occurred in parallel with the consumption of yolk. Resorption of the oil globule, however, occurred predominantly after hatching and mainly after yolk resorption. The combined data suggest that approximately 70% of the FAA are utilized as an energy substrate, while the rest are polymerized into body proteins. FAA become a significant energy substrate in the early egg stage and account for 100% of the aerobic energy dissipation 2 d after Fertilization then decrease to ca. 60% at the time of hatching. Lipids derived from the oil globule seem to be the main fuel after hatching and account for ca. 90% of the energy dissipation at the onset of first-feeding. Thus, the energetics of fish embryos which contain an oil globule seems to be different from those that depend exclusively on the nutritional reserves of the yolk.

Utilisation of yolk fuels in developing eggs and larvae of European sea bass (Dicentrarchus labrax)

Abstract Developing eggs and larvae of European sea bass ( Dicentrarchus labrax ) maintained in filtered sea water (40 gl −1 ) at 18°C, were measured for oxygen uptake, contents of free amino acids (FAA), protein, fatty acids (FA), and volumes of yolk-sac and oil globule. Newly spawned eggs had a dry weight of 90 μ g egg −1 and an egg diameter of 1.14±0.03 mm. The yolk was quickly absorbed during the embryonic and the early larval stages and was 95% depleted by 100 h post fertilisation. The depletion rates of the FAA were somewhat faster than the rate of absorption of the general yolk matter and were almost complete by 80 h post fertilisation. The oil globule was mainly absorbed after hatching following yolk absorption, and occurred concurrently with catabolism of FA from neutral lipids. Approximately 30% of the oil globule was still present at the presumed onset of exogenous feeding. Overall, FAA appeared to be a significant energy substrate during the egg stage and the early yolk-sac stage while FA from neutral lipids derived from the oil globule seemed to be the main metabolic fuel after hatching. Amino acids from protein seemed to be mobilised for energy in the last part of the yolk-sac stage. This investigation supports accumulating evidence for a common sequence of catabolic substrate oxidation in marine pelagic fish eggs that contain oil globules.

Compartmental distribution of free amino acids and protein in developing yolk-sac larvae of Atlantic halibut (Hippoglossus hippoglossus)

Free amino acids (FAA) and protein have been measured in whole laboratory-readed halibut larvae and on dissected individuals separated into yolk and body compartments. At hatching both FAA and protein are mainly located in the yolk compartment. During the first 12 d of the yolk-sac stage more than 70% of the FAA pool disappeared from the yolk without any significant changes in the yolk protein pool. This suggests different uptake mechanisms for FAA and protein from the yolk, and a sequential utilisation of the endogeneous reservoirs of free and protein amino acids in Atlantic halibut larvae. The data suggest that in the early yolk-sac stage FAA enter the embryo from the yolk and are utilised both for energy and protein synthesis. Later on when the free pool cannot fulfil the nutritional requirements, additional amino acids are recruited from yolk protein. Of the total amino acids (free + protein amino acids) present at hatching ca. 60% will be used as precursors for body protein synthesis while the remaining 40% are used as fuel in the larval energy metabolism.

Respiration and nitrogen metabolism of Atlantic halibut eggs (Hippoglossus hippoglossus)

Naturally spawned and fertilized eggs of Atlantic halibut,Hippoglossus hippoglossus L., were analysed for protein, free amino acids (FAA), ammonium ions and energy content. The chemical composition was found to be size-dependent but varied little during egg development. Ammonium ions did, however, accumulate during the late embryonic stage, and the trend in FAA content was downward during the same period. Rates of O2 uptake and NH3 excretion followed exponential patterns. A total of 1µmol O2 was consumed and 120 nmol NH3 excreted between the time intervals of fertilization and 1 d post hatch. Derived O:N ratios indicated that the dominant portion of the energy metabolism was lipid- or carbohydrate-based during the mid-development period but switched to FAA as hatch was approached.

The osmotic response of salmon louse,Lepeophtheirus salmonis (Copepoda: Caligidae), during the transition from sea water to fresh water

SummaryThe osmotic changes in haemolymph and body tissues of the ectoparasitic salmon louse,Lepeophtheirus salmonis, have been studied upon transfer from sea water (SW) to dilute sea water (37% SW), and then to fresh water (FW). The parasite shows osmoconformity in SW but hyperosmotic regulation in 37% SW regardless of whether it is attached to the salmon host or free swimming in the water. The same conclusion is reached by haemolymph Cl− measurements. In FW, the osmotic tolerance and response of attached and free swimming parasites differ: Attached animals maintain steady haemolymph osmolality and Cl− concentration and survive for at least 1 week, while free swimming parasites quickly become diluted and start to die within 8 h.Acclimation to 37% SW is accompanied by changes in body tissue water content and in the content of ninhydrin positive substances and specific amino acids which suggest the presence of cell volume regulation. Glycine is the dominating free amino acid in the cephalothorax tissues but alanine, proline and taurine also occur in high amounts. Lysine is found to increase significantly during FW acclimation of attached parasites. A breakdown of cell volume regulation is suggested to limit the survival of attached salmon louse in fresh water.

Calorespirometry of developing embryos and yolk-sac larvae of turbot (Scophthalmus maximus)

The rates of oxygen consumption and heat dissipation were simultaneously measured and related to contents of glucose, glycogen and lactate in order to determine whether anaerobic processes contributed significantly to the energy metabolism of developing turbot embryos and larvae. The results suggest that metabolism is fully aerobic between Days 0 and 12 post fertilisation. The data further suggest that glycogen is the sole metabolic fuel during the first 18 to 19 h post fertilisation. After the commencement of epiboly, carbohydrates play an insignificant role in the energy metabolism of the developing embryo and yolk-sac larva.