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The present study is focused to elucidate the main characteristics of the digestive function of this carnivorous fast-growing fish living at high temperatures. With this aim, we have examined the effects of an increased temperature from 30 to 34 °C on the daily pattern of gastrointestinal pH, enzymatic proteolytic digestive activity and the feed transit time in early juveniles of cobia (Rachycentron canadum), a species living in tropical and subtropical waters with an increasing aquaculture production. Fish were fed two meals a day. Gastric luminal pH was permanently acidic (mean pH values: 2.76-4.74) while the intestinal pH increased from neutral/slightly acidic to slightly alkaline when the digesta was present, with an increasing alkalinity from proximal to distal intestine (mean pH values: 6.05 to 7.69). The temperature did not affect the gastric pH but a slightly higher acidity was induced in the intestine at 34 °C. Pepsin activity showed a daily rhythm at 30 °C with maximum in the middle of the light period, while at 34 °C some hourly changes coinciding with feed adding without a clear daily trend during the 24-h period were observed. The trypsin activity exhibited a daily rhythm at both temperatures with an increase after morning feeding to reach a maximum several hours later. Average pepsin activity during the daily cycle was slightly higher at 34 °C (6.1 and 7.3 U mg -1 BW at 30 and 34 °C respectively), but values were significantly different only at 8 and 24 h after the morning meal. Similarly, the trypsin activity was significantly affected by the temperature only at 8 and 16 h after the morning meal, but daily activity averages were similar (1.20 and 1.29 U g -1 BW at 30 and 34 °C respectively). The partial transit rates of the first meal in the stomach for each period inter-samplings were higher during the first 4-h period and decreased progressively along the rest of the 24-h cycle at both temperatures, but no significant differences were detected at 30 °C. In addition, the transit was notably faster at 34 °C particularly during the first 8 h after feeding, with rates between 100 and 65% of total volume displaced (intake or released) during each 4-h period. In the intestine the transit rate was relatively constant and similar at both temperatures during 12 h after feeding. Then the rates remained very low during the following 12 h. Residence time of the first meal was longer at 30 than at 34 °C, particularly in the stomach (12 h:02 min vs 4 h:54 min respectively). In the intestine the difference was not so large (8 h:18 min vs 6 h:24 min respectively). In a parallel study under same conditions, cobia reared at 30 °C grew faster and showed a more favorable feed conversion ratio than those at elevated temperature (34 °C). The present results indicate that at 34 °C, a subtle increase of proteolytic activity cannot compensate for the faster gut transit rate. Therefore, 30 °C is more appropriate temperature for the early on-growing of cobia because at higher temperatures the digestion efficiency decrease being one of the causes for a lower growth., (Copyright © 2019 Elsevier Inc. All rights reserved.)

The hypothesis of the present study was that cod larvae have a limitation in lipid digestion, and that absorption of lipids would increase by pre-hydrolysation. The diets used were designed to contain 15% lipid, of which 40% was phosphatidylcholine (PC) and 60 % was TAG. Cod larvae (40d post hatch (dph)) were fed a single meal where either PC or TAG was radioactively labelled, and the labelled PC or TAG was either intact or hydrolysed (pre-digested). The larvae were then incubated individually in chambers with collection of CO2 for 10 h. The following fractions were analysed for radioactivity: the incubation water (evacuated feed); the intestine; the body; the CO2 trap. The larvae ate a 16-29 μg diet, equivalent to 3·4-5·2 % of dry body weight. In the whole population, 0-16% of the lipid was evacuated. The larvae that had eaten less than 1·9-2·7 μg lipid absorbed close to 100% of the lipid, absorption being defined conservatively as the amount contained in the carcass and CO2, excluding the intestinal tissue. In these larvae, approximately 100 % of the absorbed lipid was also catabolised. In the larvae that ingested more than 1·9-2·7 μg lipid, there was a linear reduction in lipid absorption to a minimum of 55% at the highest lipid intakes parallel to an increasing retention of lipids in the carcass. There were only minor differences in digestion, absorption, retention and metabolism of lipids between the larvae fed the different diets, and the larvae tended to retain lipid classes as they were present in the feed. The study shows that 40-dph Atlantic cod larvae have an efficient utilisation of dietary lipids supplied as intact PC and TAG.

Abstract: Further improvement of growth performance in fish larviculture is closely linked to better understanding of the dietary amino acid (AA) requirements, and therefore of the processes involved in AA metabolism. In recent years, major advances in the understanding of fish larvae amino acid metabolism have been accomplished, in particular through the use of tracer studies. Modelling is a holistic approach to integrate knowledge on growth and metabolism and identify gaps in current understanding. A dynamic mechanistic model that simulates AA metabolism of fish larvae was developed. It aims to improve the understanding of larval digestion and absorption of dietary AA, and the postprandial AA metabolism and growth. The model also assists in the interpretation of results obtained from tracer studies. The model is driven by amino acid intake, with the absorbed dietary AA being used for energy production or for biosynthetic processes. The model is implemented for Senegalese sole (Solea senegalensis) larvae fed Artemia, and was parameterized using literature data. The model allows to integrate the results obtained after feeding a single meal with tracer AA, and following these tracer AA in the free AA and protein pools of larval gut and larval body at different time points after the meal. Model simulations suggest that there is a sharp dynamic change in the FAA pool after a meal while the protein pool is little affected. This suggests that the AA composition of the food has a major contribution to the FAA pool composition. This implies that sole larvae is highly sensitive to dietary AA imbalances, having high AA unavoidable losses unless the dietary AA profile is well balanced. The model also suggests that rates of protein synthesis and AA catabolism rapidly increases after the meal, with the peak for this postprandial metabolism occurring only 1h after the meal, and the rates returning to “basal” values 2h after the meal. This suggests a rapid processing of the Artemia protein by the larvae, and supports the need for feeding sole larvae at a high frequency in order to fully use its growth potential. Mechanistic modelling is useful and an important complement in evaluation of metabolism kinetics in nutrient flux studies. Moreover, due to its mechanistic nature, the present model can be used with different AA tracers, and also for other fish species. [Copyright &y& Elsevier]

Abstract: For the majority of marine fish larvae a fully developed digestive tract, including gastric digestion, is acquired weeks to months (depending on species) after onset of exogenous feeding. Still, the processing capacity (capability to degrade and absorb dietary nutrients) of the larval gut is sufficient to support fast larval growth by digesting prey naturally available in the sea. However, the physiological constraints of the gut with respect to digestion of cultivated live prey and particularly formulated starter feeds still remain to be elucidated. This paper reviews some recent findings in the areas of control and efficiency of digestive function of marine fish larvae. For studies on the hormonal control, the peptide hormone cholecystokinin (CCK) has been targeted, since it is believed to play an important role in controlling digestion in vertebrates. Recent work on the processing capacity include studies of the digestibility and transfer kinetics of macronutrients from live prey and experimental work on absorption of protein, peptides and free amino acids from the lumen of the digestive tract into the tissues of larval fish and how this changes during ontogeny. The molecular and in vitro characterization of transporters is currently being integrated with ongoing in vivo studies. [Copyright &y& Elsevier]

The digestion, absorption and post absorptive metabolism of a radiolabelled triacylglycerol (TAG; triolein) and a free fatty acid (FFA; oleic acid), delivered by tube feeding, was studied in herring Clupea harengus larvae, using metabolic chambers and video analysis. In general, a large amount of the delivered lipid was evacuated. Most of the evacuation occurred between 2 and 6 h after tube feeding although a group of larvae responded by rapidly evacuating the lipid (>50% before 2 h). The volume of the tube-fed lipid affected its utilization. A small volume of triolein (9·2 nl, representing c. 6% of gut filling capacity) resulted in a lower proportion of fast evacuating larvae and improved utilization (lower evacuation and higher absorption: body incorporation and catabolism) compared with 50·6 nl ( c. 17% of gut filling capacity). Increases in the volume of tube fed triolein enhanced only marginally label absorption and led to a steep rise in evacuation. At a comparable high volume (50·6 nl), oleic acid, which does not require digestion, was better absorbed and less evacuated than triolein. The video observation of the lipid digestive process revealed a considerable gut contractile activity that appeared effective in processing the tube fed lipid. Also, the gut wall seemed very sensitive to physical pressure. Signs of chemical degradation during lipid digestion were also noted. The metabolic studies, together with video image analysis, suggested that the limiting step for the utilization of high dietary lipid levels may have been the lipid absorption into the enterocytes and transport into the body, rather than lipid digestion. The results support the notion that the rate of lipid digestion and absorption in fishes is slower than that of mammals. [ABSTRACT FROM AUTHOR]

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. [Copyright &y& Elsevier]

Abstract: A growth depressing effect of high dietary neutral lipid levels in marine fish larvae has been reported. This may be a result of a decrease in the efficiency or activity of digestive enzymes, a reduction in absorption efficiency and/or a decrease in food intake. The present work reviews recent studies carried out on commercially valuable species (Atlantic herring, Senegalese sole, European seabass and gilthead seabream) that investigated the effects of neutral lipid level and lipid source (fatty acid composition) on some of these key factors influencing larval growth. The results seem to collectively indicate that lipid transport from the enterocytes into the body may be more problematic in larval stages dealing with high neutral lipid diets than lipolytic enzymatic capacity, although both factors are likely to intervene. In seabass, lipase activity was significantly affected by the source of dietary lipid but not by its dietary level. Lipid sources differing in chain length and degree of saturation of their fatty acids may thus affect the efficiency or synthesis of neutral lipase through effects on substrate specificity. Phospholipid digestion appears to be more efficient than that of neutral lipids, although neutral lipase synthesis might not be a limiting factor for larval growth. High neutral lipid larval diets result in the accumulation of large lipid droplets in the enterocytes, which may in turn reduce fatty acid absorption efficiency and ultimately larval growth. Nonetheless, not all fatty acids are equally affected and mechanisms of specific essential fatty acid absorption probably exist. Food intake in seabream larvae is not strictly regulated by total lipid content of the diet and lipid source may have an important role in controlling ingestion. Therefore, the neutral lipid level in diets for marine fish larvae has a significant impact in several factors influencing growth but clearly it cannot be dissociated of its fatty acid composition, which appears to play a central role on the nutritional and physiological effects of dietary lipid, at the ingestion, digestion and absorption levels. [Copyright &y& Elsevier]