Astyanax mexicanus has gained importance as a laboratory model organism for evolutionary biology. However, little is known about its intermediary metabolism, and feeding regimes remain variable between laboratories holding this species. We thus aimed to evaluate the intermediary metabolism response to nutritional status and to low (NC) or high (HC) carbohydrate diets in various organs of the surface-dwelling form of the species. As expected, glycaemia increased after feeding. Fish fed the HC diet had higher glycaemia than fish fed the NC diet, but without displaying hyperglycaemia, suggesting that carbohydrates are efficiently used as an energy source. At molecular level, only fasn ( Fatty Acid Synthase ) transcripts increased in tissues after refeeding, suggesting an activation of lipogenesis. On the other hand, we monitored only moderate changes in glucose-related transcripts. Most changes observed were related to the nutritional status, but not to the NC versus HC diet. Such a metabolic pattern is suggestive of an omnivorous-related metabolism, and this species, at least at adult stage, may adapt to a fish meal-substituted diet with high carbohydrate content and low protein supply. Investigation to identify molecular actors explaining the efficient use of such a diet should be pursued to deepen our knowledge on this species.

The present study investigated the effect of low stocking density on growth, survival, feed parameters and physiological responses (blood metabolites, welfare indicators, immune biomarkers, and transcriptomic responses of stress and immune-related genes) on juvenile rainbow trout (Oncorhynchus mykiss) reared under a recirculating aquaculture system during 12 weeks. Fish (average weight 29.64 g) were reared in triplicate under four initial densities: nine fish per tank (D9, 3.76 ± 0.06 kg/m3), 18 fish per tank (D18, 7.66 ± 0.18 kg/m3), 27 fish per tank (D27, 9.67 ± 0.01 kg/m3) and 36 fish per tank (D36, 12.94 ± 0.14 kg/m3). Results showed that lower stocking density D9 significantly altered survival with several fish dying during the experiment and an alteration of growth and feed efficiency for the remaining fish. In parallel, the study revealed that low stocking density induced a chronic stress altering the physiological responses of trout by dysregulation of the inflammatory, immune system, and indolamine/catecholamine brain levels. In conclusion, regarding all the variables observed, low stocking density (D9) alters survival, growth and feed efficiency of rainbow trout with alteration of their physiological responses. Selecting appropriate fish density relating to rearing conditions proved to be an essential concern to improve welfare in an aquaculture context.

An increase in egg incubation temperature was previously shown to enhance the metabolism of mule ducks and increase liver fattening after overfeeding, through a metabolic programming mechanism. Here we examined whether fasting (F) followed by refeeding (RF) in 11-week-old mule ducks, could become an accelerated model to study the mechanisms of metabolic programming following embryonic thermal manipulation. This study investigated the hepatic response of mule ducks subjected to 23 hours of fasting and 1 hour of refeeding, in control or thermally programmed animals (with an increase of 1°C, 16h per day from day 13 to 27 of embryogenesis). Liver weight and energy composition, hepatocyte structure, plasma parameters and gene expression levels were measured at 1, 2, and 4 hours after RF. All these parameters were strongly affected by RF, while significant impacts of embryonic programming were measured in cell size (+1 µm on average), lipid composition (+ 4.2 % of saturated fatty acids 4h after the meal), and relative gene expressions (including HK1, SCD1, ELOVL6 and FASN). In addition to confirming previously identified molecular targets of thermal manipulation, this study revealed new ones, thanks to kinetic sampling after RF. Finally, the detailed description of the impact of the F/RF challenge on the liver structure, composition and gene expression, but also on plasma parameters allowed us to draw a parallel with these same traits measured during overfeeding. This comparative analysis suggests that this protocol could become a pertinent model to study the mechanisms involved in embryonic liver thermal programming, without overfeeding.

Rainbow trout are considered as a poor user of dietary carbohydrates, displaying persistent postprandial hyperglycaemia when fed a diet containing high amounts of carbohydrates. While this phenotype is well-described in juveniles, less attention was given to broodstock. Our objective was to assess for the first time the short-term consequences of feeding mature female and male, and neomale trout with a low-protein high-carbohydrate diet on glucose and lipid metabolism. Fish were fed for two days with a diet containing either no or 32% of carbohydrates. We analysed plasma metabolites, mRNA levels and enzymatic activities of glycolysis, gluconeogenesis, de novo lipogenesis and β-oxidation in the liver. Results demonstrated that the glucose and lipid metabolism were regulated by the nutritional status in all sexes, irrespective of the carbohydrate intake. These data point out that carbohydrate intake during a short period (5 meals) at 8 °C did not induce specific metabolic changes in broodstock. Finally, we demonstrated, for the first time, sex differences regarding the consequences of two days of feeding on glucose and lipid metabolism.