Targeting the Mild-Hypoxia Driving Force for Metabolic and Muscle Transcriptional Reprogramming of Gilthead Sea Bream (Sparus aurata) Juveniles.

On-growing juveniles of gilthead sea bream were acclimated for 45 days to mild-hypoxia (M-HYP, 40-60% O2 saturation), whereas normoxic fish (85-90% O2 saturation) constituted two different groups, depending on if they were fed to visual satiety (control fish) or pair-fed to M-HYP fish. Following the hypoxia conditioning period, all fish were maintained in normoxia and continued to be fed until visual satiation for 3 weeks. The time course of hypoxia-induced changes was assessed by changes in blood metabolic landmarks and muscle transcriptomics before and after exhaustive exercise in a swim tunnel respirometer. In M-HYP fish, our results highlighted a higher contribution of aerobic metabolism to whole energy supply, shifting towards a higher anaerobic fitness following normoxia restoration. Despite these changes in substrate preference, M-HYP fish shared a persistent improvement in swimming performance with a higher critical speed at exercise exhaustion. The machinery of muscle contraction and protein synthesis and breakdown was also largely altered by mild-hypoxia conditioning, contributing this metabolic re-adjustment to the positive regulation of locomotion and to the catch-up growth response during the normoxia recovery period. Altogether, these results reinforce the presence of large phenotypic plasticity in gilthead sea bream, and highlights mild-hypoxia as a promising prophylactic measure to prepare these fish for predictable stressful events.

Ontogeny of Expression and Activity of Digestive Enzymes and Establishment of gh/igf1 Axis in the Omnivorous Fish Chelon labrosus.

Thick-lipped grey mullet (Chelon labrosus) is a candidate for sustainable aquaculture due to its omnivorous/detritivorous feeding habit. This work aimed to evaluate its digestive and growth potentials from larval to early juvenile stages. To attain these objectives the activity of key digestive enzymes was measured from three until 90 days post hatch (dph). Expression of genes involved in digestion of proteins (try2, ctr, pga2, and atp4a), carbohydrates (amy2a), and lipids (cel and pla2g1b), together with two somatotropic factors (gh and igf1) were also quantified. No chymotrypsin or pepsin activities were detected. While specific activity of trypsin and lipase were high during the first 30 dph and declined afterward, amylase activity was low until 57 dph and increased significantly beyond that point. Expression of try2, ctr, amy2a, and cel increased continuously along development, and showed a peak at the end of metamorphosis. Expression of pla2g1b, pga2 and atp4a increased until the middle of metamorphosis and decreased afterwars. Most of these trends contrast the usual patterns in carnivorous species and highlight the transition from larvae, with high protein requirements, to post-larvae/juvenile stages, with omnivorous/detritivorous feeding preferences. Somatotropic genes, gh and igf1, showed approximately inverse expression patterns, suggesting the establishment of the Gh/Igf1 axis from 50 dph.

Tissue-Specific Orchestration of Gilthead Sea Bream Resilience to Hypoxia and High Stocking Density.

Two different O2 levels (normoxia: 75-85% O2 saturation; moderate hypoxia: 42-43% O2 saturation) and stocking densities (LD: 9.5, and HD: 19 kg/m3) were assessed on gilthead sea bream (Sparus aurata) in a 3-week feeding trial. Reduced O2 availability had a negative impact on feed intake and growth rates, which was exacerbated by HD despite of the improvement in feed efficiency. Blood physiological hallmarks disclosed the enhancement in O2-carrying capacity in fish maintained under moderate hypoxia. This feature was related to a hypo-metabolic state to cope with a chronic and widespread environmental O2 reduction, which was accompanied by a differential regulation of circulating cortisol and growth hormone levels. Customized PCR-arrays were used for the simultaneous gene expression profiling of 34-44 selected stress and metabolic markers in liver, white skeletal muscle, heart, and blood cells. The number of differentially expressed genes ranged between 22 and 19 in liver, heart, and white skeletal muscle to 5 in total blood cells. Partial Least-Squares Discriminant Analysis (PLS-DA) explained [R2Y(cum)] and predicted [Q2Y(cum)] up to 95 and 65% of total variance, respectively. The first component (R2Y = 0.2889) gathered fish on the basis of O2 availability, and liver and cardiac genes on the category of energy sensing and oxidative metabolism (cs, hif-1α, pgc1α, pgc1ß, sirts 1-2-4-5-6-7), antioxidant defense and tissue repair (prdx5, sod2, mortalin, gpx4, gr, grp-170, and prdx3) and oxidative phosphorylation (nd2, nd5, and coxi) highly contributed to this separation. The second component (R2Y = 0.2927) differentiated normoxic fish at different stocking densities, and the white muscle clearly promoted this separation by a high over-representation of genes related to GH/IGF system (ghr-i, igfbp6b, igfbp5b, insr, igfbp3, and igf-i). The third component (R2Y = 0.2542) discriminated the effect of stocking density in fish exposed to moderate hypoxia by means of hepatic fatty acid desaturases (fads2, scd1a, and scd1b) and muscle markers of fatty acid oxidation (cpt1a). All these findings disclose the different contribution of analyzed tissues (liver ≥ heart > muscle > blood) and specific genes to the hypoxic- and crowding stress-mediated responses. This study will contribute to better explain and understand the different stress resilience of farmed fish across individuals and species.