Karine Pichavant

Effects of hypoxia on growth and metabolism of juvenile turbot.

The effects of hypoxia on growth, feed efficiency, nitrogen excretion, oxygen consumption and metabolism of juvenile turbot (120 g) were studied in a 45-day experiment carried out in sea water at 17.0±0.5°C and 34.5 ppt salinity. Fish were fed to satiation at O2-concentrations of 3.5±0.3, 5.0±0.3 mg l−1 (hypoxia) and 7.2±0.3 mg l−1 (normoxia). Both feed intake (FI) and growth were significantly lower under hypoxia than under normoxia, with no significant differences being observed between 3.5 and 5.0 mg O2 l−1. During the first 2 weeks of the experiment, FI was halved under hypoxic conditions, and there were large differences among treatments in feed conversion ratio (FCR), i.e., it was 3.2, 1.5, and 0.9 in turbot exposed to 3.5, 5.0, and 7.2 mg O2 l−1, respectively. Thereafter, FCR was not significantly affected by O2-concentration. Nitrogen excretion and oxygen consumption of feeding fish were significantly higher under normoxia than under hypoxia, but following 7 days of feed deprivation oxygen consumption was similar under normoxia and hypoxia. Plasma osmolarity, ionic balance, and acid-base status were not affected by the two hypoxic conditions tested. Overall, our results indicate that turbot have some capacity to adapt to relatively low ambient O2-concentrations.

Effects of O2 supersaturation on metabolism and growth in juvenile turbot (Scophthalmus maximus L.)

Abstract Effects of O2 supersaturation on metabolism and growth were studied in juvenile turbot (Scophthalmus maximus L.). When fish were reared for 30 days in water containing O2 at 147% or 223% air saturation, there were no significant differences in food intake, growth, food conversion or protein utilization compared to fish exposed to normoxia (100% air saturation in water outlet). Exposure to hyperoxia resulted in increased body fat deposition. Daily rates of O2 consumption of resting fish were not affected by O2-concentrations, and there were no significant differences in rates of nitrogenous excretion among fish exposed to the different O2-concentrations. Turbot tolerated severe hyperoxia, 350% air saturation, for 10 days. There were changes in acid–base balance that compensated for the respiratory acidosis resulting from O2 supersaturation. Blood pH was regulated within 24 h (it averaged 7.69 over the 30-day experiment) by significant increases in plasma CO2 content and pCO2. Plasma CO2 was dose dependent averaging 11.3 and 18.9 mmol l−l under 147% and 224% O2 saturation, respectively, compared to 6.7 mmol l−l under normoxia. Over the 30-day experiment, the only change in hydromineral balance was a slight, but non-significant decrease in plasma chloride content in fish exposed to hyperoxia (137 mmol l−l compared to 139 under normoxia). There were no changes in haematocrit, haemoglobin and red blood cell counts (they averaged 18.3%, 3.7 g dl−1 and 1.37×106 mm−3, respectively) and no signs of stress (plasma cortisol averaged 3.8 ng ml−1) related to exposure to O2-supersaturation for 30 days.

Pituitary Growth Hormone Secretion in the Turbot, a Phylogenetically Recent Teleost, Is Regulated by a Species-Specific Pattern of Neuropeptides

In mammals, growth hormone (GH) is under a dual hypothalamic control exerted by growth hormone-releasing hormone (GHRH) and somatostatin (SRIH). We investigated GH release in a pleuronectiform teleost, the turbot (Psetta maxima), using a serum-free primary culture of dispersed pituitary cells. Cells released GH for up to 12 days in culture, indicating that turbot somatotropes do not require releasing hormone for their regulation. SRIH dose-dependently inhibited GH release up to a maximal inhibitory effect of 95%. None of the potential stimulators tested induced any change in basal GH release. Also, neither forskolin, an activator of adenylate cyclase, nor phorbol ester (TPA), an activator of protein kinase C, were able to modify GH release, suggesting that spontaneous basal release already represents the maximal secretory capacity of turbot somatotropes. In contrast, forskolin and TPA were able to increase GH release in the presence of SRIH. In this condition (coincubation with SRIH), pituitary adenylate cyclase-activating polypeptide (PACAP) stimulated GH release, whereas none of the other neuropeptides tested (GHRHs; sea bream or salmon or chicken II GnRHs; TRH; CRH) had any significant effect. These data indicate that inhibitory control by SRIH may be the basic control of GH production in teleosts and lower vertebrates, while PACAP may represent the ancestral growth hormone-releasing factor in teleosts, a role taken over in higher vertebrates by GHRH.

Effects of hypoxia on respiratory physiology of turbot, Scophthalmus maximus

Variations in respiratory and acid-base status were studied in turbot (Scophthalmus maximus) during progressive severe hypoxia followed by recovery under normoxic conditions. The first behavioural strategy of turbot under hypoxia was an increase in amplitude and frequency of ventilation. Consequently, standard O2 consumption remained unchanged over a broad range of O2 tensions, until a low critical level of 30 mmHg. The hyperventilation induced a moderate blood alkalosis, compensated by a lactic acidosis. The fact that blood pH did not decrease below control values could be explained by the retention in white muscle of most of the lactate produced and by a high capacity for H+ excretion. During the recovery period, the marked increase in O2 uptake corresponding to an oxygen debt repayment, was partly related to the lactate elimination. When total energy contributions of aerobic and anaerobic processes were assessed in terms of ATP, the anaerobic contribution, estimated at the deepest hypoxia level, was higher than 20% of the total energy budget and appeared to totally compensate for the decline in aerobic metabolism. Moreover, the high value of O2 tension in arterial blood in normoxia and during recovery from hypoxia showed high diffusing capacity of gills in turbot. Our results explain the high tolerance of turbot for O2 deficient waters.

Effects of repeated hypoxic shocks on growth and metabolism of turbot juveniles

Turbot juveniles (45 g) were exposed for 41 d (17 °C, 34‰ salinity) to constant normoxic (100–100% air saturation, 100–100) or moderate hypoxic (75–75% air saturation, 75–75) conditions and to repeated hypoxic shocks (20% saturation for 1 h, 5 d per week) from normoxic (100–20% air saturation, 100–20) or moderate hypoxic (75–20% air saturation, 75–20) conditions. A normoxic group was feed restricted (100-FR). Mass increase of 100–100 and 75–75 groups fed to satiation was not significantly different. In comparison, it was significantly lower in the 100–20 and 75–20 groups (NS between the two hypoxic shocks groups). Intermediate results were obtained in the 100–100-FR group. The lowest mass increase under hypoxic shocks was explained by a significant decrease in both feed intake and food conversion efficiency (FCE). FCE was lower in the two hypoxic groups, but only the 75–20 group was significantly different from all the other groups. There was no sign of stress and no change in the physiological status of fish in any group. When challenged, pre-conditioning of turbot to regular hypoxic shocks extended survival time, slightly but significantly, for 50% of the population. It wa s 8 h longer in starved than in fed fish. When reared for 1 year in normoxic water, the growth rate of post-challenged survivors was dependent on pre-conditioning: day 0–375 specific growth rate was significantly higher in the two groups acclimated to repeated hypoxic shocks. In the second experiment, it was shown that exposure to 20% air saturation for 12 h led to major physiological changes within 4 h: a significant decrease in plasma total CO 2 and increase in plasma lactate contributing in maintaining blood pH stable, and a significant increase in osmolarity and chloride concentration. When returned to normoxic water, the recovery capacity of the fish was high: plasma osmolarity and total CO 2 returned to pre-exposure levels within 1 h. The results are discussed in terms of turbot capacity to cope with repeated hypoxic shocks and to acclimate.