This paper is aimed at underlining the limited knowledge available about the physiology of triploid fishes compared with diploids. Whereas many aspects (induction, detection, growth, resistance to diseases, etc.) in the production and rearing of triploid fishes have widely been developed and described in the literature, other numerous questions of ecophysiology remain in abeyance, and the study of triploid cells physiology is still in its infancy. Triploid fishes can be considered as models worthwhile for physiological investigations not only for the economical stake in relation to the development of triploid fishes rearing, but also for the cytological and molecular features of their tissues and organs. The functional implications of these features have been poorly studied although they are potential areas of applied and/or fundamental studies.

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.

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Hormonal changes, substrate mobilization and energy metabolism were studied in turbot Scophthalmus maximus exposed to 3 hypoxic conditions (oxygen partial pressure in water, PwO 2 = 90, 60 and 30 mm Hg) followed by recovery under normoxia. Measurements of the blood pH, total CO 2 concentration, arterial oxygen partial pressure, hematocrit, glucose, lactate, and 'stress' hormones (cortisol, adrenaline and noradrenaline) plasmatic concentrations were performed. Highenergy phosphorylated compounds, glycogen, glucose and lactate concentrations were also determined in liver and white muscle tissues. Exposure to 90 or 60 mm Hg did not induce any major physiological change, as hyperventilation by itself could compensate for the decrease in water oxygen tension. At 30 mm Hg, marked increases in cortisol, adrenaline and noradrenaline concentrations, associated with a decrease in blood arterial oxygen partial pressure, were observed. During exposure to 30 mm Hg, turbot resorted to anaerobic metabolism, resulting in liver glycogen depletion and lactate production. This mechanism appeared to be efficient enough to produce energy, as no significant change in phosphorylated compounds and adenylate energy charges in muscle and liver could be observed. These results indicate an absence of metabolic depression in turbot down to 30 mm Hg and confirm the high capacity of this species to cope with low water oxygen tension.

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