Control of the somatic growth in turbot

The development of a recombinant turbot somatotropin (rtuGH) in Escherichia coli and an homologous antibody in rabbit has produced a sensitive and specific RIA allowing the measurement of blood levels and pituitary contents of tuGH and their responses to environmental change. Turbot IGF‐1 (insulin growth factors) and IGF type 1 receptor (R) cDNA have also been cloned and sequenced. The deduced IGF‐1 R primary sequence contains all the topological features characteristics of mammalian IGF‐1 R and the turbot R appears highly conserved, compared to its mammalian counterpart, particularly within the catalytic domain. IGF‐1 R mRNA polyadenylation status varies during development with polyadenylation occuring in oocytes and early stage larvae. It disappears in later stages and in adult somatic tissues, indicating that IGF‐1 R mRNA undergoes complex post‐transcriptional regulation. Thyroid hormones (TH, both T 3 and T 4 ), are also involved in growth control, but are less affected by environmental changes than GH, except in very extreme conditions. The thyroid stimulating hormone (TSH) β‐subunit cDNA fragment has also been cloned and sequenced. mRNA expression levels have been quantified under conditions where blood circulating TH were modified by dietary treatments or hormone supplementation. Specific growth rate in turbot is related to food intake and is doubled with a temperature increase from 8 to 20° C. Lipid storage by contrast was higher at low temperatures. Over a long time course, growth rate in turbot can be improved in brackish water (8–20 psu) but it cannot adapt to very low salinities. Photoperiod appears to have little influence on growth when feeding is unrestricted. Under intensive farming conditions, ammonia, the major nitrogen waste product in teleosts, may increase to levels that can reduce growth. Safe ambient levels for growth are <2–3 mg l −1 TAN above which a 10–20% decrease in growth was recorded within 3 months. Over 20 mg l −1 , feeding stops and no growth occurs. Turbot juveniles can cope efficiently with relatively large fluctuations in ambient oxygen, but major disruptions to physiological functions occur below 20% of air saturation. Under moderate hypoxia growth is reduced by 20% within 1.5 months. The primary response to hypoxia or ammonia accumulation is always a decrease of food intake.