Abstract
Hormonal control of growth was examined in juvenile Mozambique tilapia Oreochromis mossambicus under different environmental salinities. When juve nile tilapia were acclimated to fresh water (FW) or to full-strength sea water (SW) for 3 weeks, growth rate of the SW-acclimated fish doubled compared to FW fish. Pituitary mRNA levels of growth hormone (GH) were significantly higher in SW-adapted tilapia compared with FW fish, while the gene expression of prolactin (PRL188 and PRL177) in SW fish was lower than in the FW group. The release and synthesis of insulin-like growth factor-1 (IGF-1) were significantly elevated in the SW fish. This in dicates that the GH/IGF-1 axis is up-regulated in juvenile tilapia in SW and likely leads to the observed elevation in growth. Transfer of the juveniles from FW to 70% SW, while PRL188 mRNA levels were significantly elevated after transfer of the 70% SW fish to FW. Our findings indicate that the GH/IGF-I axis plays important roles both in growth and in osmoregulation in juvenile tilapia in SW.
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