Abstract

The Mozambique tilapia grows significantly faster and larger when raised in seawater (SW) than in fresh water (FW). Furthermore, there is a gender difference in the growth rates, with males growing significantly larger than females. The main objective of these studies was to investigate the endocrine mechanisms that regulate these differences in growth.

Tilapia raised in SW grew significantly larger than those raised in FW, and treatment with 17β-methyltestosterone (MT) augmented growth in both salinities. There was a significant correlation between pituitary growth hormone (GH) mRNA and body weight. By contrast, plasma GH levels did not vary significantly among the groups. Plasma levels of insulin-like growth factor-I (IGF-I) and fish size varied in a correlated pattern, however not significantly. These results indicate that SW rearing and MT treatment stimulate the GH/IGF-I axis, and suggest that pituitary GH mRNA, at this stage of development, is a better indicator of growth than plasma levels of GH and IGF-I. Transfer from SW to FW for forty days, however, resulted in a significant decrease in growth rate compared with the SW control. The activity of the GH/IGF-I axis was elevated after transfer of fish from SW to FW, suggesting a greater metabolic cost of osmoregulation in FW than in SW in tilapia.

Tilapia exhibit a sexually dimorphic pattern of growth, males growing larger than females. Estrogen (E₂) treatment in vivo induced a female-like GH/IGF-I profile in males, elevated plasma and mRNA levels of GH and vitellogenin (VTG), and suppressed plasma and liver mRNA IGF-I levels. In contrast, the androgen, 5α-dihydrotestosterone (DHT), induced a male-like GH/IGF-I profile in females, elevated plasma and liver mRNA levels of IGF-I, and suppressed plasma and mRNA levels of GH. Estradiol and DHT exhibited direct effects on liver production of VTG and IGF-I in a similar fashion to that observed in the in vitro studies. These findings suggest that E2 re-directs metabolic energy from growth (IGF-I production) and toward reproduction (VTG production). Collectively these data provide evidence that the increased growth observed in SW and in males is a result of more available metabolic energy directed toward growth.