SEX, SALINITY, AND SAMPLING PERIOD MODULATE GROWTH HORMONE EXPRESSION IN MOZAMBIQUE TILAPIA Oreochromis mossambicus

Tilapia has become a widely farmed finfish, in part due to its tolerance to a wide range of environmental conditions and its sexually dimorphic growth. Growth in vertebrates, including teleosts , is largely controlled by growth hormone (GH). In tilapia, environmental salinity directly modulates growth through the activation of GH and associated growth factors .  In our previous study, we found that t ilapia reared in a tidally-changing  salinity regime (TR) grew faster than fish reared in steady-state fresh water (FW) or seawater (SW) .  Moreover, pituitary gh mRNA expression was found  to  be a better indicator of body weight than circulating GH and insulin-like growth factor 1 (IGF-1) .  It is unknown, however  how salinity regime may influence the sex-specific regulation of growth . The objective of this study was to  determine the  integrated  effects of salinity  regimen and daylight period  on  pituitary gh mRNA expression  in male and female Mozambique tilapia.

Tilapia  adults  were  reared in FW , SW and TR,  which is  characterized by salinities  that change between FW and SW every six hours, over a 24 h period. gh expression was greater in fish reared in SW and TR compared with those in FW, in both sexes .  Pituitary gh expression was also higher in fish sampled during dark hours, compared with those sampled in daylight hours.  Sex-specific differences in p ituitary gh expression was only observed in fish reared in SW and TR.  These results  indicate that sex-specific  patterns in gh expression  is modulated  by salinity regime and daylight period in Mozambique tilapia. [Supported by HATCH (#HAW02051-H), NOAA/ UH-Sea Grant (#NA14OAR4170071, R/SS-12), NOAA (#NA18OAR4170347),  NIH (1R21DK111775-01) and NSF (IOS-1755016)]