Limited feed, ammonia, 17α-ethinylestradiol and temperature cause variable effects on growth and gene expression in the growth hormone-insulin-like growth factor-1 pathway in juvenile mummichog (Fundulus heteroclitus)

Fish may be exposed to multiple environmental stressors that are known to impact growth, including environmental changes or contaminants. This study aimed to examine the effects of limited food availability and elevated total ammonia or 17α-ethinylestradiol (EE₂) exposure under two temperatures (20 and 25 °C) on the growth of juvenile mummichog (Fundulus heteroclitus). Key genes in the growth hormone (GH)-insulin-like growth factor-1 (IGF-1) pathway were examined in muscle and liver tissues to explore their potential role in mediating growth. We conducted a 21-day experiment in which, compared to excess-fed fish, limited-fed fish exhibited decreased growth concomitant with reduced liver igf1 and muscle ghra expression. In two separate experiments of 21 and 33 days using NH₄Cl treatment to elevate total ammonia levels, expression of GH-IGF-1 pathway genes (igf1, igf1ra, igf1rb, ghra, ghrb) in liver and muscle was not affected similarly between exposures, despite significant reductions in growth compared with control fish. Growth (body weight, specific growth rate) of fish held at 25 °C was greater than 20 °C, with NH₄Cl inhibition of growth observed at both temperatures; however, the higher temperature attenuated the response. The temperature-induced increase in the growth of control fish did not affect the expression of GH-IGF-1 pathway genes after 21 days. The increased temperature resulted in lower actual EE₂ exposure concentrations, but EE₂ did not affect growth at either temperature. Overall, further studies are required to characterize the role of the GH-IGF-1 pathway in mummichog and the mechanism by which temperature and contaminants such as ammonia alter juvenile fish growth.