Who's Hot and Who's Not: The Effect of Temperature on Sex Determination in Southern Flounder From the Texas Gulf Coast

In marine flatfish of the genus Paralichthys, temperature plays a large role in sex determination.  The most susceptible areas to temperature fluctuation are shallow water environments, particularly estuaries, which serve as essential nursery habitats for juvenile southern flounder.  While in the estuaries, juveniles develop, and sex is determined.  Southern flounder exhibit genotypic sex determination; however, the sex of females is highly influenced by temperature, which can result in phenotypic sex reversal.  The temperature-sensitive enzyme complex responsible for estrogen biosynthesis in vertebrates is aromatase cytochrome P450 (P450_arom), a critical component in ovarian differentiation.  In this study, mRNA expression of P450 _arom was used to identify presumptive males and females exposed to a gradient of temperatures (18, 21, 24, 27, 30 °C) beginning at 35-40 or 65-70 mm total length (TL).  This research identifies that sex is influenced by temperature between 35 and 65 mm total length (TL) and establishes that increases in temperature from 18 °C during this size range produce increasingly male skewed sex ratios in southern flounder from Texas.  The findings presented here are critical for optimizing production of females in culture and for developing stock enhancement programs of southern flounder in Texas.