Thermal thresholds and critical period of thermolabile sex determination in two atherinid fishes, Odontesthes bonariensis and Patagonina hatcheri

Atherinidae is a family of fishes known to include at least three species with documented thermolabile sex determination. In Odontesthes bonariensis, fish exposed to about 17°C from hatching to juvenile stage become all-female, whereas groups exposed to about 25°C become male-biased. In contrast, sex in another atherinid, Patagonina hatcheri, appears to be under stronger genotypic control because the sex ratios are always balanced (1:1) within the same range of temperatures. The main purpose of this study was to determine the effect of temperature on the sex ratios of both species over their full range of viable temperatures. In addition, we determined the critical time of thermolabile sex determination in O. Bonariensis at female- and male-forming temperatures. O. Bonariensis larvae survived at temperatures between 13 and 29°C, but growth was negligible at 13 and 15°C. The proportions of females were 100% at 13–19°C, 95% at 21°C, 81.2% at 23°C, 29.4% at 25°C, 10% at 27°C, and 0% at 29°C. These results indicate a gradual transition from female- to male-forming temperatures evenly spread over the range of viable temperatures and hence the absence of any marked thermal threshold for thermolabile/genotypic control of sex; they also suggest weak or no genotypic control of sex in this species. In addition, prolonged exposure to 29°C caused disappearance of germ cells. The critical time of thermolabile sex determination in O. Bonariensis was estimated to be 3–5 wk, 2–4 wk, and 1–4 wk after hatching at 17, 19, and 27°C, respectively. Survival of P. Hatcheriwas limited to temperatures between 13 and 27°C (only 8.7% at the latter temperature) with slow but steady growth at 13 and 15°C. The proportions of females were 88.9 and 89.5% at 13 and 15°C, respectively, about 50% at temperatures between 17 and 23°C, and 30.8% at 25°C, hence, showing one clear threshold for thermolabile/genotypic control of sex at 15–17°C and possibly another at or around 25°C. These results confirmed that gonadal sex in this species is under genotypic control at intermediate temperatures but also showed that this control can be overridden at low (and possibly also at high) temperatures. J. Exp. Zool. 278:167–177, 1997.© 1997 Wiley-Liss, Inc.