Some Like it Hot: Temperature Selection during the Critical Period of Thermolabile Sex Determination in Pejerrey ( Odontesthes bonariensis )

Abstract

Introduction: Many fish species have thermolabile sex determination (TSD) and the sex ratios of wild populations may be influenced by the thermal conditions during the reproductive season. The pejerrey (Odontesthes bonariensis) is an Atherinopsid fish that shows a remarkable level of TSD at environmentally relevant temperatures [1]. Thus, gonadal sex is determined between 1 and 5 weeks after hatching and the frequency of males varies from 0% at temperatures below 19°C to 100% at 29°C [2]. As a first step to model the effects of temperature on the sex ratios of wild populations and to clarify the adaptive significance of TSD, we examined the thermal preference of pejerrey larvae during the time of gonadal sex determination and how sex ratios vary under a situation where larvae can freely select the temperature. Methods: Newly hatched pejerrey larvae were stocked in a horizontal thermal gradient consisting of five interconnected aquaria kept at constant temperatures of 17, 20, 23, 26, and 29°C. Larvae were able to move freely through the entire gradient. Similar amounts of live food (Artemia nauplii) were delivered to each aquarium three times a day regardless of the presence of larvae. Larval behavior was monitored 24-hour/day for 5 weeks using overhead cameras and the number of larvae and time spent at each temperature was calculated. Sex ratios were analyzed at the end of the experiments by light histology. Results and Conclusion: The behavioral analysis revealed a preference for high, masculinizing temperatures (26-29°C) and the resulting sex ratios were highly male-biased (80-100%). We also observed a diurnal rhythm of temperature selection, with fish preferring higher temperatures at night compared to daytime. These results provide important clues to understand the ecology of TSD for this species and predict the sex ratios of wild populations as a function of temperature. They could be also useful for modeling the effects of global warming on pejerrey resources.