Effects of rearing temperature on sex differentiation in the European sea bass (Dicentrarchus labrax L.)

Abstract

Under culture conditions, sex ratios in the European sea bass (Dicentrarchus labrax L.) are consistently skewed in favor of males. Moreover, no heteromorphic sex chromosomes have been shown for this species. This bias towards males could be due to a possible effect of environmental factors on sex differentiation, as has already been described in other fish species. To test this hypothesis, sexually undifferentiated sea bass were reared under two different thermal regimes or two different photoperiodic regimes, resulting in four groups: Group SP = short photoperiod (9 hours light [L]:15 hours dark [D]), Group LP = long photoperiod (15 L:9 D), both reared under natural temperature (24°C, average); Group LT = low temperature (15°C), Group HT = high temperature (25°C), both reared under natural photoperiod (13.5 L:10.75 D, average). Treatments lasted 80 days, from 57 to 137 days post fertilization, coinciding with the time when gonads are more sensitive to the action of exogenous sex steroids. The resulting sex proportions were determined by periodic histological examinations of the gonads during the first 2 years of age. No females were found in group LT at any sampling, whereas in the other groups, female proportions never reached values higher than 27%. Moreover, sex differentiation in group LT was delayed with respect to the other groups, as no fish completed sex differentiation until 16.4 months of age, compared to the other groups in which some individuals completed sex differentiation at 9 months of age. In addition, growth was significantly (P > 0.05) delayed in group LT with respect to the other three experimental groups and especially marked during the first year of life. Furthermore, the low values for the gonadosomatic index (GSI), together with the decrease of the number of spermatogenic males, denote a delay in gonadal maturation with respect to the rest of the experimental groups. The results of this study suggest that temperature can play an important role in the process of sex differentiation and growth of the sea bass. However, the mechanism(s) underlying this sexual lability remain unknown. J. Exp. Zool. 281:207–216, 1998. © 1998 Wiley‐Liss, Inc.

A Polygenic Hypothesis for Sex Determination in the European Sea Bass Dicentrarchus labrax

“…Menu, personal communication) or of females (Arias 1980), but as a whole do not contradict the hypothesis of balanced sex ratios in the wild. The sea bass is an important species in Mediterranean aquaculture, and it appears that, in all aquaculture populations, sex ratios are strongly biased toward males (75-95%, e.g., Blazquez et al 1998;Saillant et al 2002Saillant et al , 2003a, which is a problem for farmers as males mature earlier and grow less than females. Temperature has been shown to have a major effect on sex determination in sea bass (Blazquez et al 1998;Pavlidis et al 2000;Saillant et al 2002;Mylonas et al 2005).…”

“…The sea bass is an important species in Mediterranean aquaculture, and it appears that, in all aquaculture populations, sex ratios are strongly biased toward males (75-95%, e.g., Blazquez et al 1998;Saillant et al 2002Saillant et al , 2003a, which is a problem for farmers as males mature earlier and grow less than females. Temperature has been shown to have a major effect on sex determination in sea bass (Blazquez et al 1998;Pavlidis et al 2000;Saillant et al 2002;Mylonas et al 2005). The effect of temperature is not fully understood, as two studies show an increased proportion of males with cold temperature (15°: Blazquez et al 1998;13°: Saillant et al 2002) while the other two studies show an increased proportion of females at 13°and 15° (Pavlidis et al 2000;Mylonas et al 2005).…”

Temperature-Dependent Sex Determination in Vertebrates

“…Low temperatures produce female-biased sex ratios, and high temperatures produce mostly males in studies by Pavlidis et al (2000) and Koumoundouros et al (2002), but the reverse pattern has been reported by Blázquez et al (1998b) and Saillant et al (2002). The reasons for such wide discrepancies in results are not clear.…”

“…In most cases, the percentage of females declines with increasing temperature (Table 2.1), although in one genus of flatfishes (Paralichthys) the percentage of females is maximal at intermediate temperatures, and in another species (Dicentrarchus labrax) different investigators have reported opposite results (Pavlidis et al 2000;Koumoundouros et al 2002;Blázquez et al 1998b;Saillant et al 2002). In Oreochromis niloticus, XX genotypes tend to be masculinized by high temperature, while YY genotypes tend to be feminized (Abucay et al 1999;Kwon et al 2002).…”

“…The existence of TSD has been well established in the European sea bass Dicentrarchus labrax (Blázquez et al 1998b), but results vary dramatically among studies. Low temperatures produce female-biased sex ratios, and high temperatures produce mostly males in studies by Pavlidis et al (2000) and Koumoundouros et al (2002), but the reverse pattern has been reported by Blázquez et al (1998b) and Saillant et al (2002).…”