Elsevier

Theriogenology

Volume 88, 15 January 2017, Pages 145-151
Theriogenology

Research article
Water temperature influences growth and gonad differentiation in European sea bass (Dicentrarchus labrax, L. 1758)

https://doi.org/10.1016/j.theriogenology.2016.09.028Get rights and content

Abstract

The effect of rearing temperature on gonad differentiation and growth was evaluated in sea bass (Dicentrarchus labrax). One control group (CG, n = 60) and two experimental groups (EG1, n = 30; EG2, n = 30) were selected. CG was reared at 15.0 °C during 10 days post-hatch (dph) and at 19.0 °C throughout the remaining larval and post-larval development. EG1 was exposed to 14.5 °C from 1 to 50 dph, followed by an increase to 20 °C until sampling (176 dph). EG2 was exposed to 14.5 °C from 1 to 37 dph, followed by an increase to 20 °C until sampling (226 dph); 30 fish from CG were randomly sampled at 176 dph (CG1, control for EG1) and 30 fish were sampled at 226 dph (CG2, control for EG2). Weight, total and fork length measurement, and gonad collection were performed. All biometric indices were higher in EG1 and EG2 than in CG1 and CG2, respectively. Histologic analysis reported 100% of undifferentiated gonads in CG1 and; EG1 reported 80% of undifferentiated gonads and 20% of gonads at early differentiation toward the male line. CG2 reported 34% of undifferentiated gonads, 10% of gonads at early differentiation toward the female line, 23% of gonads at early differentiation toward the male line, and 33% of gonads with intratesticular oocytes. EG2 reported 47% of gonads differentiated in testis, 33% gonads differentiated in ovary, and 20% of gonads with intratesticular oocytes. These results suggest that water temperature plays an important role in the process of gonad differentiation and growth of sea bass.

Introduction

Sex differentiation in fish is controlled by major sex-determining genes that, in turn, are affected by other minor genes; however, in contrast with other taxa, sexual development in teleosts is protracted and plastic. Genotypic and phenotypic sex does not necessarily coincide, and, in several species, interactions among the genome, environmental conditions, and internal factors may determine sex, which suggests that fish may have a polygenic sex-determining system [1].

Under intensive aquaculture conditions, the sex ratio of cultured fish is consistently skewed in favor of males, and a significant proportion of precocious males is always present [2], [3]. This is undesirable for the fish farmers because male fish exhibit reduced somatic growth, resulting in an 18% to 40% smaller body weight at 2 years of age [3]. During the last 10 years, for scientific and economic reasons, research has been focused on the genetic and endocrine basis of sexual differentiation in European sea bass Dicentrarchus labrax (D. labrax) [4], [5]. All female stocks in D. labrax can be produced by direct estrogen treatment [2], [6]. Using direct estrogen treatment for aquaculture would raise consumer concerns. Thus, understanding environmental influences on D. labrax sex ratio has practical interest as a possible nonhormonal approach to produce highly female-biased stocks. Among the environmental factors implicated in sex differentiation, temperature is the most studied. Rearing temperature has been shown to affect sex ratios in sea bass, in a way that could be used to achieve a more desirable sex ratio (i.e., more females). The water temperature plays an important action on the metabolic pathways for steroid biosynthesis or on the brain that has been proposed to be the initial site of sex differentiation in fish [7], [8], [9]. Although many authors found that rearing temperature strongly affects sex ratios in sea bass, the results available in the literature have been equivocal. It has been demonstrated that high temperature during early life phases had a strong masculinizing effect, whereas low temperature had a feminizing effect [1], [10]. Low temperature, however, also had a strong masculinizing effect if long-term exposure was given during the period encompassing sex differentiation [2], [11]. The studies on the water temperature influence on sex differentiation in sea bass examined either long-term exposure [2], [11] or exposure immediately after fertilization [1], [10]. Unfortunately, exposure to very low water temperature immediately after fertilization may negatively influence the development of muscle fibers and, consequently, the swimming performance [12], whereas long-term exposure can reduce growth [2], [11], making both these approaches problematic for commercial aquaculture production. In all studies cited earlier carried out on sea bass, fish reared at high temperature attained higher weights than those reared at low temperature by the end of the treatment. Interestingly, in many cases, differences in size among treatments were no longer apparent when examined toward the end of the first year [1], [10], which prompted Mylonas et al. [13] to specifically search for, and find, compensatory growth; however, this only occurred for fish exposed to low temperature during the larval stages.

It is well established that understanding of gonadal differentiation processes in teleost fish can be advanced by histologic observations [14], [15]. On the basis of this knowledge, and considering that most of the studies carried out on sex differentiation in sea bass show genetic or hormonal evidence, the aim of this study was to evaluate the effect of the water temperature increase from 14.5 °C to 20 °C during the larval and post-larval phases on gonadal sex differentiation in Dicentrarchus labrax by histologic analysis and on growth rate.

Section snippets

Animals and rearing conditions

A total of 120 fingerlings sea bass (D. labrax) were selected and sampled from a Sicilian aquaculture center (36°43′10″92 N; 15°5′29″40 E). All fish came from a single, synchronous batch obtained from one population of broodstock of the same genetic origin maintained at the same aquaculture center.

Maturation and spawning occurred spontaneously under local photoperiod and temperature conditions. Released eggs passed through a water overflow pipe into a collector (500-L volume) equipped with a

Results

Body weight, total length, and fork length values recorded in D. labrax groups with the related statistical significances are reported in Table 2. In particular, all biometric indices were statistically significant higher in EG1 than in CG1 (P < 0.0001) and in EG2 than in CG2 (P < 0.001). The histologic analysis of gonad sections from fish reported undifferentiated gonads, both immature females and males, gonads fully differentiated, as well intersex subjects. In particular, gonad sections from

Discussion

Environmental conditions influence fish's phenotype during the entire life cycle, and among the environmental factors, water temperature during larvae developmental period can modify fish's external morphology [16], [17], rate of ontogeny [18], and sexual differentiation [1], [10], [19]. The phenotypic sex, as well as body size, can be altered by the thermal conditions prevailing during the period of primary sex differentiation, a phenomenon known as thermolabile sex determination [20], [21].

Acknowledgments

This study was partially supported by PON02 00451 3362185 INNOVAQUA—Innovazione Tecnologica a Supporto dell'Incremento della Produttività e della Competitività dell'Acquacoltura Siciliana.

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