Abstract
The European sea bass (Dicentrarchus labrax) exhibits female-biased sexual size dimorphism (SSD) early in development. New tagging techniques provide the opportunity to monitor individual sex-related growth during the post-larval and juvenile stages. We produced an experimental population through artificial fertilization and followed a rearing-temperature protocol (~16°C from hatching to 112 days post-hatching, dph; ~20°C from 117 to 358 dph) targeting a roughly balanced sex ratio. The fish were tagged with microchips between 61 and 96 dph in five tagging trials of 50 fish each; individual standard length (SL) was recorded through repeated biometric measurements performed between 83 to 110 dph via image analyses. Body weight (BW) was modelled using the traits measured on the digital pictures (i.e. area, perimeter and volume). At 117 dph, the fish were tagged with microtags and regularly measured for SL and BW until 335 dph. The experiment ended at 358 dph with the sexing of the fish. The sex-ratio at the end of the experiment was significantly in favor of the females (65.6% vs. 34.4%). The females were significantly longer and heavier than the males from 103 dph (~30 mm SL, ~0.44 g BW) to 165 dph, but the modeling of the growth curves suggests that differences in size already existed at 83 dph. A significant difference in the daily growth coefficient (DGC) was observed only between 96 and 103 dph, suggesting a physiological or biological change occurring during this period. The female-biased SSD pattern in European sea bass is thus strongly influenced by very early growth differences between sexes, as already shown in previous studies, and in any case long before gonadal sex differentiation has been started, and thus probably before sex has been determined. This leads to the hypothesis that early growth might be a cause rather than a consequence of sex differentiation in sea bass.
Highlights
The phenomenon of sexual size dimorphism (SSD) is common in animal species, and it is represented by the differences in average body size of adult males and females [1]
We used a combination of these two tagging methods to efficiently identify the fish during the post-larval stage with microchips and during the juvenile stage with microtags
At the end of the experiment, the individual growth data were related to the sex to gain knowledge about early sex-related growth patterns in the European sea bass
Summary
The phenomenon of sexual size dimorphism (SSD) is common in animal species, and it is represented by the differences in average body size of adult males and females [1]. Female-biased SSD is linked to increased fecundity of larger females and decreased male-male competition [8]; it has been observed, among the others, in turbot Psetta maxima [9,10] and in the European eel Anguilla anguilla [11]. Female-biased SSD is a characteristic of the European sea bass (Dicentrarchus labrax L.), one of the major aquaculture species in the Mediterranean area. The females of this species are known to be about 30% heavier than the males from 300–400 g until over 1000 g [12,13]. Previous studies exploiting individual tagging suggested that females are already significantly heavier than males from 105 dph (1024 degree days above 10 ̊C), with a stable 40% difference from 197 to 289 dph [15]
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