| Light is a key environmental parameter which can influence fish throughout various stages of their life,from embryonic development to sexually mature of adults.The European sea bass(Dicentrarchus labrax)is a economically important fish in European and Mediterranean aquaculture industries,which introduced to China for aquaculture in 2010.The biology and ecology of sea bass are characterized by migration from shallow areas in coastal lagoons and estuaries,where they grow,to deeper zones in the open coast for reproduction.During the migration,the light condition of their surrounding environment is changed accordingly,and a better understanding of light environment required for each developing stage will help to improve rearing methods for European sea bass.Most previous studies of the influence of light on European sea bass mainly foucus on juveniles and adults.However,the light requirement of the larvae has not been thoroughly investigated until now.Therefore,in the present study,the larvae at 30 days after hatching(dah)were exposed to 12 lighting regimes(0.3,1.0,and 2.0W/m~2 full spectrum white,blue,red,and green light)for 66 days.This study aimed to assesse the effects of light spectrum and intensity on the post-larval growth,development,and survival of European sea bass,and to examine whether their light requirements changed at different developmental stage.Moreover,to obtain a more complete picture of how light conditions may affect larval performance in European sea bass,the differential expression patterns of stress-related,growth-related and circadian rhythm control-related genes were compared with the biometric parameters investigated in the experimental groups.Finally,histological and electron techniques have been used to examined whether variations in light affected the visual system of the larvae,including any negative impacts on the retina.The main results are as follows:1.By day 66 of exposure,growth performance was significantly better in larvae reared under green or red light compared with white or blue light(P<0.05),while it was significantly better at 2.0 W/m~2 compared with 1.0 or 0.3 W/m~2(P<0.05).Larvae reared under white or blue light had significantly more deformities(P<0.05)than did larvae reared under light of other spectra,while larvae reared under 0.3 W/m~2 light intensity had a significantly higher proportion of jaw malformations than reared under 1.0 and 2.0 W/m~2(P<0.05).D.labrax larvae subjected to green light showed significantly lower survival compared with the other light spectra(P<0.05).Moreover,both light spectrum and intensity significantly affected the expression of related genes(P<0.05).At day 66,aanat1a,aanat1b,mellc and tsh expression of the brain were significantly higher in larvae subjected to blue,green and red light compared with the white light(P<0.05).Mt1,mt2 and crf expression were significantly higher in larvae exposed to red light than those exposed to other light spectra (P<0.05).Gh expression was significantly higher in red and white groups than that in blue and green groups(P<0.05).Aanat1a,aanat1b,mellc,mt1,mt2,tsh and crf expression were significantly higher in larvae exposed to 2.0 W/m~2 compared with those exposed to 0.3 and 1.0 W/m~2.Significant differences were also found in the expression of gh,with the higher levels being observed under 0.3 and 1.0 W/m~2(P<0.05).Therefore,the best growth performance under red and green light groups(or under 2.0 W/m~2)may be related to high melatonin,GH and(or)TSH secretion,the lower proportion of jaw malformations,and lower survival rate(in green group).2.Although light did not affect the total thickness(TT)of the retina,the thickness of the retinal pigment epithelium layer(PRE),photoreceptor layer(PRos/is),outer nuclear layer(ONL),and inner nuclear layer(INL),and the PRE:TT and ONL:TT ratios were all significant higher in larvae exposed to blue light than in larvae exposed to white light.Additionally,the thickness of PRE and ONL,and the RPE:TT and ONL:TT ratios of larvae exposed to 2.0 W/m~2 were significantly lower than in larvae exposed to 0.3 W/m~2.By contrast,the INL:TT ratio in larvae exposed to 2.0 W/m~2 was significantly higher than in larvae exposed to 0.3 W/m~2.Additionally,the INL and ganglion cell layer nuclei density of larvae exposed to 2.0 W/m~2 were significantly higher than in those exposed to 0.3 W/m~2(P<0.05).Transmission electron microscopy revealed different levels of abnormalities in the photoreceptor layers in all treatment groups.Although different levels of abnormalities in photoreceptor layers and differences in retinal structures were observed among treatments, it appears that the continuous LED light conditions used in the present study did induced damage to the photoreceptor cells,but that did not adversely affect the growth performance of the larvae.Given that fish can detect light intensity and spectral changes via their retinal and extraretinal photoreceptors,the effect of light on growth might occur via another signal pathway in extraretinal photoreceptors.The results obtained here also support the high plasticity of retinal development in response to altered environmental light.In summary,the current study highlights the fact that lighting conditions are crucial factors influencing European sea bass growth,development and survival.These results should be considered when designing rearing protocols for European sea bass larvae in aquaculture systems. |
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