Retinal development and the expression profiles of opsins genes during larvae development in Takifugu rubripes

The light sensitive capacity of fish larvae is determined by the structure of the retina and the opsins expressed in the retinal and non-retinal photoreceptors. In this study, the retinal structure and opsin expression patterns during the early development stage of Takifugu rubripes larvae were investigated. At around two days after hatching days (dah), the yolk sac of T. rubripes disappeared, the mouth was clearly visible and the larvae started swimming and feeding on rotifers. Histological examination showed that at 1 dah, seven layers were observed in the retina of T. rubripes larva, including the pigment epithelial layer (RPE), photoreceptor layer (PRos/is), outer nuclear layer (ONL), outer plexiform layer (OPL), inner nuclear layer (INL), inner plexiform layer (IPL) and ganglion cell layer (GCL). At 2 dah, optic fiber layer (OFL) can be observed, and all eight layers were visible in the retina. By measuring the thickness of each layer, opposing developmental trends were found in the thickness of ONL, OPL, INL and IPL, GCL and OFL. The nuclear density of ONL, INL and GCL and ratio of ONL/INL, ONL/GCL and INL/GCL were also measured and the ratio of ONL/GCL ranged from 1.9 at 2 dah to 3.4 at 8 dah and no significant difference was observed between the different developmental stages ( p > 0.05). No significant difference was observed for the INL/GCL ratio between the different developmental stages, which ranged from 1.2 at 2 dah to 2.0 at 18 dah ( p > 0.05). The results of quantitative real-time PCR showed that the expression of RH1 , LWS , RH2 , green opsin , SWS2 , rod opsin , opsin3 and opsin5 could be detected from 1 dah. These results suggested that the well-developed retina and early expression of the opsins of T. rubripes during the period of transition from endogenous to mixed feeding might critical for vision-based survival skills during the early life stages after hatching.