Visual system development and changes in hatching performance in hybrid grouper embryos under different light conditions

Light greatly impacts the performance of teleost embryos. In this study, the development of visual system and performance of Epinehelus moara × Epinephelus lanceolatus embryos under different light spectra (red, orange, green, blue and full spectrum) and dark conditions were assessed. The development of visual system was not completed during embryonic development, characterized by the absence of structural integrity in the retina, indicating the absence of visual photosensitivity. However, nonvisual system may play an important role in photosensitivity. Light conditions contributed significantly to embryo performance, including hatching rate, deformity rate and Wnt signaling pathway expression. There was no statistical difference in the oxygen consumption rate of embryos in different groups during heartbeat onset stage. Embryos in dark group had a significantly lower hatching rate than other groups. The hatching rate in full light spectrum group was significantly higher than red spectrum group, and no significant differences were observed among the other groups. The deformity rate of newly hatched larvae exposed to orange light was significantly lower than those under full spectrum and dark conditions. The mRNA expression level of frizzled2, dickkopf2, limb bud and heart homolog, and natural kermes dye inhibitor of Wnt signaling pathway 1 were detected. The results suggested that dark conditions could lead to the abnormal expression of the Wnt signaling pathway, characterized by significantly lower mRNA expression levels of these factors, which in turn may cause decreased hatchability. In order to increase the hatchability and performance of embryos, and reduce the deformity rate, orange light should be utilized and dark conditions should be avoided during the embryonic development of E. moara × E. lanceolatus.