Effects of light intensity and photoperiod on the embryonic development of Sepia pharaonis.

Abstract

We investigated the effects of lumination on hatching of fertilized eggs of Sepia pharaonis, to reveal the best light conditions for its embryonic development. A single-factor experiment was carried out to examine the effects of different light intensities (10, 30, 50, 70, 90 μmol·m-2·s-1) and different photoperiod L:D (24 h:0 h, 18 h:6 h, 12 h:12 h, 6 h:18 h, 0 h:24 h) on the embryonic development. The results showed that the effects of light intensity on the hatching rate, fractured yolk sac rate, incubation period, mass of newly hatched larvae and mantle length was significant. There was no significant effect on hatching period and survival rate after hatching 7 days. With the increases of light intensity, the hatching rate, incubation period, mass of newly hatched larvae and mantle length first increased and then decreased, while the fractured yolk sac rate gradually increased. The optimum light intensity was 30 μmol·m-2·s-1. Exposed to this light intensity, the hatching rate, fractured yolk sac rate, incubation period, hatching period, mass of newly hatched larvae, mantle length and survival rate after hatching 7 days were (90.0±4.1)%, (7.3±1.5)%, (25.50±0.35) d, (8.10±0.89) d, (0.213±0.011) g, (1.013±0.022) cm, (97.1±4.0)%, respectively. The effects of photoperiod on the hatching rate, incubation period, hatching period were significant, but there was no significant effect on fractured yolk sac rate, mass of newly hatched larvae, mantle length and survival rate after hatching 7 days. With the increases of illumination time, the hatching rate and hatching period first increased and then decreased. The optimum photoperiod was L:D (12 h:12 h). When exposed to this photoperiod environment, the hatching rate, fractured yolk sac rate, incubation period, hatching period, mass of newly hatched larvae, mantle length and survival rate after hatching 7 days were (88.7±1.8)%, (8.7±1.8)%, (25.00±0.50) d, (7.00±3.20) d, (0.209±0.005) g, (0.998±0.026) cm, (96.8±7.1)%, respectively. In conclusion, embryo hatchability of S. pharaonis preferred to low light intensity (30 μmol·m-2·s-1) and normal photoperiod L:D (12 h:12 h). In production practice, sunproof measures should be taken to keep the eggs in weak light condition.