Role of the pineal organ in the photoregulated hatching of the Atlantic halibut.

The timing of hatching in the Atlantic halibut (Hippoglossus hippoglossus) has been suggested to be regulated by environmental light conditions. However, the photosensory organ that perceives the triggering light has not been identified. In the present study, we investigated the morphogenesis of the pineal organ and the neurochemical differentiation of photoreceptors in the pineal organ and the retina of the Atlantic halibut during embryonic development. Immunocytochemical techniques were used for detection of integral protein components of the phototransduction process: opsins, arrestin (S-antigen) and alpha-transducin. We also studied the expression of serotonin (5-HT), a precursor of the neurohormone melatonin known to be synthesized by pineal photoreceptors. In the pineal anlage, opsin immunoreactive (ir) cells appear at 11 days post-fertilization (pf), arrestin, alpha-transducin and serotonin ir cells appear at 14 days pf; hatching took place 15 days pf. The retina contained no immunoreactive cells in embryos or in newly hatched larva. During this period, the pineal anlage is morphologically discernible only as a wedge-shaped region in the diencephalic roof, where elongated cells are aligned with their long axes converging toward a centrally located presumptive pineal lumen. The results show that the pineal photoreceptors contain serotonin and molecules involved in the phototransduction cascade before hatching. We suggest that the pineal organ has the capacity to perceive and mediate photic information before hatching in halibut embryos, and may thereby influence the timing of hatching.