Development of hatchability in halibut (Hippoglossus hippoglossus) embryos

Halibut (Hippoglossus hippoglossus) eggs raised in darkness hatched between days 14.5 and 16 after fertilization. Eggs incubated in white light (3.2 microE/sr1/m2) remained unhatched, so that time of intra ovo development could be doubled. Photo-arrest of hatching was non-diapausal since embryonic growth continued. Transfer of photo-arrested eggs to darkness induced rapid and synchronous hatching. This procedure allowed analysis of development of hatchability. Hatching was not observed prior to day 14. Nonsynchronous hatching over three days was seen when eggs were induced on day 14 + 1 h, or on day 14 + 9 h. However, darkness-induction on day 14 + 22 h produced synchronous hatching within 140 min. This high rate of inducibility persisted until day 18, before declining slowly. Hatching-induction was not observed beyond day 22. Low hatchability in long-term photo-arrested embryos apparently reflects a loss of the anatomical prerequisites for the rim-hatching mechanism. Altered hatchability and morphogenesis after prolonged intra ovo development indicate that hatching in halibut is possible only at an early, defined ontogenetic stage.