Staging of Atlantic halibut ( Hippoglossus hippoglossus L.) from first feeding through metamorphosis, including cranial ossification independent of eye migration

To determine developmental stages independent of eye migration, a highly plastic feature in Atlantic halibut, 180 sibling halibut larvae reared at an average temperature of 11.9 °C from first feeding and for a further 46 days were examined, cleared and stained for ossification and the cranial development was recorded. Morphological development and cranial ossification generally coincided. The order of ossification of cranial structures was: jaw structures, hyoid arch, opercular bones and structures of the neurocranium. The Frontale exhibited torsion correlated with eye migration, but calcification began earlier and full calcification was independent of ocular displacement. The appearance of ossified elements was used to group larvae into Stages 5–9, comprising premetamorphosis to climax metamorphosis, with significant morphometric differences between stages. The trajectory of juvenile development appears fixed by Stage 8. There was a linear relationship between stage and myotome height ( R 2=0.86) and stage and standard length ( R 2=0.80). The stage definitions were validated on two groups ( n=23, n=101) of commercially produced larvae. Because metamorphosis is protracted in halibut, use of these stages and especially myotome height should help standardize sampling and analysis between experiments and between producers.