Ultrastructural study of the olfactory organ in adult and developing European sea bass, Dicentrarchus labrax

The olfactory organ of the European sea bass (Dicentrarchus labrax) in adults and during development has been studied by light microscopy and by transmission and scanning electron microscopy. This organ includes two cavities, each extended by an accessory sac and opening to the outside through two nostrils. It contains a rosette consisting of about forty lamellae. The olfactory epithelium is characterized by the presence of two types of receptor cells, ciliated or with microvilli, and numerous ciliated nonsensory cells. Rod cells, essentially found in the altered epithelia of farmed bass, and rodlet cells are also observed. The olfactory organ forms very early in the developmental process. Two olfactory pits holding both types of sensory receptors appear 24 h before hatching. The ciliated nonsensory cells only appear at the end of the endotrophic period, shortly before the mouth opens. Although it is rather unspectacular during the larval stage, the development of the olfactory organ is characterized at the start of the juvenile stage by three important events: the formation of the nostrils, the hollowing of the accessory sacs, and the development of the rosette. This is created by raising the floor of the cavity and forming successive folds, which are the lamellae where the sensory epithelium is found.