Macro- and micro morphology of the olfactory organ of African bonytongue, Heterotis niloticus (Cuvier 1829), compared with other species of the family Osteoglossidae (Teleostei)

Osteoglossiformes (bonytongue fishes) possess many morphological specializations associated with functions such as airbreathing, feeding, and electroreception. The olfactory organ also varies among species, notably in the family Osteoglossidae. Herein, we describe the olfactory organ of an osteoglossid, Heterotis niloticus, to compare it with the olfactory organs of other osteoglossiforms. We demonstrate the presence of an olfactory rosette within the olfactory chamber. This structure consists of a short median raphe surrounded by olfactory lamellae, which possess dorsal lamellar processes. On the surface of the olfactory lamellae, there are secondary lamellae formed by the olfactory epithelium. Within the olfactory epithelium, two zones can be distinguished: parallel brands of sensory cells located in the cavities between the secondary lamellae and a nonsensory area covering the remaining part of the olfactory lamellae. The olfactory epithelium is formed by ciliated and microvillus olfactory sensory neurons, supporting cells, goblet cells, basal cells and ciliated nonsensory cells. Additionally, rodlet cells were observed. The results confirm large variability in terms of the olfactory organ of Osteoglossiformes, particularly of Osteoglossidae, and support the secondary lamellae evolution hypothesis within this family.