Morphological Variety of Fish Olfactory Organ in Terms of Phylogenetic Position and Adaptations to the Environment Illustrated by Lesser Spiny Eel Macrognathus aculeatus Bloch 1786 (Mastacembelidae, Teleostei) With Revision of Its Olfactory Organ Description

The morphology of the olfactory organ of fish exhibits considerable variability, which reflects taxonomical position and also adaptations to the diverse environmental conditions in which these animals live. The findings of studies on the representative of the Mastacembelidae family, Macrognathus aculeatus , provide a further illustration of this phenomenon. This species is a carnivorous, bottom‐dwelling fish that frequently buries itself in the substrate. The olfactory organ present in M. aculeatus consists of left and right elongated canals, each with an olfactory rosette. The olfactory canal, situated posterior to the tubular incurrent nostril, is lined with non‐sensory squamous epithelium. In the subsequent broad part of the canal, the epithelium forms an olfactory rosette. The structure is composed of a small median raphe and olfactory lamellae positioned on either side of the raphe. The olfactory epithelium, which lines the olfactory lamellae, contains several types of olfactory receptor neurons (ORNs). These include ciliated and crypt ORNs, as well as ORNs with long microvilli. On the level of the excurrent nostril, there is a vent of a voluminous ethmoid accessory nasal sac. We provide a revised characterization of the olfactory organ of M. aculeatus , complementing earlier descriptions.