Beyond Terrestrial Carnivores: Olfactory Bulb Anatomy in a Semi-Aquatic Mammal

The organization of the olfactory systems remains poorly understood in mustelids, particularly in lutrines adapted to semiaquatic environments. This study provides the first detailed anatomical, histological, lectin-histochemical, and immunohistochemical characterization of the main (MOB) and accessory (AOB) olfactory bulbs of the Asian small-clawed otter (Aonyx cinereus). Serial transverse and sagittal sections were examined using Nissl and Tolivia staining, a panel of lectins, and neuronal and olfactory immunohistochemical markers. Macroscopically, the olfactory bulbs were prominent and elongated, although no distinct AOB could be identified. Histologically, the MOB displayed a well-developed laminar organization, abundant mitral and tufted cells, and a clearly defined lateral olfactory tract. By contrast, the AOB was small, located dorsocaudally within the olfactory peduncle, and lacked the canonical lamination observed in many other mammals. Its identification was supported by the selective binding of Ulex europaeus agglutinin and strong Gαi2 immunoreactivity in glomerular-like structures, whereas Gαo labeling was weak. LEA and STA labeled the superficial layers of both bulbs, while VVA and SBA recognized only scattered MOB glomeruli. Neuronal markers, including NCAM, NSE, NeuN, PGP 9.5, and β-tubulin III, confirmed the neuronal nature of the AOB, while calretinin strongly labeled its glomerular structures and projection neurons. These findings reveal a marked contrast between a highly developed MOB and a small, poorly laminated but neurochemically differentiated AOB. This organization indicates that the vomeronasal pathway is retained in A. cinereus, although in a structurally reduced form, and provides a comparative basis for investigating chemosensory evolution in semiaquatic carnivorans.