Morphology and Distribution of Olfactory Receptor Neurons in Southern Bluefin Tuna, Thunnus maccoyii (Teleostei):...

Olfactory receptor neurons (ORNs) of teleosts interact with waterborne odorants and serve as a crucial first step in detecting and transmitting the olfactory signal to the brain. Despite the significance of olfaction in feeding among pelagic teleosts, very little is known about the spatial distribution pattern of the density and types of ORNs on the olfactory lamellar surface. A scanning electron microscopy (SEM) investigation was conducted on southern bluefin tuna, Thunnus maccoyii, to quantify the types and density of ORNs in different parts of the olfactory lamellar surface. SEM revealed two types of ORNs: ciliated olfactory receptor neurons (cORNs) and microvillous olfactory receptor neurons (mORNs). The cORNs are dominant over mORNs with mean densities of 84,000 mm^-2 and 5000 mm^-2, respectively. Ciliated non-sensory cells (cNSs) are present at the periphery of the olfactory epithelium, with a mean density of 800 mm^-2. The mORNs display a distinct, heterogeneous spatial distribution, being highly concentrated in the rostral region of the lamellar surface, especially at the entry of the interlamellar space. Based on the hydrodynamics of the incoming flow of seawater within the interlamellar spaces and the topography of the olfactory epithelium, the rostral distribution of mORNs is organised to optimise interactions with water-borne stimuli. The mean density of ORNs on the lamellar surface of T. maccoyii (90,000 mm^-2) is one of the highest among teleosts reported to date, which may serve as an anatomical indicator of high olfactory ability.