Comparative analysis of the olfactory organs in selected species of marine sharks and freshwater batoids

Abstract

We present comparative analysis of the micro- and macro-morphology of the olfactory organ of six neoselachians living in different habitats: two freshwater benthopelagic batoids, Potamotrygon motoro and Potamotrygon hystrix (Potamotrygonidae) as well as four species of sharks occupying the outer continental shelves and upper slopes Mustelus mustelus (Trikidae) and Scyliorhinus canicula (Scyliorhinidae), and deep-water Galeus melastomus (Scyliorhinidae) and Etmopterus spinax (Etmopteridae). Our research used light and electron (TEM, SEM) microscopes as well as macroscopic observations. The position of the nostrils in relation to the mouth varied in these species but all of them possessed elongate olfactory rosettes with primary lamellae arranged over two rows. The types of primary lamellae were distinguished based on their arrangement on the opposite sides of the median raphe as well as morphology of secondary lamellae and distribution of sensory and non-sensory regions of olfactory epithelium lining the lamellae; these were different between batoids and sharks. Analysis of olfactory epithelium revealed the presence of microvillous olfactory sensory neurons as the only type of receptor cells in all studied species whereas ionocytes were recognized only in sharks. All species possessed one type of goblet cells (Alcian Blue or Periodic Acid Schiff positives) except for P. motoro in which both types of goblet cells were present. Moreover, apical parts of non-sensory microvillous cells that lined the basal parts of the primary lamellae, possessed PAS-positive mucous granules; these were present in both species of batoids. This analogous region was abundant in goblet cells in the species of sharks studied here. Results indicated that differences between freshwater and marine species as well as between marine species occupying various habitat zones include mainly nostrils position, density of occurrence of ionocytes and goblet cells within olfactory epithelium as well as biochemical character of mucus.