The development of the olfactory organ in the rainbow fish, Nematocentris maccullochi, was studied using scanning and transmission electron microscopy; it was compared with the developmental process in other teleosts, especially in the closely related atherinids and cyprinodonts. The formation of the nares parallels that in atherinids, salmonids, cyprinids and heterosomats, but differs from that found in cyprinodonts. Another ontogenetic feature in which the olfactory organs of the rainbow fish and also of atherinids differ from those of cyprinodonts, is the occurrence of transitory kinociliary cells which disappear during the postlarval period. The divergent evolutionary pathways are discussed with reference to experimental investigations. During development, ciliated and microvillous receptor cell types occur. At the primary larval stage ciliated receptor neurons are exclusively present. At a later stage the microvillous type develops and becomes equal in frequency. Thus, the microvillous receptor represents a separate type of olfactory neuron and is not a progenitor of the ciliated receptor cell.

Development of the olfactory organ in the rainbow fish Nematocentris maccullochi (Atheriniformes, Melanotaeniidae).

This study investigated the relationship between olfactory morphology, habitat occupancy, and lifestyle in 21 elasmobranch species in a phylogenetic context. Four measures of olfactory capability, that is, the number of olfactory lamellae, the surface area of the olfactory epithelium, the mass of the olfactory bulb, and the mass of the olfactory rosette were compared between individual species and groups, comprised of species with similar habitat and/or lifestyle. Statistical analyses using generalized least squares phylogenetic regression revealed that bentho-pelagic sharks and rays possess significantly more olfactory lamellae and larger sensory epithelial surface areas than benthic species. There was no significant correlation between either olfactory bulb or rosette mass and habitat type. There was also no significant difference between the number of lamellae or the size of the sensory surface area in groups comprised of species with similar diets, that is, groups preying predominantly on crustaceans, cephalopods, echinoderms, polychaetes, molluscs, or teleosts. However, some groups had significantly larger olfactory bulb or rosette masses than others. There was little evidence to support a correlation between phylogeny and morphology, indicating that differences in olfactory capabilities are the result of functional rather than phylogenetic adaptations. All olfactory epithelia exhibited microvilli and cilia, with microvilli in both nonsensory and sensory areas, and cilia only in sensory areas. Cilia over the sensory epithelia originated from supporting cells. In contrast to teleosts, which possess ciliated and microvillous olfactory receptor types, no ciliated olfactory receptor cells were observed. This is the first comprehensive study comparing olfactory morphology to several aspects of elasmobranch ecology in a phylogenetic context.

Morphometric and ultrastructural comparison of the olfactory system in elasmobranchs: The significance of structure–function relationships based on phylogeny and ecology

The olfactory organ of juvenile clearnose skates (Raja eglanteria) was studied with the light and electron microscopes. The organ is ovoid in shape, and its free surface is complicated by the presence of some 20 lamellae. Each lamella has a folded surface lined by a typical neurosensory olfactory epithelium. Bipolar olfactory receptor neurons, ciliated sustentacular cells, and basal cells are the pre-eminent cellular components of the epithelium. Two types of receptor neurons, both bearing microvilli but not cilia, were identified. The type 1 neuron is similar to that previously described in other fishes. The type 2 neuron has a characteristic morphology justifying a separate description. Its dendritic knob is larger than that of type 1, and its microvilli, which are shorter and thicker, are straight and regularly arranged. Tight bundles of filaments provide a skeleton to each microvillus, and these filament bundles reach more than 5 μm down into the dendrite. Type 2 receptor neurons have a well-developed Golgi complex and sparse rough endoplasmic reticulum (rER), whereas type 1 receptor neurons have a less well-developed Golgi complex and a conspicuous sytem of rER lamellae. The mucous layer on the epithelial surface is provided by the secretion of goblet cells that are situated mostly in the peripheral regions of each lamella. Secretory granules in the sustentacular cells and glands in the lamina propria were not observed.

Microscopic structure of the olfactory organ of the clearnose skate, Raja eglanteria

Olfaction in vertebrates is the sense used in the processing of chemical information that is detected and transmitted to the central nervous system by bipolar neurons that compose the olfactory nerve (cranial nerve I). Gustation is the sense used in the processing of chemical information that is detected by taste cells and transmitted centrally by facial (cranial nerve VII), glossopharyngeal (IX), or vagal (X) nerves. Receptor cells of both systems must discriminate relevant chemical stimuli from background chemical “noise” that exists in the immediate environment of all vertebrates. This ability of chemoreceptors to act as “peripheral filters” by detecting and preferentially passing information concerning biologically important chemical stimuli occurs through the evolutionary development of different receptor molecules with different chemospecificities built into the membranes of the receptor cells. This chapter will review the general organization of the olfactory epithelium and taste bud structure in fishes and summarize recent physiological findings as they relate to the sensitivities and specificities of the “filtering elements” (i.e., transduction processes and types of receptor sites) of the receptive olfactory and gustatory receptor cells. For books specific to chemoreception in aquatic organisms, see Kleerekoper (1969), Mackie and Grant (1974), Hara (1982), and Hasler and Scholz (1983).

Peripheral Filters and Chemoreceptor Cells in Fishes

The structure of the receptor cells in the olfactory organ of the rays Raja clavata and Dasyatis pastinaca and spiny dogfish Squalus acanthias is similar to that of the olfactory cells in other Vertebrates, although their tops are invariably crowned by microvilli, not flagella. In the peripheral process of these cells of Elasmobranchs and some other vertebrates represent a special type of the olfactory receptors which originated as the result of differentiation of primary olfactory cells supplied both by flagella and by microvilli. In the olfactory epithelium of Elasmobranchs, together with the supporting cells crowned by cilia, numerous goblet cells were observed. These glands produce a mucus which protects the olfactory epithelium of Elasmobranchs from the direct contact with the sea water. Similar to that in all other vertebrates, this mucus represents an intermediate milieu for the molecules of odorous substances.

Fine structure of the olfactory organ of elasmobranchs (Elasmobranchii)

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Fine structure of the olfactory organ of elasmobranchs (Elasmobranchii)