Olfaction in fish

1. Recent progress in the studies on olfaction in fish, with particular emphasis on electrophysiological and behavioral responses to biological odors and related chemicals, is reviewed. 2. One of the most characteristic features in fish olfaction is that it takes place entirely in the aquatic environment. The carrier of stimulant molecules is not air but water; therefore, chemicals that are detected olfactorily by fish need not be volatile, but must be soluble in water. 3. The olfactory organs of fishes are diversely developed. At one extreme they are well developed (macrosmatic) such as in sharks and cels, and at the other they are poorly developed (microsmatic) such as in pike and stickleback. 4. The nasal cavity is lined with the olfactory epithelium, which is raised from the floor of the organ into a series of lamellae to make a rosette. The arrangement, shape and degree of development of the lamallae in the rosette vary considerably from species to species. 5. It is doubtful whether simple relation exists between the surface area of the olfactory epithelium and sensitivity to odors, since the sensory epithelium is not distributed uniformly over the surface of the olfactory lamellae. 6. The olfactory epithelium of fish, like other vertebrates, consists of three cell types: receptor cells, supporting cells and basal cells. 7. The receptor cell, which is a bipolar primary sensory cell, sends a slender cylindrical dendrite toward the surface of the epithelium and is directly connected with the olfactory bulb by its axon. The dendrite terminates in a minute swelling (olfactory knob) which bears a variable number of cilia. 8. The information from the receptor cell is conveyed into the olfactory bulb, the first relay station, where signals are processed and integrated. The dominant feature of the bulb is the synaptic contact between the primary and secondary olfactory neurones in the form of glomerulus. 9. All the available evidence points to a great acuity of the olfactory sense in many fish species both in the capability and discriminating odorous chemicals. However, much discrepancies exist among data obtained by behavioral and electrophysiological techniques mainly because of the lack of systematic investigations. 10. Electrophysiological studies of olfaction have been hampered by the extremely small size of the olfactory neurones. 11. A slow negative monophasic potential is induced in the olfactory epithelium when stimulated with odorous chemicals (electro-olfactogram, EOG). It is a graded nonconducted response, the time course and amplitude of which are closely related to the parameters of the olfactory stimuli. 12. When the nasal mucosa is stimulated with odor, the olfactory bulb develops a slow potential which is the activity of the secondary bulbar neurone, possibly a presynaptic potential generated in the glomeruli. 13. Infusion of odorous stimulants into the nares also induces large rhythmic oscillations in the olfactory bulb (induced waves); the spontaneous electrical activity (intrinsic waves) is immediately interrupted by oscillatory waves that are terminated on cessation of the stimulus. 14. A computer technique has been developed to analyze frequency contents of the olfactory bulbar responses. Preliminary experiments indicate that the olfactory bulbar activities are tuned specifically to active components of chemical stimuli. 15. The afferent responses in the olfactory bulb are controlled by influences originating in the higher central nervous system (centrifugal control). 16. Olfaction plays an important and sometimes decisive role in fish behavior such as procurement of food, recognition of sex, discrimination between individuals of the same or different species, defense against predators, parental care and orientation and migration. 17. Amino acids have been identified in the active fraction of natural food extracts which induce exploratory and feeding behavior in some species. 18. “Schreckstoff” or alarm substance, a pterin-like compound, released from injured skin of ostariophysan fishes causes fright reaction in other members of the same or related species (warning of danger). 19. “Pheromones”, secreted in skin mucus, act as chemical signals in recognition of individuals of conspecifics and in cohering schools. 20. Migrating salmon use olfactory cues to locate their homestream when coming in from the sea to spawn. Homing is not a genetically determined character, but depends on “learning” during the early life of salmon. 21. Structure-activity relationships of amino acids in olfactory stimulation have been established; effective amino acids inducing olfactory stimulation being simple, short and straight-chained, with only certain attached groups. 22. A hypothetical receptor site has been proposed which involves two charged centers, one cationic and one anionic, capable of interacting with ionized groups of stimulatory amino acids.