Fish olfactory organ

Muller J. and Marc R. (1984). Three distinct morphological classes of receptors in fish olfactory organs. J Comp Neurol 222, 482-495. 

Elasmobranchs have their paired olfactory organs on the ventral side near the mouth. As the fish takes the respiratory water current into the mouth, water passes through the olfactory sacs. Thus, elasmobranches use the respiratory water current for supplying the olfactory organ with waterborne stimuli. 

By contrast, bony fish have their olfactory organs on the dorsal side of the snout at some distance from the mouth. The olfactory system in fish involves the first (olfactory) cranial nerve, while the ninth (glossopharyngeal) and other nerves serve the sense of taste. 

FIGURE 5.2 The olfactory organ in fish, (a) The nostril positions in sculpin (Cottidae) b) nostril position in spiny eel (Mastacembelidae) (c) a skin flap separates in- and outflow, an arrangement typical for bony fish (here Catastomidae) (d) the olfactory lamellae are located in the floor of the olfactory capsule (here minnow, Phoxinus). (From C. E. Bond Biology of Fishes.)... 

Hamdani El H, Doving KB (2002) The alarm reaction in crucian carp is mediated by olfactory neurons with long dendrites. Chem Senses 27 395-398 Hamdani El H, Doving KB (2006) Specific projection of the sensory crypt cells in the olfactory system in crucian carp, Carassius carassius. Chem Senses 31 63-67 Hamdani El H, Doving KB (2007) The functional organization of the fish olfactory system. Prog Neurobiol 82 80-86... 

On the other hand, the anatomical situation is completely different in the olfactory system of fish. In the fish nose, there is only a single type of olfactory organ... 

The neural pathways mediating alarm responses were examined in the crucian carp (Carassius carassius L.). In these fish, two olfactory tracts convey information from the olfactory bulbs [adjacent to the olfactory organs (i.e., nostrils)] to other parts of the brain. One courses along the midline (the medial olfactory tract) and the other along the side (the lateral olfactory tract). The medial olfactory tract further divides into two bundles (the medial and the lateral bundles of the medial olfactory tract). Severing the medial bundle of the medial olfactory tract eliminated the alarm responses to skin extract, whereas severing the lateral bundle of the medial olfactory tract diminished the feeding behavior (Hamdani et al. 2000). 

It is true that most of the compounds that normally excite the olfactory organ in fish differ from those to which air breathing vertebrates are exposed, but there is no evidence that the basic transduction mechanisms in air and water differ significantly. It is known, for example, that the same odorants delivered in the aqueous phase, are as effective as when delivered in the vapor phase as judged by the slow voltage shift recorded when a macroelectrode tip was positioned in the nasal cavity of a box turtle during odorant stimulation ( ). 

Yamamoto, M., 1982, Comparative morphology of the peripheral olfactory organ in teleosts, "Chemoreception in Fishes," T. J. Hara, ed., Elsevier, Amsterdam. 

Breucker, H., Zeiske, E., and Melinkat, R., 1979, Development of the olfactory organ in the rainbow fish, Nematocentris maccullochi (Atherini-formes, Melantaeniidae), Cell Tis. Res., 200 53. 

In fish, both taste and olfactory stimuli are waterborne. However, taste involves the seventh, ninth or tenth cranial nerves, in contrast to the first cranial nerve for smell. Elasmobranchs have their taste buds in the mouth and pharynx, but in bony fish they occur around the gills, on barbels and pectoral fins, and also scattered over the rest of the body surface. They crowd particularly in the roof of the mouth, forming the palatal organ. The taste receptor cells are arranged as a bundle to form a taste bud. Like other vertebrates, fish have receptors for sweet, sour, salty, and bitter. For instance, goldfish reject quinine-treated food pellets (Jobling, 1995). Many fish species are particularly sensitive to acidic taste characteristics. The responses of fish to amino acids will be discussed in Chapter 12. 

In fish the olfactory sense is important, in a species-specific way, during various situations and behaviors. Chemical cues released from heterospecific and conspe-cific individuals provide information about the presence of food items, predators, competitors, mates and allow the recognition of kin (e.g. reviews Hara 1994 Olsen 1999). Several species of fish are dependent on the olfactory sense to detect sex pheromones during reproduction (review Stacey and Sorensen 2006). Pollutants can have effects on one or more links in the chemical communication chain between the individuals (see Fig. 26.2). The chemicals can affect the sender of the pheromone or the individual that detects the signal, the receiver. It is also possible that the pheromone is affected by biotic and abiotic factors, such as bacteria, organic materials and pH, when released into the water. 

Copper exposures at 20 pg/L or higher induce degenerating effects on the olfactory receptor cells in fish (Saucier and Astic 1995). Since it is a normal process that receptor cells are regenerating in the olfactory epithelium of fish and other vertebrates as long as basal cells are present, new functional olfactory cells will be continuously produced and the animal can recover its sense of smell (e.g. Zippel 1993). There will, however, be problems if the fish remains in contaminated water and the olfactory epithelium does not acclimate and protect the receptor cells from metal toxicity (e.g. by metal-lothioneins, mucus production). It has been shown that olfactory receptor neurons can be a transport route of metal ions and organic molecules to the olfactory bulbs and the brain in vertebrates, fish included, with severe disturbing effects on the function of the CNS (e.g. Tjalve and Henriksson 1999 Persson et al. 2002). 

A further advantage in using fish is anatomical. In air-breathing vertebrates the olfactory chamber extends from the respiratory airway in most fish, however, it is a separate organ divorced from respiratory functions. This feature, and the presence of an aqueous medium, allows us to place a conductivity electrode at the inlet and one at the outlet of the nasal chamber. If electrolytes are used as odorants, their arrival and departure from the chamber can then be measured by conductivity changes. Since conductivity is proportional to concentration we can specify odorant concentration, within known limits, close to the receptors - something which cannot be done with the Intact nasal chamber in air-breathing vertebrates. It is also possible to deliver the odorant in a way that closely imitates that in which it normally arrives ( ). 

We believe that the hormonal stimuli which have been found to affect fish physiology and behavior should be considered pheromonal in nature because they stimulate the olfactory system in highly specific manners at low concentrations and, when tested on the whole organism, often evoke specific, adaptive responses which do not appear to be learned (Sorensen, 1996). At least in fishes, hormonal products can exert special pheromonal actions on conspecifics without being specialized. This being the case, we define a sex pheromone as a substance, or mixture of substances which is released by an individual and which evokes in conspecifics a specific and adaptive reproductive response, the expression of which does not require specific learning. ... 

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