Physiology of Finfish and Shellfish (2+1)

3.3.2.1 Olfaction

The sense of smell is important in almost all fishes. In fishes, the olfactory organs are still more sensitive of the 2 means of chemoreception. The olfactory or nasal organ of fishes is located on the dorsal surface of the snout. The olfactory receptors are usually located in olfactory pits i.e., the nasal sac which have incurrent and excurrent channels divided by a flap of skin. Water is induced to flow through olfactory pits, by movements of cilia within the pit by the muscular movement of the branchial pump, by swimming or by a combination of these. Odours are perceived when the dissolved chemical makes contact with the olfactory rosette, a multi folded epithelium rich in receptor cells, located in the olfactory pit. Fishes relying heavily on olfactory cues have elongated rosettes with many receptor cells located in elongate olfactory pits.

Olfactory stimuli are communicated to the olfactory lobe of the brain via the first cranial nerve. Species which rely heavily on olfactory information, such as anguillid eels, moray eels and sharks display oversized olfactory lobes. Certain eels with tiny eyes depend mostly on smell for location of food.
A few fishes such as some of the puffers have greatly reduced lobes consistent with the evolutionary loss of the olfactory organs and pits. These puffers probably rely entirely on sight for feeding. Odour also serves as an alarm system. Many fishes, especially various species of freshwater minnows, react with alarm to the body fluids produced by an injured member of their own species.

Olfactory cues have been shown to be of importance to salmon in locating their natal stream, once they are in the vicinity of the river mouth. Thus, salmon are thought to be imprinted with odours as pre-smolts and smolts migrate down to the rivers and streams to a larger body of water where they spend the majority of their adult life. In sharks and rays, the paired pits that lead into the internally folded nasal sac are usually on the ventral side of the snout and divided by a fold of skin into an inlet and an outlet. Studies on nurse sharks show a distinct gradient searching activity for food detection. By detection of a stronger chemical concentration in the olfactory rosette on one side of its head, the shark turns in that direction.