This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Neuroscience. Please check back later for the full article.
Chemoreception is the physiological capacity whereby organisms detect the varied external and internal chemical information required for survival and is the most primitive sensory process. Fish living in water have gill, gustatory, and olfactory chemosensory systems that detect water-soluble chemical cues. Gill chemoreception detects changes in the levels of three respiratory gases: O2, CO2, and NH3. While gustation, which involves several taste receptor genes, is primarily involved in the tasting of foods, olfaction, which involves between 15 and 150 olfactory receptor genes, is involved in a variety of important biological functions such as food recognition, crisis avoidance (alarm action), individual identification, kin recognition, intraspecies (conspecific) recognition, dominance hierarchies, symbiotic behavior, territorial behavior, schooling behavior, sexual behavior, and migratory behavior. The olfactory functions are primarily controlled by hormones secreted from various endocrine glands that are the key mediators and integrators of external and internal information in organisms. Conversely, olfactory stimuli cause changes in hormone conditions.
Among the roughly 30,000 species of fish, only 165 species are categorized as migratory. Migratory fish possess many important biological functions that make possible the long-distance migrations between nursery habitats, feeding habitats, and spawning habitats that map their complex life histories. Salmon in particular are recognized for their amazing ability to memorize information related to their natal stream during downstream migration so that, after they travel thousands of kilometers in the ocean, over many years during feeding migration, they are able to use their homing abilities to migrate precisely to their natal stream for reproduction. Olfactory memory formation and retrieval of natal stream odors in salmon, which are primarily controlled by the brain-pituitary-thyroid and brain-pituitary-gonad hormones, respectively, are essential to imprinting and homing migration. Salmon olfactory systems can discriminate seasonally—and yearly—stable compositions of dissolved amino acids produced by biofilms in the riverbed in their natal streams. Recently, ocean and freshwater ecosystems may have been affected by climate change-related CO2-induced acidification that impairs olfactory-mediated neural and behavioral responses in migratory fish.