Olfaction is the most primitive and important chemosensory system for detecting a wide variety of environmental changes and is involved in various functions required for survival. Salmon have an efficient olfactory system; their olfactory abilities are inferior to those of dogs but superior to those of humans. Salmon olfactory systems can discriminate seasonally and yearly stable compositions of dissolved free amino acids in their natal streams produced by biofilms in the riverbed, and salmon can imprint on single amino acids before and during smoltification and discriminate these amino acids in the spawning season several years later. This chapter describes the structure and functions of the olfactory systems in salmon, previous olfactory hypotheses for salmon homing, properties of natal stream odorants, and biochemical and molecular biological studies on salmon olfactory functions.
TopIntroduction
Olfaction, which is the most primitive sense of organisms, is the physiological capacity to detect various types of environmental information required for survival by chemoreception. More than 500 million years ago, organisms developed chemosensory systems to detect a wide variety of environmental changes (Hara, 1992). There are five essential sensor systems in vertebrates: audition (hearing), vision (seeing), somatosensation (touching), gustation (tasting), and olfaction (smelling). The numbers of receptor genes for each sensor are range from 1 for audition, to 3~4 for vision, 9 for somatosensation, 20~30 for gustation, and 150~1,500 for olfaction, indicate that olfaction can detect the most forms of environmental information. Fish living in water have both olfactory and gustatory systems to detect water-soluble cues. While gustation mainly involves the tasting of foods, olfaction is involved in a variety of functions: 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. As described in the previous chapter, hormones are secreted to adapt to environmental changes and to maintain homeostasis in cooperation with the ANS. Olfactory stimuli cause changes in hormone conditions. Conversely, hormonal changes also influence olfactory functions.