Hormones are important signaling molecules produced and secreted in the endocrine system that show interesting close similarities between humans and salmon. They are transported to target organs where they bind to their receptors and control physiological regulation and behavioral activity to maintain homeostasis via feedback mechanisms. Various hormones control freshwater and seawater adaptations to maintain water and salt balances. The juvenile imprinting migration and adult homing migration of salmon are mainly controlled by the brain (thyrotropin-releasing hormone)-pituitary (thyrotropin)-thyroid (thyroid hormones) axis and the brain (gonadotropin-releasing hormone)-pituitary (gonadotropin)-gonad (steroid hormones) axis, respectively. This chapter describes hormone species and actions, hormonal control of freshwater and seawater adaptations, and hormonal changes during juvenile imprinting migration and adult homing migration in salmon.
TopIntroduction
Hormones are defined as signaling molecules that are produced and secreted by endocrine cells in endocrine glands and are transported by several circulatory systems to target cells in target organs, where they exert their functions by binding to specific receptors to control physiological regulation and behavioral activity in organisms. Hormones are the key mediators and integrators of environmental and internal information in organisms for the maintenance of “homeostasis”, which is the optimal steady state of internal conditions controlling various essential functions that support life in organisms. Hormones are classified by their secretion and transportation patterns and chemical structures and show close similarities between humans and salmon. Although some endocrine glands disappeared during the evolution of animals from life under water to life on land, many endocrine glands have essential common important functions in vertebrates.
Salmon show dynamic and complex life cycles; they are born in freshwater, undergo downstream migration into seawater as juveniles for growth, during which they develop seawater adaptability, and undergo upstream migration into freshwater as adults for reproduction, during which they develop freshwater adaptability. Moreover, salmon exhibit amazing natal stream imprinting and homing abilities during downstream migration in juveniles and upstream migration in adults, respectively. These dramatic physiological and behavioral changes are mainly controlled by many hormones secreted from various endocrine glands for freshwater and seawater adaptation, natal stream imprinting during downstream migration in juveniles and natal stream homing during spawning migration in adults. Using anadromous chum salmon and lacustrine sockeye salmon as the main species, the hormonal control mechanisms of seawater and freshwater adaptability as well as the hormonal changes associated with juvenile imprinting migration during downstream migration and adult homing migration during spawning migration are explained in this chapter.