Since the work of Homer W. Smith (1930), it is generally accepted that teleosts in fresh water (FW) scarcely drink water; however, in seawater (SW) they drink continuously and absorb water together with monovalent ions from the intestine to compensate for osmotic water loss in marine environment. He used American eel (Anguilla rostrata) as an experimental animal, and described that “Eel is admirably suited to experimental work, and there is no reason to believe that results obtained upon it are not generally applicable to other marine or fresh water fi sh”. Eels can live in both FW and SW, thus being euryhaline teleosts. Therefore, eels have been used as an experimental material in osmoregulation research. In both FW and SW environments, eels can maintain plasma osmotic pressure and electrolyte concentration at levels largely independent of the ionic concentration and composition of their environments. In order to maintain water and ion balance, eels are equipped with specialized ion-and water-transport epithelia situated in the gill, kidney, esophagus and intestine likely as in other teleosts (for

Laboratory of Physiology, Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan. aE-mail: [email protected] bE-mail: [email protected] *Corresponding author

a more extensive review of fi sh osmoregulation see Marshall and Grosell 2006, Evans and Claiborne 2009). In euryhaline fi shes, the function of these osmoregulatory organs must change according to environmental demands, and various endogenous factors are known to play a signifi cant role as mediators in the necessary physiological adaptation (Takei and MacCormick 2012). In this chapter, we will review initially how eels obtain water from hypertonic SW after processing through the digestive tracts, and secondly how the intestinal absorption is controlled by endogenous regulators.