ABSTRACT
In teleost fi shes, including the eel, the composition of body fl uids is infl uenced by the environmental water, since, either in freshwater (FW) or in seawater (SW), the fi sh are in direct contact with the environmental medium via the body surface (Marshall and Grosell 2006). Nevertheless, body fl uid osmolality is maintained near a setpoint value that is typical of each species regardless of environmental salinity. The setpoint values are achieved by balancing the gain and loss of water and ions. In regard to water, in FW water excretion is more important than ingestion because these fi shes continuously face the osmotic gain of water. On the other hand, SW fi shes must acquire water constantly to compensate for the osmotic water loss across body surfaces. Although the relative importance of water ingestion is different between SW and FW fi shes, it is drinking that is a primary means of water intake both in SW and FW fi shes.
