Evaluation of water intake in seawater adaptation in eels using a synchronized drop counter and pulse injector system

A new system has been developed for continuous measurement of drinking rate in eels in which drunk water exteriorized via an esophageal fistula was re introduced into the stomach by a pulse injector synchronized with a drop co unter. Using intact fish (controls), esophagus-cannulated fish whose drunk water was drained (drained fish), and esophagus and stomach-cannulated fish whose drunk water was reintroduced into the stomach (reintroduced fish), t he validity of this system was examined by monitoring the changes in drinki ng rate and hydromineral balance after exposure to seawater (SW). In reintroduced fish, the SW exposure was followed by an immediate burst of drinking and a subsequent cyclic pattern of drinking. The drained fish exh ibited a similar pattern of drinking but at much higher rate. The plasma Na concentration and osmolality increased linearly for one day and then decre ased gradually to a steady SW level in 5-6 days in control and reintroduced fish. However, both parameters increased linearly for 4-5 days in drained fish until they died at plasma osmolality of ca. 500 mOsm. The initial incr ease in plasma Na and osmolality was steeper for a day in control and reint roduced fish than in drained fish. Hematocrit scarcely changed for one week in control and reintroduced fish, but it increased abruptly from the secon d day in drained fish, suggesting severe hypovolemia. These results show that the water and electrolyte balance of reintroduced f ish were normal as in intact fish after exposure to SW. Thus, the drinking rate measured by the current system may represent actual drinking. The pres ent study also provides first direct evidence to show that drinking plays a key role in SW adaptation in fish.