Ph Profile along the Gut of the Silver Eel and Its Modification during Seawater Acclimation

It is generally accepted that the gut plays an essential role in the water economy of marine and seawater-acclimated teleosts, water losses being balanced by continuous ingestion of drinking water (e.g. Maetz, 1970). Desalting of the ingested sea water (SW) takes place in the oesophagus, which is very permeable to Na and Cl but relatively impermeable to water, and Na, Cl and water resorption continues in the intestine (Hirano, Morisawa, Ando & Utida, 1976; Kirsch & Meister, 1982). Since 1930, it has been believed that the intestinal fluids of marine teleosts are alkaline (Smith, 1930). This has since been confirmed by Hickman (1968) for the rectal fluid of SW-adapted southern flounders and by Oide (1973) for SW-adapted Japanese eels, whose intestinal fluid was shown to have an average pH of 8*7. Oide has also shown that an increase in the pH of the bathing medium results in greater water movement across everted eel intestine. Hence the alkalinity of intestinal fluid has been invoked as a possible factor involved in water and salt resorption in marine or in SW-adapted teleosts. However, the precise role, or the origin, of this alkalinity has not been elucidated. Moreover, pH measurements in samples collected from dead animals and exposed to the air should always be considered with extreme care, especially in this case, since alkaline pH values are somewhat uncommon in biological fluids. Our aim here is to report on the pH profile in the gut of silver eels (Anguilla anguilla L.) acclimated either to fresh water (FW) or to SW, using a flexible glass pH probe (Pharmacia Electronics microelectrode), in order to re-evaluate the importance of the alkaline nature of the intestinal fluid in ion and water absorption. Silver eels caught in Meuse river tributaries were kept in FW or in SW tanks at 20 °C for 3 weeks. The pH of the tanks was checked once a week and kept between 7-5 and 7-8. After decapitation, each animal was pithed and the microelectrode was immediately introduced into the oesophagus. The advancement of the pH probe along