The mechanism of adaptation to varying salinity the common eel and the general problem of osmotic regulation in fishes

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Abstract
Teleost fishes can live in fresh water (hypotonic) or in sea water (hypertonic). This paper deals (1) with weight changes and (2) with blood concn. changes during adaptation to one or the other environment. The normal fasting eel (An-guilla vulgaris) in sea water loses 0.2-0.5% of its total weight per day. If the esophagus is blocked by a balloon so that water can not be swallowed, the weight loss is 3.0-4.0% per day, and death intervenes in 3-4 days. In fresh water, blocking the esophagus produces .7% weight loss per day and the eel lives normally. On changing normal eels from fresh water to sea water there is an initial passive phase (10 hrs.) of weight loss due to osmotic adjustment followed by an active phase (40-50 hrs.) during which weight is gained and finally a steady state is reached at a lower weight level. Amytal delays the onset of the 2d phase prolonging the 1st phase and increasing the loss of weight. Fasting eels transferred from sea water to fresh water showed an initial gain in weight followed by a loss in weight and finally a steady state, again below that at the outset of the exp. Changes in blood serum concn. were detd. by measurements of freezing point depressions, chloride concns., and refractive indices. From these, total salt and total protein were computed. These values show that the water content of the serum is higher in fresh water than in sea water. The data of this paper further clarify the mechanism of adaptation in the eel. In fresh water a large volume of dilute urine is formed with retention of salts. In sea water very little urine is formed (high colloid osmotic pressure), water is swallowed, absorbed from the gut and excess salt secreted by the gills.