CHANGES IN DENSITY, WEIGHT, CHLORIDE, AND SWIMBLADDER GAS IN THE KILLIFISH, FUNDULUS HETEROCLITUS, IN FRESH WATER AND SEA WATER

¹ Department of Biology, Dalhousie University, Halifax

1. The adjustment of the killifish, Fundulus heteroclitus, to fresh water involves significant changes in the swimbladder gas, chloride content, weight, and density of the fish. Adaptation to fresh water is complete after 24 hours.

2. The gain in weight is only temporary, returning to normal after 18 hours The gain is due to taking on water from the hypotonic medium. The subsequent loss may be the result of kidney excretion and inanition.

3. The chloride decreases markedly during the first 12 hours but some appears to be regained after 24 hours. This irregularity corresponds to the short period of weight gain and loss, and appears because the water taken on by the fish is included in the calculation of chloride on the basis of weight. After 4 days in fresh water the fish have lost approximately 60 per cent of their normal chloride content.

4. When the fish are first put into fresh water they sink immediately to the bottom because fresh water is less buoyant than sea water. In order to adjust to the change and regain normal buoyancy the fish deposit oxygen and, to a lesser degree, carbon dioxide into the swimbladder. The volume of gas measured at barometric pressure is greater after adjustment to fresh water, showing that the amount of gas in the swimbladder has increased.

5. All the above adjustments tend to decrease the density of the fish to approximately the density of the water within 24 hours.

6. When fish are replaced in sea water after two days in fresh water, they regain their normal chloride, density, and swimbladder gas within six hours, or four times as fast as the previous adjustment to fresh water. Possible reasons for this difference in rate of adjustment are discussed.