Extra-branchial pathways of salt exchange in a teleost fish

Abstract

1. 1. Effects of ligation of the dendritic organ of the catfish Cnidoglanis macrocephalus were examined at several salinities. 2. 2. Significantly reduced survival was found at certain salinities. 3. 3. The plasma sodium concentrations of experimental fish (dendritic organ ligated) were significantly higher than control fish (intact) in hyperosmotic salinities, and significantly lower in hypoosmotic salinities. 4. 4. In a number of instances urine taken by direct puncture of the bladder was shown to be hyperosmotic to the plasma. In one case the urine was also more concentrated than the external medium. 5. 5. The high osmotic concentration of the bladder-urine was not always due to high sodium concentration, indicating the presence of some other osmotically active constituent. There were instances, however, of bladder-urine more concentrated with respect to sodium than the plasma. 6. 6. It was suggested that in Cnidoglanis there may be several major pathways of sodium exchange with the environment.