Regulation of plasma Na + and K + concentrations in five African species of Tilapia fishes

Abstract

1. 1. Five species of the genus Tilapia were collected from five localities in the vicinity of Nairobi, Kenya. Two species, T. alcalica and T. grahami, were obtained from concentrated “soda” lakes (Lakes Natron and Magadi respectively) while a second sample of T. grahami was obtained from a less concentrated “soda” lake. The plasma Na + and K + levels in these three groups of “soda” lake fish were compared with those in the three fresh water species ( T. zillii, T. nigra and T. leucostictd). 2. 2. The highest plasma Na + concentration was found in T. alcalica from Lake Natron which was significantly ( P<0·01) greater than in any other group. The two groups of T. grahami (from Lakes Magadi and Nakuru) had similar plasma Na + levels and were significantly greater than in all three fresh water species. 3. 3. Plasma K + levels were highest in T. alcalica and T. grahami (from Lake Magadi) and were significantly ( P<0·01) higher than in any other group. 4. 4. The plasma Na +/K + ratio was not related to ambient salinity. The ratio in T. zillii was significantly ( P<0·01) larger than that in T. nigra and that in T. grahami from Lake Nakuru was significantly greater ( P<0·01) than in the same species from Lake Magadi. 5. 5. Both T. grahami and T. alcalica showed high mortalities under laboratory conditions although T. alcalica appeared to be more successful. 6. 6. T. alcalica appeared to tolerate a marked loss of plasma Na + in fresh water conditions whereas T. grahami was more effectively able to maintain plasma Na + levels. 7. 7. Fresh water T. zillii and T. nigra succumbed within 48 hr to ambient salinities in excess of 2·5% NaCl. In 2·0% NaCl the fish had significantly ( P<0·01) higher plasma Na + levels than similar fish in tap water or in 0·6% NaCl but the levels were lower than the ambient Na + concentration.