Physiological adaptation of Oreochromis niloticus and O. aureus to salinity

Experiments in Ivorian lagoons on the tolerance of tilapias to brackishwater showed significant differences among species and hybrids of Oreochromis, particularly in their survival rate. To document these differences, the physiological adaptation to salinity was studied by choosing the species most tolerant to lagoon environment, O. aureus, and the least tolerant, O. niloticus. This study was conducted on populations from Côte d'lvoire (field strain: FS) and on a population of O. niloticus of the same origin; but cultured in laboratory conditions (laboratory strain: LS). The populations were transferred from freshwater (FW) to brackishwater (BW) both progressively (FW--> BW 10 ppt --> BW 20 ppt) and rapidly (FW--> BW 20 ppt--> BW 30 ppt). Four parameters were used to assess the adaptability to saltwater: mortality rate, plasma natremia, gill Na+/K+ -ATPase activity and the presence of gill interlamellar chloride cells. Results showed that O. niloticus (LS) is the only strain to adapt rapidly to 30 ppt while O. niloticus (FS) had difficulties tolerating the progressive transfer to 20 ppt. However, such a transfer was well-tolerated by O. aureus. Plasma natremia increased (15%) in each strain only when the maximum level of salt tolerance was approached. Mortalities occurred at 190 µM natremia. Gill Na+/K+ -ATPase activity was stimulated, but its kinetics was different from that of the natremia. Interlamellar chloride cells, apparently rare in freshwater, occurred after transfer to brackishwater. These cells were found in large numbers in O. niloticus (LS) and O. aureus. Based on the results, the following classification was established for progressive adaptation to salinity: O. niloticus (FS) < O. aureus (FS) < O. niloticus (LS). A number of hypotheses are derived from these findings. (Résumé d'auteur