THE ONTOGENY OF SALINITY TOLERANCE IN THE AMERICAN SHAD, ALOSA-SAPIDISSIMA

Larval and juvenile American shad (Alosa sapidissima) raised from eggs in the laboratory were subjected to biweekly 24-h seawater (35 ppt) c hallenges. There was no survival in seawater before 36 days post-hatch , and most mortalities occurred within 2 h of transfer. Twenty-four ho ur survival reached 89% in seawater at 45 days post-hatch (when larval -juvenile metamorphosis occurred), 96% at 58 days post-hatch, and 92-1 00% from 58 to 127 days post-hatch. Survival in seawater for 24 h was a good indicator of long-term survival and growth. Seawater tolerance was associated with gill development and increased gill Na+,K+-ATPase activity, which occurred at the onset of the larval-juvenile metamorph osis (3 months prior to the peak of migration). Gill Na+,K+-ATPase act ivity increased threefold in juvenile shad acclimated to 35-ppt seawat er, reached peak levels 5 days after transfer, and remained elevated w ith respect to controls. Plasma sodium and chloride increased 12 and 1 1%, respectively, within 48 h of seawater exposure. Plasma sodium reco vered to initial levels and plasma chloride stabilized at a level 10% higher than initial levels after 5 days. Ionic perturbations that occu rred at elevated salinities stabilized when gill Na+,K+-ATPase activit y increased.