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) challenges. There was no survival in seawater before 36 days post-hatch, and most mortalities occurred within 2 h of transfer. Twenty-four hour survival reached 89% in seawater at 45 days post-hatch (when larval-juvenile metamorphosis occurred), 96% at 58 days post-hatch, and 92-100% 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 metamorphosis (3 months prior to the peak of migration). Gill Na + ,K + -ATPase activity increased threefold in juvenile shad acclimated to 35-ppt seawater, reached peak levels 5 days after transfer, and remained elevated with respect to controls. Plasma sodium and chloride increased 12 and 11%, respectively, within 48 h of seawater exposure. Plasma sodium recovered to initial levels and plasma chloride stabilized at a level 10% higher than initial levels after 5 days. Ionic perturbations that occurred at elevated salinities stabilized when gill Na + ,K + -ATPase activity increased.