Effects of low salinity on metamorphosis in estuarine colonial ascidians

We studied the effects of brackish water on larval attachment, events of me tamorphosis, and juvenile mortality in three colonial ascidian species that live in a Florida coastal lagoon. Eudistoma olivaceum and Eudistoma hepati cum are restricted in their adult distribution to areas of relatively high and constant salinity near inlets, whereas Ecteinascidia turbinata extends more than 20 km into the Indian River, where salinity can be much more vari able. In all three species, metamorphosis proceeded more quickly at 33 ppt than at lower salinities. The thresholds for successful metamorphosis diffe red among species in a manner that corresponded to the adult distributions, with E. turbinata being capable of completing metamorphosis at salinities as low as 22 ppt, E. hepaticum as low as 24 ppt, and E. olivaceum as low as 26 ppt. Larvae of both Eudistoma species delayed settlement in very low sa linity water, whereas those of E. turbinata settled very quickly, then fail ed to complete metamorphosis. Juvenile mortality at salinities lower than 2 2 ppt was 100% for all three species. Survival in salinities higher than 22 ppt was strongly correlated with salinity in E. olivaceum and E. hepaticum , but not E. turbinata.