YOUNG, C.M.*; VASQUEZ, E.; SVANE, I.: Early Life-History Strategies of Sessile Invertebrates in Stratified Fjords and Coastal Lagoons

The larvae of sessile marine animals are faced with the difficult challenge of selecting a habitat that will be appropriate for the entire adult life. Many processes occurring in the water column prior to settlement help determine whether larvae will have the opportunity to make an appropriate settlement choice. Most ascidians are stenohaline marine animals that penetrate short distances into water of low or variable salinity. In coastal lagoons of Florida, the diversity of ascidians is high near inlets but decreases rapidly as a function of distance from the inlets. In laboratory pycnoclines, larvae of colonial ascidians become inactive when swimming into brackish surface layers. This causes them to sink toward the bottom where salinity is highest. Metamorphosis is delayed or inhibited in brackish water and species that are best able to metamorphose at low salinities are also able to penetrate into brackish portions of the lagoons. In Gullmar Fjord, Sweden, most ascidians are found below the permanent pycnocline and few are ever found in brackish “Baltic” water that lies above the pycnocline. Experimental transplants and lab incubations indicate that adults of Ascidiella spp. tolerate brackish water but that low salinity inactivates or kills larvae and inhibits metamorphosis. A dense band of Ascidiella aspersa found just below the pycnocline is explained by flotation of embryos and by larval responses to the pycnocline layer. The occasional appearance of Ascidiella scabra in shallow water appears to result from short-term mixing events which coincide with times when larvae are present in the water column. In both the fjord and lagoon system, the tolerances and responses of ascidians to low salinity change with ontogeny, and processes occurring in the embryonic and larval phases help position the adults in appropriate salinity regimes.