Responses of compound ascidian larvae to haloclines

Abstract

Larval swimming behavior in response to sharp laboratory haloclines was studied in 4 species of subtropical colonial ascidians: Eudistoma hepaticum, Eudistoma olivaceum, Ecteinascidia turbinata and Polyandrocarpa zorritensis. Larvae were observed as they swam upward from seawater of high salinity (33 or 28 ppt) into a layer of lower salinity. The vertical movements of larvae in columns with haloclines were compared with movements of larvae in unstratified control columns. Dilute seawater at 10 and 16 ppt acted as a barrier to tadpoles of all species swimming toward the surface. Seawater at 22 ppt also halted upward progression in all species except Eudistoma hepdticum. Seawater of 26 ppt was passed without difficulty by the larvae of all species. However, most larvae (with the exception of Eudistoma olivaceum) sank or swam out of 26 ppt sea water as soon as they arrived at the airwater interface, whereas larvae in 33 ppt control columns remained at the surface for significantly longer periods of time. There were interspecific differences in the larval responses, some of which could explain differences in the distances that various species penetrate into coastal lagoons. However, salinity was not a robust explanation for overall distributional patterns. The observed responses to haloclines could help explain how larvae are retained near parental habitats in the mouths of estuaries.