Roles of hydrodynamics and larval behaviour in determining spatial aggregation in the tunicate Ciona intestinalis

Much attention has been paid in the literature to the importance of gregariousness of marine invertebrate larvae and its potential role in the formation of aggregated assemblages in the field. In the waters of the Swedish west coast Ciona intestinalis L. can be highly aggregated. Laboratory experiments showed that lanrae of C. intestinalis were not stimulated to settle and metamorphose in aqueous extracts of adult tunic, nor were settlement patterns on uniform, plane substrata aggregated. Field experiments with adult C. intestinalis and adult C. intestinalis mimics indicated that recruitment was dependent on the number of adults or mimics present (higher numbers yielded higher settlement). However, recruitment density around a given number of C. intestinalis was not significantly different from that around the same number of mimics. These results indicate that formation of aggregations of C. intestinalis in the field are probably the result of hydrodynamic processes rather than gregarious larval settlement. This result is in contrast to those obtained from similar investigations of other ascidian species.