Physical Factors Influencing Larval Behaviour in Three Species of Solitary Ascidian

Three solitary ascidians, Ciona intestinalis, Ascidiella aspersa and Stycla olava coexist in great abundance in Southampton Water, an estuary on the south coast of England. There appears to be zonation of these ascidians on constant-depth submerged substrata (S. clava above A. aspersa above C. intestinalis). As the adults are sedentary, it is suggested that larval zonation in the water column may play an important role in determining the adult distribution. The physical fectors gravity, light and hydrostatic pressure are proposed as the cues effecting pre-settlement larval zonation. Adult ascidians were induced to spawn and monospecific larval populations produced. The responses of these larval populations to light, gravity and hydrostatic pressure, individually and in combination, were determined. The results are used in a qualitative model to identify the depths at which larvae are most likely to congregate; the predictions are compared with the vertical distribution of larvae found in the water column and the distribution of juvenile recruits. Juvenile mortality was eliminated as a cause of adult zonation. Adult distribution can be explained by pre-settlement larval behaviour. C. intestinalis larvae were negatively buoyant, negatively geotactic and ambivalently phototactic; it is suggested that these larvae do not concentrate at a specific depth, but circulate in the water column and settle opportunistically. A. aspersa larvae were negatively buoyant, negatively geotactic and positively phototactic; these responses entrain the larvae in a circulation cell below 2 m depth and a buoyancy anomaly concentrates them near the top of the cell, at a depth at which adults are frequently found. S. clava larvae were negatively buoyant, negatively geotactic, and positively phototactic (with high barokinesis); these responses concentrate the larvae near the surface, where adults are found in abundance. Interspecific competition is minimised by larval zonation, permitting these three species to coexist