Larval behavior of the ascidian Ecteinascidia turbinata Herdman; an in situ experimental study of the effects of swimming on dispersal

Swimming and nonswimming tadpole larvae of the ascidian Ecteinascidia turbinata Herdman were followed in situ by divers to determine whether swimming affects larval dispersal. Swimming affected neither dispersal direction nor time spent in the water column. However, the dispersal rates and distances of swimming larvae were significantly lower than those of nonswimming larvae. Potential paths of larval dispersal were modeled with subsurface drogues. Movements of both swimming and nonswimming larvae differed consistently from surface flo w direction in these shallow (1.0–1.5 m) waters, indicating that caution should be used in modeling larval dispersal with drogues, particularly when larvae do not consistently remain in surface waters. In our south Florida study site, E. turbinata colonies were present only on unanchored mangrove prop roots and survival of colonies transplanted into surroundings habitats was very low. Drogue paths demonstrated that currents could potentially carry E. turbinata between mangrove islands, but behavior of the larvae suggests that dispersal is generally very localized with larvae settling near colonies from which they were released. This behavior differs dramatically from that reported for E. turbinata larvae in a more homogeneous habitat in the northern Gulf of Mexico. The short-distance dispersal of swimming tadpole larvae inthis study may represnt a local adaptation to favor recruitment near parental habitats and to prevent advection to inappropriate sites. Long-distance exchange between isolated islands probably occurs through rafting of adult colonies on fragmented mangrove roots rather than through larval dispersal.