Larval dispersal in the ascidian Ciona intestinalis (L.). Evidence for a closed population

Dispersal in ascidians is influenced by the reproductive mode and the mobility of a non-feeding larva. All available evidence indicates that mechanisms have evolved to ensure settlement close to the adult population. Populations of the solitary ascidian Ciona intestinalis (L.) are generally found heterogeneously distributed both spatially and temporally, and in Scandinavian waters the species is usually confined to fjords and inlets. In a shallow terminal cove, Kertinge Nor, a patchy distributed Ciona population is found abundantly forming dense aggregates on the eelgrassZostera marina . The purpose of this study was to demonstrate that dispersal in a population of this solitary ascidian is limited and local. Our results showed that the Ciona population in Kertinge Nor is highly local and dispersal to the outside as well as influx of larvae through the narrow Kerteminde Fjord is most likely limited or non-existent, since recruitment only occurs in the cove proper. In model simulations with jellyfish grazing pressure some larvae may be retained in the plankton for dispersal but insignificant numbers of eggs and larvae were recorded in the field in regular plankton samples. On a smaller scale (eelgrass leaves), Ciona was found aggregated indicating epibenthic retention of eggs or egg-strings.