Natural dispersal mechanisms and dispersal potential of the invasive ascidian Didemnum vexillum

Over the past decade, several species of non-indigenous ascidians have had adverse effects on a range of coastal ecosystems, and associated industries like aquaculture. One such species, the colonial ascidian Didemnum vexillum, poses a threat to the highly-valued New Zealand green-lipped mussel industry, and there is interest in whether and to what extent its spread can be managed at a regional scale (<100 km). An important component in the decision-making process for managing human-mediated pathways of spread is an understanding of D. vexillum’s natural dispersal potential. Here we use a weight-of-evidence approach, combining laboratory and field studies, to assess the role of natural dispersal mechanisms in the spread of D. vexillum. Under laboratory conditions, >70 % of D. vexillum larvae remained viable and were able to settle and undergo metamorphosis successfully following an artificial delay of 2 h. Larval viability decreased with increasing delay duration, although 10 % of larvae remained viable following a 36 h delay. A field-based study documented larval dispersal from two discrete source populations, with recruitment consistently detected on settlement plates at 250 m from source populations at one experimental site. Exponential decay models used to predict maximum larval dispersal distances at this site indicated that dispersal greater than 250 m is theoretically possible (>1 km in some situations). That being so, we recognise that the successful establishment and persistence of populations will depend on a wide range of processes not taken into account here. Our findings are supported by surveillance of D. vexillum spread in the wider study region; there are a number of instances where the species established on artificial structures that were several kilometres from known source populations, at a time when intensive regional-scale management of anthropogenic vectors was underway. Collectively, our findings indicate that D. vexillum has the ability to spread further by natural dispersal than previously assumed; probably hundreds of metres to kilometres depending on the local hydrological conditions, which has important implications for the ongoing management of this pest species world-wide.