Biological Invasions

, Volume 15, Issue 3, pp 627–643 | Cite as

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

  • Lauren M. Fletcher
  • Barrie M. Forrest
  • James J. Bell
Original Paper


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.


Ascidian Dispersal Non-indigenous species Pelagic larval duration Propagule supply Tunicate 



We are grateful to Richard Piola, Grant Hopkins, Kirsty Smith, Weimin Jiang (Cawthron Institute) and James Brodie for their assistance with various aspects of the experiments and data analysis, as well as an anonymous reviewer whose helpful comments improved this manuscript. Sincere thanks is also expressed to New Zealand King Salmon Ltd, in particular staff at the Ruakaka Bay farm, who provided considerable logistic support for this project, as well as Aaron Pannell of Marlborough Mussels Ltd for access to the results of the monitoring program and valuable insight into the spread of this species. This work was funded by the New Zealand Ministry of Science and Innovation as part of Contract C01X0502 (Effective Management of Marine Biodiversity and Biosecurity). L. Fletcher was funded through a joint Cawthron Institute and New Zealand Tertiary Education Commission ‘Bright Futures Enterprise PhD Scholarship’.


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Copyright information

© Springer Science+Business Media B.V. 2012

Authors and Affiliations

  • Lauren M. Fletcher
    • 1
    • 2
  • Barrie M. Forrest
    • 1
  • James J. Bell
    • 2
  1. 1.Cawthron InstituteNelsonNew Zealand
  2. 2.School of Biological SciencesVictoria University of WellingtonWellingtonNew Zealand

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