The suspected occurrence of an invasive didemnid ascidian in NSW.
Creese R, Glasby T, Gilligan D and O’Rourke B (2010) The suspected occurrence of an invasive didemnid ascidian in NSW. Presentation given at the 2010 Australian Marine Science Association Conference, 4 – 8 July 2010, University of Wollongong, NSW, Australia.
In early March 2010, an encrusting ascidian was reported from several wharf pilings in Twofold Bay on the far south coast of NSW. This ascidian bore a close resemblance to Didemnum vexillum, an invasive species that has caused major environmental and economic damage in North America, Europe and New Zealand. Because of this impact, the species had been listed on the Australian trigger list for marine pests, under the heading “exotic invasive ascidians of the genus Didemnum”. Initial taxonomic investigation, based on standard morphological features, suggested that the taxon from Twofold Bay might be D. vexillum, and an emergency response was initiated. Underwater surveys along the NSW south coast suggested that the taxon was limited in distribution to Twofold Bay. Here, it was recorded commonly on some wharf pilings and mussel farm infrastructure, but only occasionally on natural rock surfaces (and then growing only on mussels). Movement controls were put in place to reduce the risk of spreading the species to other locations in Australia and a control plan was developed (based on the New Zealand experience with D. vexillum). However, subsequent molecular testing based on the cytochrome oxidase subunit 1 gene (COI gene), in Sydney and at 2 overseas laboratories, ruled out D. vexillum as the species in Twofold Bay. Although a definitive identification was not possible from the molecular testing, examination of considerably more material and additional field investigations, strongly suggested that the species was neither exotic nor invasive. The emergency response has now been cancelled and replaced with a precautionary surveillance program.