Abstract
Didemnid species are assessed as species with a high invasive potential for Australia and as such are listed as target species for both state and national monitoring programs. The presence of the sea squirt Didemnum perlucidum (Monniot, 1983) was first documented in Australia in 2010 and has since then been detected extensively throughout the state of Western Australia and in the Northern Territory. These detections have raised important questions as to the origin and potential impact of this species in Australia. The current study was initiated to review the current known global geographic range of D. perlucidum and to obtain specimens that could support molecular studies aimed at evaluating the potential origin of this species in Australia. Characterization of 5’ COI mitochondrial sequences from 286 specimens revealed a remarkably low level of genetic diversity across the current known range of D. perlucidum and the existence of one main widespread genetic haplotype. Such findings suggest that all locations sampled in this study may in fact represent introductions of D. perlucidum and that the natural native range of the species remains unknown. Our demonstration that specimens (n=187) originating from across a broad expanse of the Australian West Coast were comprised of a single haplotype also lends support to the hypothesis that D. perlucidum is a species that has been introduced recently into Australia. This hypothesis is supported by the fact that D. perlucium distribution in Australia is mostly confined to artificial structures, it has displayed invasive characteristics, and its presence is now being detected across an increasingly wide geographical area. Given the demonstrated low level of genetic COI variation across its known global distribution, lack of clarity around its native range, and limited availability of data on this species globally, we recognize the requirement for further work to more fully elucidate the exact origins and patterns of distribution of D. perlucidum in Australia. This study represents the most comprehensive mapping of the current global distribution of D. perlucidum conducted to date and will hopefully motivate further studies aimed at elucidating this species biology, origin, high-risk routes and impacts.
Highlights
The human-mediated transport of species outside of their native ranges is recognised to have dramatically increased during the last century due to industrialization, human population growth, and globalization (Elton 1958; Carlton 2011)
One sample collected in another previously-reported D. perlucidum location, Zanzibar in Tanzania (Monniot and Monniot 1997), was identified as Lissoclinium fragile (Van Name, 1902), a white colonial Didemnidae frequently mistaken with D. perlucidum during sampling
Sequences obtained from 182 samples collected at 13 sites in Western Australia (WA) and from five samples collected at one site in the Northern Territory (NT) were represented exclusively by a previously reported haplotype for WA (JQ731735) which we refer to for convenience as Hap1
Summary
The human-mediated transport of species outside of their native ranges is recognised to have dramatically increased during the last century due to industrialization, human population growth, and globalization (Elton 1958; Carlton 2011). Ascidians are among the taxa with the highest reported record of introduced species (Lambert 2002; López-Legentil et al 2015; Pagad et al 2015) and according to a risk-based approach, didemnid species are listed as target species for both state and national introduced marine pest monitoring programs in Australia. Given the absence of previous records of D. perlucidum in Australia, a country from which ascidian diversity has been extensively described (Kott 2005; Shenkar and Swalla 2011), and its demonstrated invasive characteristics (Smale and Childs 2012; Bridgwood et al 2014; Muñoz et al 2015), D. perlucidum is being managed in Australia as an introduced marine pest
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