Human-mediated transport of invasive marine species, including ascidians, has been occurring for centuries (Carlton & Hodder 1995, Carlton 2003). The rate of non-native species introductions greatly increased during the 20th century with the dramatic increase in marine travel and the globalization of world economies (Cohen & Carlton 1998). Within the last thirty years, ascidians have invaded numerous habitats worldwide, often with severe consequences.
Every continent except Antarctica has been invaded by non-native ascidians (Lambert & Lambert 1998, Hewitt et al. 2004, Robinson et al. 2005, Rocha & Kremer 2005, Abdul & Sivakumar 2007, Gittenberger 2007). While no ascidians are known to have been introduced to Antarctica (but see Lewis et al. 2005, Lewis et al. 2006), some species have spread from the Antarctic biogeographical province (e.g., Lambert 2004). In recent years ascidian introductions have been occurring with increasing frequency. For example, by 1998 thirteen species of non-native ascidians were established along the California coast, most of which (61%) had arrived within the last 15 years (Lambert & Lambert 1998). Additional species have since invaded (Lambert & Lambert 2003, Bullard et al. 2007a). Similarly, at least five ascidian species have invaded New England within the last thirty years (Whitlatch & Osman 1999, Steneck & Carlton 2001, Bullard et al. 2007a) and four species have invaded Prince Edward Island, Canada within the last eleven years (Ramsay et al. 2007). Worldwide, most coastal areas have experienced similar levels of invasion. Some native species have also increased in abundance within their normal biogeographic boundaries (e.g., Bak et al. 1996).
Ascidians are highly invasive as a group, but some species pose a greater invasion risk than others. In describing the environmental tolerances of ascidians in Spain, Naranjo et al. (1996) divided common Spanish species into three groups: regressive, transgressive and tolerant. These groups roughly correspond to invasiveness. Regressive species (not likely invasive) are largely confined to natural surfaces and cannot tolerate stress, transgressive species (potentially invasive) are pioneering species that are rarely dominant, and tolerant species (likely invasive) thrive in many settings and are very tolerant of stress. Ten out of twenty-seven Spanish species (37% overall) were considered tolerant (Naranjo et al. 1996). Thus, an exceptionally high proportion of ascidians have the strong potential to become invasive.
Some invasive ascidians have been particularly successful and have developed cosmopolitan distributions. For example, Didemnum vexillum (previously Didemnum sp. A; Stefaniak et al. 2008) has undergone massive population explosions in North American, New Zealand, Europe and Japan (Coutts 2002a, Bullard et al. 2007a, Minchin & Sides 2006, Lambert & Stefaniak 2008, Stefaniak et al. 2008). Many other species including Styela clava (Berman et al. 1992, Coutts & Forrest 2005, Davis & Davis 2007), Ciona intestinalis (McDonald 2004, Nydam & Harrison 2007, LeGresley et al. 2008) and Botrylloides violaceus (Yamaguchi 1975, Lambert & Sanamyan 2001, Steneck & Carlton 2001, Gittenberger 2007) have similar global distributions.
Prince Edward Island, Canada (PEI) provides a striking model of ascidian invasions and their impacts. Non-native ascidians began invading PEI waters in 1997. Since then a total of four species have arrived, Styela clava (1997), Ciona intestinalis (2004), and Botrylloides violaceus and Botryllus schlosseri (both appearing in 2005) (Boothroyd et al. 2002, Ramsay et al. 2007). There is great concern that a fifth species, Didemnum vexillum, will invade in the near future. These ascidians have had tremendous negative effects on PEI's shellfish aquaculture industry. By overgrowing aquaculture equipment and organisms ascidians have made it difficult to process overgrown shellfish and have added significant physical weight to aquaculture gear (e.g., cages, lines, buoys, etc; Figure 1). Prior to the invasions mussel aquaculture contributed >$36 million to PEI's GDP; though the overall economic effects of the invasions have not been tallied, non-native ascidians have increased costs associated with aquaculture due to the additional maintenance, labor and processing required by fouled cultures (Department of Fisheries and Oceans Canada 2006). In 2003 it was estimated that the control of Styela clava cost $0.115 per pound of shellfish harvested (ACRDP 2003). Costs have increased as additional species have arrived. Paradoxically it was likely the initial success of the PEI aquaculture industry that first led to the establishment of the ascidians. The benthic environment of coastal PEI is mostly muddy and has relatively little natural hard substrate. The explosive growth of aquaculture during the 1990s (Department of Agriculture, Fisheries and Aquaculture, PEI 2003), led to a massive increase in the amount of substrate available for ascidian colonization (Locke et al. 2007). Once ascidians became established, transfers of infested aquaculture organisms aided their spread (Locke et al. 2007). Development of effective, economically viable control measures has been a challenge. Despite intense individual, scientific and governmental control efforts ascidian populations in PEI and Atlantic Canada remain in bloom conditions.
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