Numerous vectors facilitate the introduction, movement and spread of invasive ascidians. Due to the short-lived nature of ascidian larvae, most introductions likely occur when viable juveniles, adults or colony fragments are transported to new habitats. The most commonly cited transport mechanisms include shipping vectors (i.e., hull fouling and ballast water) and the movement of ascidians with aquaculture organisms and equipment; a more recently identified transport vector is the movement of epibiotic ascidians attached to mobile benthic organism such as crustaceans. Additional factors (e.g., abundant uncolonized artificial surfaces) may assist with the establishment of ascidians once they have been introduced to a region.
Because ascidian larvae generally remain in the plankton for minutes to hours (Svane & Young 1989, Lambert 2005a), few ascidian introductions are caused by natural larval dispersal. Although short-lived larvae significantly hinder long distance movement of ascidians, they provide an effective mechanism for local dispersal. Once ascidians have been introduced to a new location, high levels of local larval recruitment can allow them to quickly colonize surrounding surfaces and rapidly spread from their point of origin. High larval densities and recruitment rates have been documented in areas experiencing ascidian blooms. For example, Styela clava larvae have been found at concentrations of 560 m-3 in PEI, Canada (Bourque et al. 2007) and Ciona intestinalis had weekly recruitment levels of ~165 per 100 cm in Nova Scotia, Canada (Howes et al. 2007).
Shipping vectors, especially hull fouling, likely account for most ascidian introductions (Minchin et al. 2006, Locke et al. 2007, Dodgshun et al. 2007, Darbyson et al. 2008b). Boat hulls provide an ideal hard substrate for colonization by benthic organisms, including ascidians (Darbyson et al. 2008a; Figure 7). Once attached, ascidians move with their host ship and can spread from the hull to benthic surfaces or release larvae into the environment (Minchin & Gollasch 2003). Ascidian hull fouling is common. For example, Godwin (2003) found seven ascidian species (all non-native) attached to boat hulls during a survey of only eight ocean-going vessels in Hawaii. Both recreational and commercial vessels may contribute to the spread of ascidians, but slow moving, rarely cleaned vessels are of most concern. This is demonstrated by the fact that heavily fouled barges have been implicated in several ascidian introductions (Coutts 2002a, Locke et al. 2007, Coutts & Forrest 2007). Ascidians may also be transported to new areas when small boats are trailered between locations. For example, approximately 90% of one-year-old Styela clava attached to boat hulls survived out of the water for 48 h in daytime temperatures of ~30 oC (Darbyson et al. 2008a), suggesting that they could easily survive short term trailering.
Ship ballast water may play a role in ascidian introductions (Carlton & Gellar 1993). Viable ascidian larvae have been found in ballast water samples (Chu et al. 1997) and ballast water likely assists with short range ascidian transport (Lavoie et al. 1999). However, given the brief lifespan of ascidian larvae, it is unlikely that ballast water can cause long distance ascidian introductions unless reproducing adults are carried inside the ballast tanks. It could be possible for ballast water to transport viable colony fragments over long distances (e.g., Lambert 2005b, Bullard et al. 2007b).
Invasive ascidians can be transported with infested aquaculture organisms (e.g., Naylor et al. 2001, Rocha & Baptista 2008). For example, because ascidians readily grow on bivalve shells, unless fouling organisms are removed prior to shipment, attached ascidians will be transported along with fouled bivalves. Dijkstra et al. (2007a) speculated that Botrylloides violaceus and Didemnum vexillum may have been introduced to the Gulf of Maine when Pacific Oysters (Crassostrea gigas) were imported for aquaculture. On a more regional scale, intra-province transfers of mussels are thought to have helped establish populations of Styela clava, Botrylloides violaceus and Botryllus schlosseri throughout PEI, Canada (Locke et al. 2007).
Natural flora and fauna also serve as transport vectors for invasive ascidians. For example, ascidians are transported on detached eelgrass blades and may be rafted many kilometers in this way (Worcester 1994). Another recently identified ascidian transport mechanism is movement on crustacean carapaces. Bernier et al. (2008) discovered Botrylloides violaceus and Molgula sp. growing on several species of crustaceans including Atlantic rock crabs (Cancer irroratus), northern lady crabs (Ovalipes ocellatus) and American lobsters (Homarus americanus). Didemnum vexillum has also recently been found on lobster carapaces and hermit crab-carried gastropod shells on Georges Bank (Valentine personal communication). Attached ascidian hitchhikers may be deposited into new benthic habitats with molts and abandoned shells and larvae and gametes are likely released as the crustaceans move. Such movement might be considerable as American lobsters are capable of moving 100s km over several years (Cambell & Stasko 1986).
Additional factors may enhance the establishment and spread of invasive ascidians. Invasive ascidians are often more abundant on artificial than natural surfaces (Glasby et al. 2007) and man-made surfaces (docks, pilings, boat hulls, buoys, aquaculture equipment, etc) have become ubiquitous in marine habitats (Glasby & Connell 1999, Connell 2001, Minchin et al. 2006). Artificial structures may aid the establishment of invasive ascidians in several ways. First, artificial structures provide novel surfaces that may negate any "home court" competitive advantages possessed by native species (e.g., Tyrrell & Byers 2007). Second, man-made structures provide additional physical space that can be colonized by invasive ascidians (Locke et al. 2007). Third, if artificial structures are not present, ascidians may be prevented from invading native communities either because they are less likely to reach the area (fewer docks would support less boat traffic and thus less risk from hull fouling) or because limited free space is available for their establishment. For example, in Martha's Vineyard, Massachusetts native species (scallops, eelgrass and algae) adjacent to artificial surfaces with invasive ascidians were all fouled by ascidians, while scallops and plants near artificial surfaces without invasive ascidians were ascidian-free (Carman et al., 2008a).
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