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Abstract
Over the past three decades the colonial ascidian Didemnum vexillum has been expanding its global range, significantly impacting marine habitats and aquaculture facilities. What biological features make D. vexillum so highly invasive? Here, we show that juxtaposed allogeneic D. vexillum colony fragments (‘ramets’) may, initially, form chimeric entities. Subsequently, zooids of the differing genotypes within such chimeras coordinately retreat away from fusion zones. A few days following such post-fusion retreat movements there is further ramet fission and the formation of zooid-depauperate tunic zones. Using polymorphic microsatellite loci to distinguish between genotypes, we found that they were sectorial at the fusion zones and the subsequent ramet movements resulted in further spatial separation of the paired-genotypes indicating that the fusion events observed did not lead to formation of long-term, stable chimeras. Thus, movements of D. vexillum colony ramets from initial fusion zones lead to progressive segregation of genotypes probably minimizing potential somatic/germ-cell competition/parasitism. We speculate that relatively fast (≤10 mm/day) movement of D. vexillum colonies on substrates along with frequent, and perhaps unrestrained, transient allogeneic fusions play significant roles in this species’ striking invasiveness and capacity to colonize new substrates.
Highlights
Historic, and on-going, biological invasions that have accompanied human activities have resulted in many contemporary ecosystems including a significant percentage of non-native species (Ruiz et al, 1997; Kolar & Lodge, 2001; Arim et al, 2006; Ricciardi, 2007; Lowry et al, 2013)
This is true for organisms, where invasive species out-compete native species using competitive traits associated with space acquisition during inter-specific interactions (Lowry et al, 2013; Gamradt, Kats & Anzalone, 1997; Arens et al, 2011), while attenuating competitive intra-specific interactions (Tsutsui et al, 2000; Tsutsui, Suarez & Grosberg, 2003; Torchin et al, 2003; Mangla et al, 2011)
D. vexillum fragments were harvested from submerged man-made structures (0.3–1.5 m below sea surface) in the Nelson city marina (New Zealand: 41◦S, 173◦E) and maintained in the laboratory as described in Rinkevich & Fidler (2014)
Summary
On-going, biological invasions that have accompanied human activities have resulted in many contemporary ecosystems including a significant percentage of non-native species (Ruiz et al, 1997; Kolar & Lodge, 2001; Arim et al, 2006; Ricciardi, 2007; Lowry et al, 2013). Researchers have explored the ecological, genetic (Pérez-Portela, Turon & Bishop, 2012) and life-history traits of highly invasive species (Ruiz et al, 1997), with less emphasis being placed on behavioral (Holway & Suarez, 1999) and immunology associated traits (Lee & Klasing, 2004), including allorecognition processes (Payne, Tillberg & Suarez, 2004) This is true for organisms, where invasive species out-compete native species using competitive traits associated with space acquisition during inter-specific interactions (Lowry et al, 2013; Gamradt, Kats & Anzalone, 1997; Arens et al, 2011), while attenuating competitive intra-specific interactions (Tsutsui et al, 2000; Tsutsui, Suarez & Grosberg, 2003; Torchin et al, 2003; Mangla et al, 2011). To date, only a limited number of simple generalisations have emerged regarding those traits that favour the bioninvasive propensity of organisms (Kolar & Lodge, 2001; Arim et al, 2006; Hastings et al, 2004; Simberloff et al, 2013)
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