Colonial Ascidian Didemnum Vexillum Research Articles

Seasonal variability in a nascent population of a non-indigenous colonial ascidian (Didemnum vexillum) near Winchester Bay, Oregon

The non-indigenous colonial tunicate Didemnum vexillum plagues many shellfish aquaculture operations around the world by smothering crop and gear and displacing juvenile bivalves. There are two populations of D. vexillum in Oregon, but little is known regarding their spatial extent, seasonal dynamics, annual variability, reproductive biology, or ecology. SCUBA divers monitored the seasonal variation and spatial extent of the nascent D. vexillum population in its current foothold at Umpqua Aquaculture, a longline oyster farm adjacent to Winchester Bay, Oregon. From May 2011 to November 2016, divers performed biannual subtidal surveys to measure the cover of D. vexillum on anchor lines. We found that this population of D. vexillum exhibited extensive fluctuation in colony during this period, especially between fall and spring. With one exception, cover was greater in the fall than the spring, though this fluctuation cannot be explained by our proxies for salinity or temperature. Seasonally, on average, D. vexillum covered the greatest proportion of line between 5.5 and 6.0 m depth in both the spring (37.5 ± 18.8%) and fall (45.1 ± 16.1%). During individual surveys, the depth of maximum percent D. vexillum cover was more variable, spanning ~ 4.5–6.5 m depth. Overall, we document a net increase in D. vexillum cover at Umpqua Aquaculture between the first and last surveys during the 5-year period. This study is the first published multi-year analysis of D. vexillum in Oregon and represents the longest continuous monitoring period of an in situ population of D. vexillum to date.

High fusibility and chimera prevalence in an invasive colonial ascidian

The formation of chimeric entities through colony fusion has been hypothesized to favour colonisation success and resilience in modular organisms. In particular, it can play an important role in promoting the invasiveness of introduced species. We studied prevalence of chimerism and performed fusion experiments in Mediterranean populations of the worldwide invasive colonial ascidian Didemnum vexillum. We analysed single zooids by whole genome amplification and genotyping-by-sequencing and obtained genotypic information for more than 2,000 loci per individual. In the prevalence study, we analysed nine colonies and identified that 44% of them were chimeric, composed of 2–3 different genotypes. In the fusion experiment 15 intra- and 30 intercolony pairs were assayed but one or both fragments regressed and died in ~45% of the pairs. Among those that survived for the length of the experiment (30 d), 100% isogeneic and 31% allogeneic pairs fused. Fusion was unlinked to global genetic relatedness since the genetic distance between fused or non-fused intercolony pairs did not differ significantly. We could not detect any locus directly involved in allorecognition, but we cannot preclude the existence of a histocompatibility mechanism. We conclude that chimerism occurs frequently in D. vexillum and may be an important factor to enhance genetic diversity and promote its successful expansion.

Genotypic variability following fusion in the invasive colonial tunicate Didemnum vexillum

AbstractFusion to form a chimera has been documented in many marine invertebrate taxa, including poriferans, cnidarians, bryozoans, and colonial ascidians. Allogenic interactions in chimeric ascidian colonies vary widely across taxonomic groups but are poorly characterized in the invasive colonial ascidian Didemnum vexillum. The moderate level of discrimination expressed in the fusion–rejection response of D. vexillum suggests that there is some integration of cells beyond the fusion line in a chimeric colony. We tracked the shifts in representation of microsatellite alleles between fused colonies of D. vexillum to elucidate the extent of genotypic integration in fused colonies and the patterns of changes to the genotypic composition of colonies immediately following chimera formation. By genotyping colonies before and after fusion, we found that allogeneic fusion in D. vexillum may lead to genotypic changes beyond the visible fusion line. Alleles from one colony were found in multiple tissue samples in the chimera 7–10 days after fusion had occurred. In some instances, alleles that were in a single colony prior to fusion were lost following fusion. We observed multiple patterns of allelic change, including both the unidirectional transfer and reciprocal exchange of alleles between fused colonies. Our findings suggest that tissue or cells are exchanged following allogeneic fusion between colonies of D. vexillum and that the genotypic composition of chimeric colonies may be fluid.

Single zooids, multiple loci: independent colonisations revealed by population genomics of a global invader

Assessing genomic diversity and population structure of non-indigenous species is crucial to develop adequate management strategies. However, in species with scarce material for DNA extraction, applying genomic techniques can be a difficult task. Here we set a protocol for small DNA samples combining whole genome amplification (WGA) and genotyping-by-sequencing (GBS). This protocol was applied to the worldwide invasive colonial ascidian Didemnum vexillum using a single zooid per colony. WGA–GBS performance was tested using half zooids, providing empirical demonstration for genotyping reliability. We analysed 296 individuals from 12 localities worldwide including native and the main invaded areas. Polymorphic loci datasets generated by locality, area and globally, identified genetic differentiation at all levels. The two groups found in Japan, the native area, matched Cytochrome Oxidase I clades and were strongly differentiated at the genomic level suggesting reproductive isolation. Our genomic analyses confirmed that only one clade spread worldwide. We also detected some clones, always within the same locality. Genetic diversity was high in both the introduced and in the native area. Three independent colonisation events determined the global distribution of the species, although population pairwise comparisons within each introduced genetic cluster were significant. Human-mediated transportation seems to drive the distribution pattern of this species worldwide and regionally, as there is a lack of isolation by distance within introduced areas. Diverse and well differentiated populations point to a high expansion potential of this worrisome worldwide invader.

Population genomics revealed independent colonisation events of a global ascidian invader

Event Abstract Back to Event Population genomics revealed independent colonisation events of a global ascidian invader Xavier Turon1*, Maria Casso1, 2 and Marta Pascual2 1 Center for Advanced Studies of Blanes, Superior Council of Scientific Investigations, Spain 2 Department of Genetics, Microbiology and Statistics, Faculty of Biology, University of Barcelona, Spain Invasive species constitute a major threat to biodiversity and can cause high economic and ecological negative impacts. To assess genomic diversity and population structure of non-indigenous species it is crucial to develop adequate management strategies. We developed a protocol combining whole genome amplification (WGA) and genotyping-by-sequencing (GBS). WGA is suitable for small sized organisms, colonial forms with small zooids, or studies where only trace material (scales, hairs…) is available, as is often the case for elusive and/or endangered species. This protocol was applied to the worldwide invasive colonial ascidian Didemnum vexillum using a single zooid per colony to avoid any genetic heterogeneity (chimeras). The obtained reads were filtered and analysed using the GIbPSs toolkit to get the loci dataset. WGA-GBS performance was first tested using half zooids, providing empirical demonstration for genotyping reliability. The Percentage of Shared Genotypes (PSG) was calculated from the table of haplotypic genotypes to determine the reliability of the WGA-GBS protocol. Then, we analysed 296 individuals from 12 localities worldwide including native and main invaded areas. PSG identified some clones, always within the same locality, and only one sample per clone pair was kept for further analyses. Polymorphic loci datasets generated by locality, area and globally, identified genetic differentiation at all levels. The two groups found in Japan, the native area, matched Cytochrome Oxidase I (COI) clades and were strongly differentiated at the genomic level suggesting reproductive isolation and a speciation process. Our genomic analyses confirmed that only one COI clade spread worldwide. Genetic diversity and differentiation was high in the introduced localities as well as in the native area. MDS and STRUCTURE analyses showed that three independent colonisation events determined the distribution of the species, although all population pairwise comparisons were significant. Human-mediated transportation seems to drive the distribution pattern of this species worldwide and regionally, as there is a lack of isolation by distance within introduced areas. Diverse and well differentiated populations point to a high expansion potential of this worrisome worldwide invader. Acknowledgements We are indebted to all people that contributed samples for this study: Gretchen Lambert, Ian Davidson, Judith Pederson, and Gaku Kumano. We specially thank Carles Bori, Davide Tagliapietra, Marco Sigovini and Irene Guarneri for their help during sampling in Ebro Delta and Venice. This research was funded by project PopCOmics (TM2017-88080) from the Spanish Government. This is a contribution from the Consolidated Research Group “Benthic Biology and Ecology” SGR2017-1120 (Catalan Government). Keywords: invasive species, Population Genomics, genotyping-by-sequencing, whole genome amplification, Didemnum vexillum Conference: XX Iberian Symposium on Marine Biology Studies (SIEBM XX) , Braga, Portugal, 9 Sep - 12 Sep, 2019. Presentation Type: Oral Presentation Topic: Global Change, Invasive Species and Conservation Citation: Turon X, Casso M and Pascual M (2019). Population genomics revealed independent colonisation events of a global ascidian invader. Front. Mar. Sci. Conference Abstract: XX Iberian Symposium on Marine Biology Studies (SIEBM XX) . doi: 10.3389/conf.fmars.2019.08.00043 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 14 May 2019; Published Online: 27 Sep 2019. * Correspondence: Prof. Xavier Turon, Center for Advanced Studies of Blanes, Superior Council of Scientific Investigations, Blanes, Spain, xturon@ceab.csic.es Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Xavier Turon Maria Casso Marta Pascual Google Xavier Turon Maria Casso Marta Pascual Google Scholar Xavier Turon Maria Casso Marta Pascual PubMed Xavier Turon Maria Casso Marta Pascual Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Effect of substrate deployment timing and reproductive strategy on patterns in invasiveness of the colonial ascidian Didemnum vexillum

The colonial ascidian Didemnum vexillum is a high-profile marine invader, with a geographically widespread distribution after introductions to several temperate regions. D. vexillum has been the focus of several eradication and control programmes globally and the need for specific biological knowledge that relates to establishment processes, persistence, impacts and potential for spread remains. The present study describes temporal patterns of D. vexillum percent cover on experimental substrates over 1.5-years in relation to seasonality of substratum availability, in conjunction with key physical (i.e. temperature and sedimentation) and biological factors (i.e. interspecific competition) at two sites in New Zealand. Colonies showed large fluctuations in percent cover between the two study sites and with reference to timing of bare substratum availability. Colonies generally exhibited an initial lag phase, with peak levels of D. vexillum cover reached during the second summer or autumn post-deployment. The long-term competitive dominance of colonies founded from the reattachment of fragments, as opposed to ambient larval recruitment alone, was also investigated. Increases in colony size as a result of ambient recruitment alone were initially much slower. However, after 12-months colony cover exceeded that of plates inoculated with fragments, suggesting the benefit of the apparent competitive dominance conferred by fragment inoculation is restricted to the early establishment phase. This information will provide increased understanding of the population dynamics of this species, as well as assist in the implementation of effective management strategies through knowledge of environmental drivers of prolific infestations.

Efficient dispersal and substrate acquisition traits in a marine invasive species via transient chimerism and colony mobility.

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.

Mechanisms for invasion success by Didemnum vexillum (Chordata: Ascidiacea): observations versus manipulations

The widespread success of the invasive colonial ascidian Didemnum vexillum has led to descriptions of the species as fast-growing and competitively dominant, with few predators in its introduced range. However, recent experiments assessing the competitive strength and predation resistance of D. vexillum suggest that this ascidian is not a stronger competitor compared to other non-native colonial ascidians (e.g., Botrylloides violaceus, Botryllus schlosseri). To examine this apparent contradiction, juvenile D. vexillum colonies of varying size and age from an invasive population were exposed to competitive interactions with other epifaunal invertebrates and predation by mobile benthic invertebrates. Results indicate that both competition and predation had a significant negative effect on D. vexillum growth as well as on the composition of and percent cover of D. vexillum in 6- to 8-week-old experimental communities. In smaller colonies, predation also had a significant negative effect on D. vexillum survival. The discrepancy between field observations describing D. vexillum as a superlative competitive dominant and recent manipulative experiments indicating the opposite may be a function of the intrinsic life history characteristics of D. vexillum: (a) frequent colony fusion in non-native regions, (b) transport of larvae over relatively long distances via tendril detachment, (c) unique formation of colony mats on pebble substrates, and (d) calcified structures embedded in the tunic. The combined effects of these characteristics could make non-native populations of D. vexillum appear to be competitively dominant species with few predators, even though manipulative experiments on individual colonies suggest otherwise.

Automated detection of ncRNAs in the draft genome sequence of a colonial tunicate: the carpet sea squirt Didemnum vexillum.

BackgroundThe colonial ascidian Didemnum vexillum, sea carpet squirt, is not only a key marine organism to study morphological ancestral patterns of chordates evolution but it is also of great ecological importance due to its status as a major invasive species. Non-coding RNAs, in particular microRNAs (miRNAs), are important regulatory genes that impact development and environmental adaptation. Beyond miRNAs, not much in known about tunicate ncRNAs.ResultsWe provide here a comprehensive homology-based annotation of non-coding RNAs in the recently sequenced genome of D. vexillum. To this end we employed a combination of several computational approaches, including blast searches with a wide range of parameters, and secondary structured centered survey with infernal. The resulting candidate set was curated extensively to produce a high-quality ncRNA annotation of the first draft of the D. vexillum genome. It comprises 57 miRNA families, 4 families of ribosomal RNAs, 22 isoacceptor classes of tRNAs (of which more than 72 % of loci are pseudogenes), 13 snRNAs, 12 snoRNAs, and 1 other RNA family. Additionally, 21 families of mitochondrial tRNAs and 2 of mitochondrial ribosomal RNAs and 1 long non-coding RNA.ConclusionsThe comprehensive annotation of the D. vexillum non-coding RNAs provides a starting point towards a better understanding of the restructuring of the small RNA system in ascidians. Furthermore it provides a valuable research for efforts to establish detailed non-coding RNA annotations for other recently published and recently sequences in tunicate genomes.Electronic supplementary materialThe online version of this article (doi:10.1186/s12864-016-2934-5) contains supplementary material, which is available to authorized users.

Colonial ascidian Didemnum vexillum Kott, 2002 is an alien species in Peter the Great Bay (Sea of Japan)

In November 2013 in Vostok Bay (Peter the Great Bay, Sea of Japan), in the fouling of the experimental installation of the Mytilus trossulus mussel culture, mass settlements of colonial ascidian not previously observed in Peter the Great Bay were registered. The taxonomic identification of colonial ascidians is rather complicated, and frequently the morphological and genetic methods of the species determination do not give a clear result. The usage of molecular-phylogenetic methods allowed us to indicate the sequences of nucleotides obtained from three colonies of ascidians from Vostok Bay as inherent to the invasive species Didemnum vexillum Kott, 2002. This species is known as an invasive in aquaculture settlements of mussels and oysters in various regions of the World Ocean in different years.

Distribution of the non-indigenous colonial ascidian Didemnum vexillum (Kott, 2002) in the Bay of Fundy and on offshore banks, eastern Canada

The invasive colonial ascidian Didemnum vexillum (Kott, 2002) was initially discovered in the fall of 2013 near Parrsboro, Nova Scotia. A rapid response survey was conducted in April 2014 to confirm the presence of the species and to determine its distribution near the original detection site. Subsequent surveys between May and August 2014 examined the dredge contents collected during sea scallop stock assessment surveys on German Bank, northern Browns Bank, eastern Georges Bank and in the Bay of Fundy region. The presence of D. vexillum was confirmed at 22 of 30 stations sampled in four areas of the Minas Basin and Minas Channel region in the northern Bay of Fundy during the rapid response survey. The scallop surveys confirmed the presence of D. vexillum at 9 of 829 stations sampled in the Bay of Fundy region, including 7 in the Minas Basin, 1 off Digby Gut and another off the coast of Yarmouth. Due to the presence of a native species, D. albidum, in the region, a PCR assay was developed to distinguish D. vexillum from all other species in the region. Once the PCR assay was validated this assay was used to confirm all positive identifications in this study. Colonies overgrew rocks, bivalve shells and seaweeds or were retrieved as large dislodged fragments. They were in an overwintering state in April, but healthy, and observed to grow into dense mats in summer. In other regions of the world, D. vexillum has been reported to foul shellfish and aquaculture gear, smother benthic organisms such as the sea scallop (Placopecten magellanicus), and overgrow substrates, suggesting this new colonial invasive ascidian poses a potential threat to Eastern Canada aquaculture and commercial benthic fisheries.

Life history attributes of a global invader: factors contributing to the invasion potential of Didemnum vexillum

The colonial ascidian Didemnum vexillum has dispersed from its native Japan to many temperate coastal regions of the world, yet many features of its life history remain poorly studied. Using D. vexillum populations in the northeastern USA, several features of growth and sexual reproduction, as well as asexual reproduction via the unusual feature of detachment of larva-laden tendrils were examined from 2008 to 2011. Recruits of D. vexillum grown on PVC pan- els in Groton, CT, between July and September 2011 reached sexual maturity in 50 to 62 d. A nat- ural, overwintering D. vexillum population in Noank, CT, sampled weekly from 12 May to 14 July 2008 first had reproductive structures on 19 May; 7 wk later the first recruits were found on recruit- ment panels at the same dock. Examination of paired samples of encrusting and tendril growth forms collected on 22 July 2010 in Newport, RI, revealed no significant differences in densities of reproductive structures between growth forms. Fifty-nine percent of tagged tendrils in Westport, MA, detached in 2 wk. One hundred percent of tendrils reattached within 48 h to PVC panels in the laboratory, while only 1 in 80 tendrils tethered to bare substrates (rock or concrete) at nearby field sites reattached to the substrate. Tendrils tethered to natural rock were rapidly consumed by preda- tory snails. Further laboratory experiments found a minimum of 8 h of undisturbed contact with the substrate was necessary for D. vexillum tendrils to reattach. Overall, we found that the tendril growth form is an important factor in the population biology of D. vexillum because it increases sur- face area for feeding and reproducing zooids in a space-limited environment.

Population control to mitigate the spread of marine pests: insights from management of the Asian kelp Undaria pinnatifida and colonial ascidian Didemnum vexillum

The control of marine pest populations in transport hubs has the potential to contain or limit spread by minimizing the infection of vessels and other vectors. We draw on New Zealand experiences with eradication or population control attempts on two marine pests, the Asian kelp Undaria pinnatifida Harvey Suringar and the colonial ascidian Didemnum vexillum Kott, to evaluate the extent to which vector infection was mitigated by management efforts. For both species we compare two levels of effort: sustained control involving intensive regional-scale eradication programs, and shorter-duration less intensive efforts that we refer to as partial control. The mean monthly proportion of vessels infected by Undaria was ~1% in two regions where sustained control reduced densities in marine habitats to 1–5% of the pre-treatment population. By contrast, 39% of vessels were infected during a partial control effort, which was comparable to the proportion of vessels infected in four ports where Undaria populations were not managed. For Didemnum, a reduction of the population to < 0.1% of the pretreatment level almost completely negated vessel infection, to the extent that few vessels were implicated in the further spread of Didemnum while sustained control was in place. By contrast, partial population control led to limited benefit in terms of vector risk reduction. More importantly, a cessation or reduction in population control efforts resulted in relatively rapid vessel re-infection. Both case studies illustrate that intensive control of marine pest populations can greatly reduce the infection of susceptible vectors, but achieving such an outcome requires an indefinite commitment of resources, and may be impractical for multiple pest species or multiple infested locations. Although slowing the spread of marine pests can lead to demonstrable benefits, for population control to have greater practicality and stakeholder support, there is a need for socially acceptable and affordable control tools that are effective across relatively large spatial scales.

Seasonal Patterns of Ascidian Settlement at an Aquaculture Facility in the Damariscotta River, Maine

Biofouling ascidians represent a serious and costly problem for aquaculture. Although the subject of intensive study for the past decade, little is still known about the timing, intensity, and duration of ascidian settlement, especially in coastal Maine. To assess ascidian settlement, PVC panels were deployed in the Damariscotta River, Maine, on a weekly basis during the summers of 2007 and 2008. Long-term fouling panels were deployed from June to October 2007 to assess the development of mature fouling communities. Settlement of fouling organisms largely occurred during 2 seasonal phases. Early summer (May to July) was dominated by settlement of hydroids and bivalve molluscs; late summer and early fall (August to September) was dominated by ascidian settlement. Overall, ascidians accounted for ∼50–80% of total settlement. The solitary ascidian Ciona intestinalis dominated mature fouling panels, but had low and intermittent settlement levels. The invasive colonial ascidian Didemnum vexillum settled later in the year (starting at the end of July) than other taxa. Specific knowledge of settling patterns of biofouling organisms may provide growers with a means of determining opportune times for deployment of gear to avoid excess fouling.

Reproductive seasonality of the invasive ascidian Didemnum vexillum in New Zealand and implications for shellfish aquaculture

AEI Aquaculture Environment Interactions Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections AEI 3:197-211 (2013) - DOI: https://doi.org/10.3354/aei00063 Reproductive seasonality of the invasive ascidian Didemnum vexillum in New Zealand and implications for shellfish aquaculture Lauren M. Fletcher1,2,*, Barrie M. Forrest1, Javier Atalah1, James J. Bell2 1Cawthron Institute, Private Bag 2, Nelson 7010, New Zealand 2School of Biological Sciences, Victoria University of Wellington, PO Box 600, Wellington 6140, New Zealand *Email: lauren.fletcher@cawthron.org.nz ABSTRACT: The global spread of invasive fouling species poses a significant barrier to the development of shellfish aquaculture, which has led to a need to understand the biological characteristics of fouling species that underpin management. One such fouling species, the colonial ascidian Didemnum vexillum, has become a very successful invader in temperate marine communities worldwide, and is proving problematic in a number of aquaculture regions. To evaluate the scope for managing risks to shellfish aquaculture around seasonal reproductive patterns, we assessed recruitment and larval development of D. vexillum in relation to water temperature, over a 20 mo period at 2 locations in central New Zealand. Our findings indicate that the reproductive season for D. vexillum in New Zealand is considerably longer than comparable northern-hemisphere populations (at least 9 mo of the year compared with 3 to 5 mo in the USA). Reproductive patterns were strongly correlated with water temperature, with a 3 mo period during the winter months (surface water temperature <12&degC) when larval recruitment was not detected at our study sites. However, during that period, late-stage larvae were often present in tissue sections, suggesting that the species has the potential to recruit year-round, albeit at very low levels during winter. Information on the duration of the reproductive season as well as critical temperature thresholds for spawning will enable more effective risk management in relation to aquaculture industry practices (e.g. timing of seed-stock deployment), as well as assist in the wider management of this species. KEY WORDS: Biofouling · Non-indigenous species · Reproductive patterns · Temporal variation · Larval settlement · Recruitment · Tunicate · New Zealand Full text in pdf format PreviousNextCite this article as: Fletcher LM, Forrest BM, Atalah J, Bell JJ (2013) Reproductive seasonality of the invasive ascidian Didemnum vexillum in New Zealand and implications for shellfish aquaculture. Aquacult Environ Interact 3:197-211. https://doi.org/10.3354/aei00063 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AEI Vol. 3, No. 3. Online publication date: April 10, 2013 Print ISSN: 1869-215X; Online ISSN: 1869-7534 Copyright © 2013 Inter-Research.

The role of species richness and spatial resources in the invasion success of the colonial ascidian Didemnum vexillum Kott, 2002 in eastern Long Island Sound

Since it was first discovered in the northwest Atlantic in the late 1980s, the non-indigenous colonial ascidian Didemnum vexillum Kott, 2002 has become a common inhabitant of coastal marinas as well as deeper water sites. This species has also been found to have an expanding global distribution leading to questions about its ability to successfully invade into a variety of communities. The goal of the present study was to build on previous knowledge and to examine the possible mechanisms that allow Didemnum to successfully invade fouling communities in eastern Long Island Sound. In separate experiments, we examined whether species richness and spatial resources (i.e. open space) influenced invasion success. Communities at 2 sites, a protected marina and an open coast site, were compared. In the first experiment, small colonies of Didemnum were transplanted into communities that contained different species richness levels (0, 2, 4, 6 spp.) and in the second, small colonies of Didemnum were transplanted into communities that varied in the amount of species cover (0, 60, 50, 100%) that was present. The growth of Didemnum, as well as species initially present and any new species from recruitment was measured each week for approximately 10weeks. Communities with high amounts of initial open space, regardless of species richness, tended to result in increased abundances of Didemnum at the open coast site and initially at the protected marina. At the marina site, growth of recruits and resident species resulted in the rapid loss of space in all treatments in both experiments, which reduced the success of Didemnum. At the open coast site, without increased competition from recruiting species, Didemnum became more abundant and eventually dominated many of the communities in all treatments. Despite being a global invader, Didemnum appears to be a relatively poor competitor for spatial resources. Its dominance seems to depend on the amount of unoccupied space present within communities it's invading. That, coupled with high recruitment from a diverse community, can overwhelm the potential invasion success of this species.

First record of the colonial ascidian Didemnum vexillum Kott, 2002 in the Mediterranean: Lagoon of Venice (Italy)

Numerous colonies of the invasive colonial ascidian Didemnum vexillum Kott, 2002 have been found in the Lagoon of Venice (Italy) in 2012, overgrowing fouling organisms on maritime structures such as docks, pilings, and pontoons. This is the first record for the Mediterranean Sea. A survey conducted in July 2012 revealed that D. vexillum is present in the euhaline and tidally well flushed zones of the lagoon, whereas it was absent at the examined estuarine tracts and at the zones surrounding the saltmarshes. Suitable climatic, physiographic and saline features together with a high volume of international maritime traffic make the Lagoon of Venice a perfect hub for the successful

Natural dispersal mechanisms and dispersal potential of the invasive ascidian Didemnum vexillum

Over the past decade, several species of non-indigenous ascidians have had adverse effects on a range of coastal ecosystems, and associated industries like aquaculture. One such species, the colonial ascidian Didemnum vexillum, poses a threat to the highly-valued New Zealand green-lipped mussel industry, and there is interest in whether and to what extent its spread can be managed at a regional scale ( 70 % of D. vexillum larvae remained viable and were able to settle and undergo metamorphosis successfully following an artificial delay of 2 h. Larval viability decreased with increasing delay duration, although 10 % of larvae remained viable following a 36 h delay. A field-based study documented larval dispersal from two discrete source populations, with recruitment consistently detected on settlement plates at 250 m from source populations at one experimental site. Exponential decay models used to predict maximum larval dispersal distances at this site indicated that dispersal greater than 250 m is theoretically possible (>1 km in some situations). That being so, we recognise that the successful establishment and persistence of populations will depend on a wide range of processes not taken into account here. Our findings are supported by surveillance of D. vexillum spread in the wider study region; there are a number of instances where the species established on artificial structures that were several kilometres from known source populations, at a time when intensive regional-scale management of anthropogenic vectors was underway. Collectively, our findings indicate that D. vexillum has the ability to spread further by natural dispersal than previously assumed; probably hundreds of metres to kilometres depending on the local hydrological conditions, which has important implications for the ongoing management of this pest species world-wide.

Increased Inter-Colony Fusion Rates Are Associated with Reduced COI Haplotype Diversity in an Invasive Colonial Ascidian Didemnum vexillum

Considerable progress in our understanding of the population genetic changes associated with biological invasions has been made over the past decade. Using selectively neutral loci, it has been established that reductions in genetic diversity, reflecting founder effects, have occurred during the establishment of some invasive populations. However, some colonial organisms may actually gain an ecological advantage from reduced genetic diversity because of the associated reduction in inter-colony conflict. Here we report population genetic analyses, along with colony fusion experiments, for a highly invasive colonial ascidian, Didemnum vexillum. Analyses based on mitochondrial cytochrome oxidase I (COI) partial coding sequences revealed two distinct D. vexillum clades. One COI clade appears to be restricted to the probable native region (i.e., north-west Pacific Ocean), while the other clade is present in widely dispersed temperate coastal waters around the world. This clade structure was supported by 18S ribosomal DNA (rDNA) sequence data, which revealed a one base-pair difference between the two clades. Recently established populations of D. vexillum in New Zealand displayed greatly reduced COI genetic diversity when compared with D. vexillum in Japan. In association with this reduction in genetic diversity was a significantly higher inter-colony fusion rate between randomly paired New Zealand D. vexillum colonies (80%, standard deviation ±18%) when compared with colonies found in Japan (27%, standard deviation ±15%). The results of this study add to growing evidence that for colonial organisms reductions in population level genetic diversity may alter colony interaction dynamics and enhance the invasive potential of newly colonizing species.

Induced spawning and culture techniques for the invasive ascidian Didemnum vexillum (Kott, 2002)

The colonial ascidian Didemnum vexillum has become relatively widespread in New Zealand, since its initial discovery in 2001. Despite the potential economic and ecological impacts of D. vexillum, there are still considerable knowledge gaps surrounding its key biological attributes. The ability to obtain larvae and culture colonies in the laboratory is crucial to research into larval longevity and dispersal potential, and the factors affecting colony survivorship and growth. Here we present methods for spawning and culture of D. vexillum under laboratory conditions. A ‘light shocking without cycles’ technique was used to stimulate larval release in adult colonies, with > 500 larvae being produced from ~ 100 g of tissue at the peak of the reproductive season. Following release, the larvae were allowed to metamorphose and the juveniles were cultured under controlled conditions for four weeks. Recruit survival during the four weeks of culture was > 85 % with the majority having formed small colonies of 4 to 6 zooids with a dense cover of white spicules throughout the tunic. The most effective laboratory spawning conditions are described with respect to light and temperature. The ability to obtain D. vexillum larvae on demand will enable increased research into several aspects of this species’ reproductive biology and ecology.