Didemnum Vexillum Research Articles

Chimerism and population dieback alter genetic inference related to invasion pathways and connectivity of biofouling populations on artificial substrata.

Disentangling pathways by which nonindigenous species expand and spread regionally remains challenging. Molecular ecology tools are often employed to determine the origins and spread of introduced species, but the complexities of some organisms may be reducing the efficacy of these tools. Some colonial species exhibit complexities by way of chimerism and winter colony regression, which may alter the genetic diversity of populations and mask the connectivity occurring among them. This study uses nuclear microsatellite data and simple GIS‐based modeling to investigate the influence of chimerism and winter regression on the genetic diversity and patterns of genetic population connectivity among colonies of Didemnum vexillum on artificial substrates. Colonies sampled in summer were shown to form a metapopulation, with high levels of admixture, extreme outcrossing, and some substructure. These patterns were consistent within the subsampled winter colonies and with the inclusion of chimeric data. However, allelic richness and diversity were significantly different between winter and summer samples, altering interpretations relating to population connectivity and pelagic larval duration. This study demonstrates the importance of including seasonal sampling and imperative life history traits in genetic studies for clear interpretations and the successful management of introduced species.

Tunic microbiome of a global ascidian invader

Event Abstract Back to Event Tunic microbiome of a global ascidian invader Xavier Turon1*, Maria Casso1, 2, Marta T. Rodrigo1, Núria Marco2 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 The tunic of the ascidians shows a high diversity of prokaryote symbionts, which may play an important role in the species’ biology, adaptation and survival. The global invader Didemnum vexillum provides a unique opportunity to assess the patterns of variability of ascidian microbiomes. In particular, do they remain stable as the host species colonises new habitats or do they shift according to local environment? We have sequenced 16S amplicons of the microbiome of 60 D. vexillum colonies from 2 native (Japan) and 10 introduced populations spanning the known distribution of the species. We found 2,984 zero-radius OTUs (ZOTUs) belonging to 36 bacterial and 3 archaeal phyla. The microbiome had a markedly different composition from surrounding seawater. Only 8 ZOTUs (0.27%) were present in all colonies and 47 (1.58%) were present in all localities. However, these ZOTUs represented the 71.31 and 89.60%, respectively, of the total number of reads obtained. Thus, the species showed a high-abundance but low-diversity core microbiome. The variable component is quantitatively much less important but comprises a highly diverse assemblage. This component differs within localities (0.59 average Bray-Curtis distance), but even more between localities (0.70) and between described genetic groups of the ascidian (0.74). The average of exclusive ZOTUs was 11.95% in each colony and 22.05% in each locality. The combination of a quantitatively dominant core component and a highly diverse variable fraction in the microbiome of D. vexillum can contribute to the success of this global invader in their expansion to different environments. 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, microbiome, Symbiosis, adaptation, Didemnum vexillum Conference: XX Iberian Symposium on Marine Biology Studies (SIEBM XX) , Braga, Portugal, 9 Sep - 12 Sep, 2019. Presentation Type: Poster Presentation Topic: Global Change, Invasive Species and Conservation Citation: Turon X, Casso M, Rodrigo MT, Marco N and Pascual M (2019). Tunic microbiome of a global ascidian invader. Front. Mar. Sci. Conference Abstract: XX Iberian Symposium on Marine Biology Studies (SIEBM XX) . doi: 10.3389/conf.fmars.2019.08.00171 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 T Rodrigo Núria Marco Marta Pascual Google Xavier Turon Maria Casso Marta T Rodrigo Núria Marco Marta Pascual Google Scholar Xavier Turon Maria Casso Marta T Rodrigo Núria Marco Marta Pascual PubMed Xavier Turon Maria Casso Marta T Rodrigo Núria Marco 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.

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.

Biosecurity implications of the highly invasive carpet sea-squirt Didemnum vexillum Kott, 2002 for a protected area of global significance

Biosecurity implications of the highly invasive carpet sea-squirt Didemnum vexillum Kott, 2002 for a protected area of global significance

A decade of invasion: changes in the distribution of Didemnum vexillum Kott, 2002 in Narragansett Bay, Rhode Island, USA, between 2005 and 2015

A decade of invasion: changes in the distribution of Didemnum vexillum Kott, 2002 in Narragansett Bay, Rhode Island, USA, between 2005 and 2015

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.

Effects of temperature and salinity stress on DNA methylation in a highly invasive marine invertebrate, the colonial ascidian Didemnum vexillum.

Environmentally induced epigenetic changes may contribute to phenotypic plasticity, increase adaptive potential in changing environments, and play a key role in the establishment and spread of invasive species in new habitats. In this study, we used methylation-sensitive amplified polymorphism (MSAP) to assess environmentally induced DNA methylation changes in a globally invasive clonal ascidian, Didemnum vexillum. We tested the effect of increasing temperature (19, 25 and 27 °C) and decreasing salinity (34, 32, 30, 28 and 26 practical salinity units (PSU)) on global DNA methylation, growth and survival rates. Exposure to 27 °C resulted in significant changes in DNA methylation over time. Growth also decreased in colonies exposed to high temperatures, suggesting they were under thermal stress. In contrast, no differences in growth nor DNA methylation patterns were observed in colonies exposed to a decreasing salinity gradient, potentially due to prior adaptation. The results of this study show that environmental stress can induce significant global DNA methylation changes in an invasive marine invertebrate on very rapid timescales, and that this response varies depending on the type, magnitude, and duration of the stressor. Changes in genomic DNA methylation and the rate of growth may act to ‘buy survival time’ under stressful conditions, expanding the distribution limits of this globally invasive species.

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.

The invasion risk of species associated with Japanese Tsunami Marine Debris in Pacific North America and Hawaii

Marine debris from the Great Tsunami of 2011 represents a unique transport vector for Japanese species to reach Pacific North America and Hawaii. Here we characterize the invasion risk of invertebrate species associated with tsunami debris using a screening-level risk assessment tool – the Canadian Marine Invasive Screening Tool (CMIST). Higher-risk invertebrate invaders were identified for each of five different ecoregions. Some of these are well-known global invaders, such as the mussel Mytilus galloprovincialis and the ascidian Didemnum vexillum which already have invasion histories in some of the assessed ecoregions, while others like the sea star Asterias amurensis and the shore crab Hemigrapsus sanguineus have yet to invade large portions of the assessed ecoregions but also are recognized global invaders. In general, the probability of invasion was lower for the Gulf of Alaska and Hawaii, in part due to lower climate matches and the availability of other invasion vectors.

Predicting global ascidian invasions

AbstractAimMany species of ascidians are invasive and can cause both ecological and economic losses. Here, we describe risk assessment for nineteen ascidian species and predict coastal regions that are more vulnerable to arrival and expansion.LocationGlobal.MethodsWe used ensemble niche modelling with three algorithms (Random Forest, Support Vector Machine and MaxEnt) to predict ecologically suitable areas and evaluated our predictions using independent (area under the curve—AUC) and dependent thresholds (true skill statistics—TSS). Environmental variables were maximum and the range of sea surface temperature, mean salinity and maximum chlorophyll. We used our niche modelling results and a modified invasibility index to compare invasion risk among 15 coastal regions.ResultsCurrently, the most invaded regions are in temperate latitudes of the Northern Hemisphere and Temperate Australasia, which are regions most prone for new invasions. In the tropics, the West and Central Indo‐Pacific are two regions of strong concern, the former with high risk of primary invasion by Botryllus schlosseri and Didemnum perlucidum. In the Southern Hemisphere, the Southwest and Southeast Atlantic are most at risk, both subject to invasion by Botrylloides violaceus, Didemnum vexillum, Molgula manhattensis and Styela clava among others. Regions most at risk of expansion of established invasive species are the Central Indo‐Pacific, Northwest Pacific, Mediterranean and West Indo‐Pacific.Main conclusionsAll regions studied have areas that are suitable and connected to receive new ascidian introductions or that may permit the spread of already established species. Risk comparison of primary introductions and expansion of established introduced ascidians among regions will allow managers to prioritize species of concern for each region both for monitoring future introductions or to enforce control actions towards established species to decrease the risk of regional expansion.

Assessing invasion risk of Didemnum vexillum to Atlantic Canada

Assessing invasion risk of Didemnum vexillum to Atlantic Canada

Seasonal patterns of settlement and growth of introduced and native ascidians in bivalve cultures in the Ebro Delta (NE Iberian Peninsula)

Ascidians are important both as invasive species and as a fouling group in artificial marine habitats, causing negative impacts in aquaculture settings and the surrounding environment. The Ebro Delta is one of the major centres of bivalve production in the Mediterranean and is affected by proliferation of ascidian species (mostly introduced forms). Knowledge of the patterns of settlement and growth of the fouling species is mandatory to attempt mitigation measures. Settlement PVC plates were deployed from May to September 2015 at different depths (0.2, 1 and 2 m) in the Ebro Delta oyster aquaculture facilities. The occurrences of all species and the area cover of a selected subset of 6 species were monitored on a monthly basis from June 2015 to December 2016. Fifteen species were found, of which 10 are introduced. There were some differences between the deployed plates and the oyster ropes in species abundance and composition, likely due to differences in substrate complexity. For instance, Didemnum vexillum and Clavelina oblonga occurred in few plates in contrast to their abundance on oysters. The most abundant species were Styela plicata and Clavelina lepadiformis, which together with Ecteinascidia turbinata showed a preference to grow on plates deployed in May and June. Most of the species grew more at 0.2 m depth than at deeper plates. Thus, to minimise fouling on bivalves, spat immersion during fall and below 1 m depth is recommended. The number of occurrences and cover of the species was found to be similarly informative; suggesting that a periodic monitoring of species occurrence on replicate plates is sufficient for detecting new introduced species as soon as possible and will provide information useful for management.

Invasive tunicate restructures invertebrate community on fishing grounds and a large protected area on Georges Bank

Marine invasive species can profoundly alter ecosystem processes by displacing native species and changing community structures. The invasive tunicate Didemnum vexillum was first found on the northern edge of Georges Bank in 1998. It can form encrusting colonies on gravel substrates that are also a preferred habitat for a number of other invertebrates. In this study we used data collected via HabCam, a vessel-towed underwater imaging system, to investigate the distribution of D. vexillum and its relationship to other epibenthic macroinvertebrates in a portion of Georges Bank that includes fishing grounds and an area protected from bottom-fishing. This novel technology provides imaging of epibenthic species distributions in areas of the benthic environment that were previously unobservable. We found that D. vexillum density is negatively correlated with the Atlantic sea scallop (Placopecten magellanicus), barnacles (genus Balanus), the tube anemone (genus Cerianthus), the green sea urchin (Strongylocentrotus droebachiensis), the globular sponge of the genus Polymastia, and Bryozoa. However D. vexillum is positively correlated with Cancer spp. Crabs, the tube forming polychaete, Filograna implexa, and Asterias spp. sea stars. The hypothesis that D. vexillum restructures the invertebrate community is supported by principal components analysis, revealing it as a primary driver of variation in the community when present. Additionally, there is an effect of the closed area as compared to fishing grounds on the structure of the invertebrate community and the abundance of certain species as consistent with previous studies, bottom-fishing affects invertebrate community structure. Principal components analysis revealed that bottom-fishing also appears to weaken clustering among species in the invertebrate community as compared to the community structure in the closed area. Biodiversity in high gravel sites of the epibenthic environment, as measured by the Shannon diversity index, also declined with increasing D. vexillum percent cover, while the open and closed areas were not significantly different in their level of biodiversity. D. vexillum appears to be the key driver of biodiversity decline in the epibenthos when present, rather than other processes such as direct disturbance and extraction from dredging. This research evaluates ecological responses to the presence of an invasive tunicate and suggests that this invasive species is a major force in shaping the ecological interactions in invaded areas.

Evaluating the interaction of the invasive tunicate Didemnum vexillum with the Atlantic sea scallop Placopecten magellanicus on open and closed fishing grounds of Georges Bank

Abstract An invasive colonial tunicate (Didemnum vexillum) was initially observed on Georges Bank in 1998, and it has since spread in benthic environments on fishing grounds and areas closed to bottom-fishing. It can form dense mats on gravel substrates that are also a preferred habitat for the Atlantic sea scallop (Placopecten magellanicus), which supports one of the most valuable commercial fisheries in the United States. We used HabCam, a vessel-towed underwater imaging system, to investigate the spatial distributions of P. magellanicus and D. vexillum in a region that includes fishing grounds and an area protected from bottom-fishing. We found a negative relationship between P. magellanicus and D. vexillum, even after controlling for substrate and management status, suggesting that D. vexillum competes for habitat with P. magellanicus. We also applied the geostatistical method of universal kriging to interpolate the distribution of D. vexllium based on the covariables gravel, depth and area. Our results indicate that D. vexillum is more common in areas open to fishing than in the areas closed to fishing, even taking bottom substrate effects into account. Didemnum vexillum appears to have spread over portions of the northern edge of Georges Bank. This research evaluates potential fish and invertebrate habitat degradation caused by an invasive species.

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.

Didemnum vexillum: invasion potential via harvesting and processing of the Pacific oyster (Crassostrea gigas) in British Columbia, Canada

Didemnum vexillum: invasion potential via harvesting and processing of the Pacific oyster (Crassostrea gigas) in British Columbia, Canada

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.

Real-time PCR detection of Didemnum perlucidum (Monniot, 1983) and Didemnum vexillum (Kott, 2002) in an applied routine marine biosecurity context.

Prevention and early detection are well recognized as the best strategies for minimizing the risks posed by nonindigenous species (NIS) that have the potential to become marine pests. Central to this is the ability to rapidly and accurately identify the presence of NIS, often from complex environmental samples like biofouling and ballast water. Molecular tools have been increasingly applied to assist with the identification of NIS and can prove particularly useful for taxonomically difficult groups like ascidians. In this study, we have developed real-time PCR assays suited to the specific identification of the ascidians Didemnum perlucidum and Didemnum vexillum. Despite being recognized as important global pests, this is the first time specific molecular detection methods have been developed that can support the early identification and detection of these species from a broad range of environmental sample types. These fast, robust and high-throughput assays represent powerful tools for routine marine biosecurity surveillance, as detection and confirmation of the early presence of species could assist in the timely establishment of emergency responses and control strategies. This study applied the developed assays to confirm the ability to detect Didemnid eDNA in water samples. While previous work has focused on detection of marine larvae from water samples, the development of real-time PCR assays specifically aimed at detecting eDNA of sessile invertebrate species in the marine environment represents a world first and a significant step forwards in applied marine biosecurity surveillance. Demonstrated success in the detection of D. perlucidum eDNA from water samples at sites where it could not be visually identified suggests value in incorporating such assays into biosecurity survey designs targeting Didemnid species.

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.

Forecasting at the edge of the niche: Didemnum vexillum in Southeast Alaska

Controlling the spread of marine invasive species is a challenging and costly task. Maps that predict the potential spread of an invader based on known habitat preferences can be extremely valuable for assessing invasion risk and prioritizing management actions for invasion control or prevention. Most maps are developed by using environmental data on the species’ known distribution to map the potential niche of the species in a new location. However, this approach is complicated when a species spreads to an area where environmental conditions are much different than in other places it is known to exist. Didemnum vexillum was discovered in Southeast Alaska in 2010, marking the northernmost known range of this species. A self-organizing map (SOM) was used to assess potential habitat for D. vexillum in other parts of Southeast Alaska using summer and winter temperature and salinity as controlling factors. This research highlights the uncertainty of using the species’ current distribution to evaluate potential spread to an environment at the edge of a species’ environmental tolerances. It also identifies gaps in our knowledge of D. vexillum thermal and salinity tolerances, including potential synergistic and additive effects of both low temperature and low salinity, which limit investigation of mechanistic modeling methods.