Evidence for self-sustaining populations of Arcuatula senhousia in the UK and a review of this species’ potential impacts within Europe

The invasive Asian date mussel (Arcuatula senhousia) inhabits diverse global coastal environments, in some circumstances posing significant ecological and economic risks. Recently recorded in the Greater North Sea ecoregion, an established population has not previously been confirmed. Combining historical and field data, we provided baseline information from the UK and recorded colonisation in a variety of habitats. Gonadal development was assessed using the gonadosomatic index (GSI) to determine if an intertidal soft-sediment population is self-sustaining. Arcuatula senhousia records from subtidal muddy/mixed-sediment within a major estuarine system from 2007 to 2016 were also analysed. First detected in 2011, spatial distribution was variable across the years within the subtidal, with individuals found at 4–9 out of 25 sites, and densities per site varying from 10 to 290 individuals per m2. The intertidal population was, in part, associated with seagrass (Zostera spp.) and attached to bivalves. In marinas, individuals were attached to concrete tiles, associated with live Mytilus edulis, and to dead Ostrea edulis. Mean GSI from the intertidal population differed across months, peaking in July before declining in September/October, but with high inter-individual variability. Arcuatula senhousia is reproducing and maintaining viable populations. Using a natural capital approach, we identify the potential impacts on Europe’s functionally important habitats, fisheries and aquaculture if its spread continues.


Assessment of spatial distribution and temporal trends. Subtidal surveys in Southampton Water
were undertaken from 2007 to 2016 when the EA carried out routine benthic surveys as part of the monitoring programme for the UK government's Water Framework Directive (WFD) 44 . Forty-five sites (2007) and 25 sites (2011, 2013 and 2016) within Southampton Water and its estuaries (Rivers Test, Itchen and Hamble) were semi-randomly selected for sampling each year by considering sediment type, accessibility and potential hazards. A site was approximately defined as a 50 m radius surrounding a target coordinate. One grab sample, using a 0.1 m 2 Day grab, was taken to assess macrofauna at each site. Macrofauna processing and identification were undertaken by a contractor using standard operating and quality control procedures used by the industry (e.g. NMBAQCS: North East Atlantic Marine Biological Analytical Quality Control Scheme) with macrofauna extracted using a 0.5 mm sieve. No specific size measurements of A. senhousia were recorded. Assessment of spatial distribution, gonad staging, habitat preference. The intertidal shore at Brownwich was surveyed in 2019 using six 600 m × 5 m transects parallel to the mean low water springtide line, evenly spaced (by 40 m) from high shore to low shore. The surveyor walked within the transect parameters locating A. senhousia that were immediately apparent on the sediment surface without sediment excavation. Arcuatula senhousia locations were recorded using a GPS device (Garmin eTrex 20x) and shell lengths measured using calipers. Every other measured specimen was transported back to the laboratory and fixed in formalin before the gonadosomatic index (GSI) measurements were obtained. For GSI, gonads and other tissue were dissected and then calculated as follows: ([gonad wet weight (g) / bodyweight without shell (g)] × 100) 45 .
Surveys not targeted at detecting A. senhousia also provided records for this species from intertidal locations within the Solent region. These surveys were conducted by researchers from the Universities of Portsmouth and Southampton, a volunteer for the Hampshire and Isle of Wight Wildlife Trust and Pisces Conservation Ltd (see Supplementary Table S1). From west to east, surveys included an intertidal macrobenthos survey at Lepe (2019), a fish push-net survey within the River Test (2016) and a seagrass quadrat survey at Portsmouth Harbour (2019). A specimen from the River Itchen (2018) was also found during an intentional search for A. senhousia on mudflats (no methodology recorded). Survey details regarding the specimen found at Chichester Harbour (2019) cannot be provided due to the commercial sensitivity of the location where it was found.
Marina and harbour surveys across the Solent. As part of the Solent Oyster Restoration Project 46 , O. edulis were purchased in 2016 from the commissioned dredge fishery in Langstone Harbour and were translocated from the seabed into broodstock cages deployed at various locations within the Solent including Saxon Wharf (River Itchen). It should be noted that oysters were not cleaned of epifauna before translocation, in part due to the sheer numbers of oysters being moved. In total, approximately 10,000 O. edulis were purchased from the fishery, with each oyster being at least 3 years in age (> 70 mm). The oysters remained in the cages throughout 2017 and 2018 until the trial concluded in November 2018. At the end of the trial deceased individuals were Statistical analysis. A Kruskal-Wallis test (SPSS v.25) was used to determine whether there was a significant difference between median densities per site of A. senhousia individuals collected from each of the three EA surveys when A. senhousia was detected (2011, 2013 and 2016). This test was chosen because data were not normal and, due to the high number of zeros, could not be transformed. This test was also used to identify significant differences between the median GSI reported for March, May, July and September/October (data were collected during the last week of September and the first three weeks of October and were, therefore, combined). In order to identify which months had a significantly different GSI, pairwise-comparisons were subsequently made using the Wilcoxon rank sum test.
Assessment of potential impacts. A literature review was conducted to gather information on A. senhousia impacts, specifically in relation to natural capital and vulnerable and protected habitats and species. To extract the relevant information, Web of Science and Google Scholar were used to search for common names and synonyms for A. senhousia as listed by CABI 5 . Other key words searched included "Zostera", "impact", "distribution", "competition", "clam", "oyster" and "reproduction". Impacts were then categorised by the relevant ecosystem services using the commonly used top level categories of Provisioning, Regulating, Cultural and Supporting e.g. 47,48 . We adapted these definitions to be the following: Provisioning services are products that people obtain from ecosystems (e.g. food and other raw materials); Regulating services are benefits that people obtain from the regulation of ecosystem processes (e.g. climate regulation and water purification); Cultural services are the nonmaterial benefits that people obtain from ecosystems (e.g. recreation and health); Supporting services are those that are necessary for the production of all other ecosystem services (e.g. habitat provision and genetic diversity).

Results
Spatial distribution, temporal trends, habitat preference. The first scientifically reported sighting of A. senhousia in the UK prior to this study was from 2017 26 , however our study confirms the presence of this species in the UK since 2011 (mean A. senhousia densities for each survey can be found in Table 1). Routine surveys undertaken by the EA throughout Southampton Water and its three estuaries recorded the presence of A. senhousia from 2011-2016. In 2007, no A. senhousia individuals were found at any of the 45 sites ( Fig. 1; sites 1-45, Supplementary Table S2). In 2011, five out of the 25 sites sampled contained A. senhousia, concentrated towards the upper reaches of the estuarine system ( Fig. 1), and densities varied from 0 to 70 individuals per m 2 (m −2 ) (mean = 7.2 + /− 18.6 SD) (sites 46-70, Supplementary Table S2). In 2013, samples from four out of the 25 sites contained A. senhousia ( Fig. 1) with densities ranging from 0 to 70 m −2 (mean = 4.0 + /− 14.1 SD) (sites 71-95 in Supplementary Table S2). In 2016, A. senhousia was found at more sites (nine out of 25) across a greater geographic area (Fig. 1). For example, it was detected for the first time in the River Hamble and near the mouth of Southampton Water. The highest density was recorded in 2016 when there was a range of 0-290 m −2 (mean = 20.4 + /−58.8 SD) (sites 96-120 in Supplementary Table S2). Nevertheless, there is no significant difference in A. senhousia median density per site between 2011, 2013 and 2016 (Kruskal-Wallis test, X 2 (2) = 3.1, p = 0.215).
In addition to the EA's surveys, there have been further reports of A. senhousia from a variety of intertidal and marina surveys in all three rivers which discharge into Southampton Water (see Table 1 for survey details and site numbers). Two individuals were found near Hythe at the mouth of the River Test ( Fig. 1; site 121), one in 2016 (17 mm length) and another in 2019 (18 mm length). In 2018, two individuals were recorded from Weston Shore in the River Itchen ( Fig. 1; site 123). Further, A. senhousia were found attached to empty adult shells of O. edulis that had been removed from oyster cages at Saxon Wharf ( Fig. 1; site 124), also in the River Itchen. Fourteen A. senhousia individuals ranging from 13-23 mm (mean = 17.6 + /− 3.0 SD) were removed from the oysters. Concrete tiles deployed in Saxon Wharf Marina ( Fig. 1; site 124) had three individuals (mean = 8.7 + /− 3.1 SD) attached to the tiles or to Mytilus edulis when recovered in 2019. Two individuals (19 mm and 28 mm in length) were also found at Shamrock Quay (2019) attached to the metal cages housing the oysters ( Fig. 1; site 129). An unknown number of A. senhousia individuals were also collected from oyster cages at Port Hamble (River Hamble). They were found attached to cockles and Ulva spp. that had been caught in cages suspended beneath the marina pontoons ( Fig. 1; site 125).
Reports from three intertidal surveys and one subtidal survey provide evidence for the conclusion that A. senhousia is distributed across the Solent region. In 2019, one individual was found in Lepe in the west of the Solent ( Fig. 1; site 126). To the east, one individual (18 mm length) was recovered from Portsmouth Harbour ( Fig. 1; site 127) growing on mixed eelgrass (Zostera marina and Z. noltei) alongside significant quantities of Ruppia spp. Another individual (4 mm in length) was found on muddy sediment in highly sheltered conditions within Chichester Harbour ( Fig. 1; site 128, but exact location cannot be disclosed due to commercial sensitivity of the survey) and another was recovered from the Isle of Wight in Newtown ( Fig. 1; site 130 Supplementary Fig. S1b). The translucent tissue corresponds to a high volume of follicle cells with collapsed or empty gametes indicating spent or developing gonads with no clear differences between sexes 19 . Arcuatula senhousia from May also resembled those collected in March, however, by July gonad tissue had substantially thickened and channels within the tissue could be seen ( Supplementary Fig. S1c, d), suggesting that the gonads were ripe or at the spawning stage 19 Fig. S1d) was also discernible confirming a 3F:2M sex ratio for the 15 A. senhousia individuals collected. By September/October there was a high inter-individual variation in reproductive state, with gonad stage appearing to range from spent to ripe/spawning. One out of the 12 individuals collected in September/October was identified as a female, although a sex ratio could not be established due to the thin gonad tissue of many of the mussels.
To support the gross anatomical observations the GSI was calculated for each month and presented in Fig. 2. Mean GSI was low for both March and May (6.0 + /− 7.2 SD, 5.9 + /− 11.0 SD, respectively), but had increased to 23.1 + /− 6.1 SD by July. By September/October the mean GSI had decreased but remained higher than for March and May (16.7 + /− 13.3 SD). A Kruskal-Wallis test confirms that there are significant differences in median GSIs between the months sampled (Kruskal-Wallis, X 2 (3) = 41.5, p = < 0.001). A pairwise-comparison of the median GSI for each month indicates that all months are significantly different from each other (Wilcoxon rank sum test, p = < 0.05) apart from March and May.

Discussion
Baseline biological data and spatial distribution. Our data suggests that A. senhousia arrived in the UK, between 2007 and 2011, which was recently confirmed by Worsfold et al. 49 . The closest (in distance) European record of A. senhousia prior to this was from Arcachon Bay (Bay of Biscay), on the Atlantic coast of France in 2002 24 . The lack of reported sightings between the UK and the Bay of Biscay suggests a direct introduction event in the Solent as opposed to natural dispersal. As stated by Barfield et al. 26 , if the French population had gradually extended northwards unaided by any direct anthropogenic vector, it is reasonable to assume that its presence would have been recorded elsewhere before it reached the UK. However, spread of A. senhousia towards UK could have gone undetected due to limited monitoring for non-native species in the region. Potential vectors for introduction to the Solent include as a hitch-hiker with aquaculture species/produce 50 , but intro- Table 1. Summary of A. senhousia population data from sites within the Solent region of the UK, recorded from 2007-2019. Site numbers correlate with Fig. 1. Gonad stages based on those of Sgro et al. 19 : "1-2" = spent or developing; "3-4" = ripe or spawning; "-" = data not collected. www.nature.com/scientificreports/ duction by shipping is most likely. This is supported by the species' ability to foul boat hulls 51 and the detection of A. senhousia DNA in ballast water of boats in Dutch harbours 52 . A phylogenetic analysis is required to fully explore the likely invasion route(s) into the Solent and contextualise the global colonisation process. Attachment to seaweeds such as Ulva spp., as found in this study, could facilitate more local spread of A. senhousia by acting as a raft for hitchhikers (e.g. 53 ). Individuals collected ranged in size from 4 mm (Chichester Harbour) to 32 mm (Brownwich shore). Whilst Huber 2 indicates an upper length of 40 mm for this species, an upper size limit of around 30-35 mm in its nonnative range is most common in the literature (e.g. 1,8,24,54 ). Linked to the small size in terms of traits of a successful invader is the short lifespan with most individuals living for only a year. Morton 17 concluded that the small fraction of the population that lives up to two years is an adaptation for the continued survival of population in a variable environment. Considering a growth rate of approximately 2 mm a month depending on environmental conditions 1,16,55 it is possible that a few individuals at Brownwich were potentially older than a year. The size ranges recorded here, combined with the fact that individuals have been recorded from three sites on multiple years ( Any self-sustaining population requires successful reproduction. While this is supported by the size ranges of A. senhousia (which spanned the 14-20 mm length maturity threshold 19,23 ) the strongest evidence comes from  Supplementary Table S2. Numbers 121-130 refer to surveys by other organisations (see Table 1). Mean densities for all surveys can be seen in Table 1. Site 128 is not a specific location but represents one individual found in Chichester harbour. Map created using ArcGIS Pro 2.6 https:// pro. arcgis. com/. The intertidal shore at Brownwich ( Fig. 1; site 122) was comprehensively surveyed in 2019. Compared to the subtidal sites in Southampton Water, the population density was low, with only 169 individuals recorded equivalent to 0.06 m −2 ( Table 1). Single individuals were found mainly on the higher part of the shore partially buried in the sediment. None were attached to seagrass (Zostera spp.), however, when removed from the sediment a number were attached by their byssal threads to dead cockles (Cerastoderma edule) (empty shells) and living individuals. Arcuatula senhousia shell lengths ranged from 9 to 32 mm (mean = 20.1 + /− 3.9 SD). The timings of these reproductive stages likely coincides with changes in water temperature; a variable which is well-documented for influencing bivalve reproduction and development 56,57 , especially in temperate regions 58 . In its native range of the Sea of Okhotsk, Southern Sakhalin (Russia), the spawning period of A. senhousia coincides with temperatures of 15-20°C 3 . This temperature range matches the inshore summer temperatures of the Solent (Watson, unpublished data) suggesting summer spawning in Europe's temperate systems, if other requirements, such as oxygen levels and salinity are met. This is likely considering A. senhousia is also tolerant of a wide range of salinity (multiple Solent sites have reduced or fluctuating salinities) and oxygen levels 21 . Colder months in the Solent, when the average temperature is < 15 °C (e.g. winter and spring) 28 , probably limit reproduction 59 . Despite the evidence indicating a summer spawning population in the Solent, there are inconsistencies in the temperature range reportedly required for A. senhousia reproduction to take place. For example, a temperature of 22.5-28 °C is well documented 5,19,60 . It is possible that this temperature range only applies to A. senhousia individuals originating from the warmer parts of its native region 61 . A lineage that is predisposed to colder waters and has high levels of polymorphism may be responsible for adaptation to the relatively cold waters of Northern Europe 61 . Research should, therefore, focus on identifying the lineage present in this area and determining the temperature limits for reproduction. In addition, the possibility of multiple and prolonged spawning events in the UK cannot be excluded since we observed high inter-individual variability of GSI data. This is not an unusual phenomenon, with prolonged spawning (more than two months) reported outside of its native range 1,10,24,55,62 .
This study highlights that A. senhousia survives in multiple habitat types present in the Solent confirming the species' capability for colonising diverse intertidal and subtidal habitats 10,34,40,51 . Due to the opportunistic collection methods for data used in this study, it is not currently possible to determine the geographical extent of the population or the rate of spread within the Solent since its arrival. Indeed, although the largest density (290 m −2 ) and greatest number of positive sites (35%) were reported from sampling of Southampton Water in 2016, there was no significant difference in median density between years. Currently, distributions in both Southampton Water and Brownwich beach appear patchy and spatially variable with lower densities than other invaded locations 10,40 . This may be in part due to limited sampling, but the A. senhousia populations in the Solent could be experiencing an extended lag phase which is typical of newly introduced species 63 . However, this does not necessarily mean densities will inevitably increase in the future. Local factors might prevent mat formation, for example, anoxia associated with warmer months can induce mass mortalities 23,64,65 . Arcuatula senhousia is also predated upon by shorebirds birds (diving ducks and oyster catchers) 8,62,66 , boring carnivorous gastropods 51,67,68 , fish 15 and probably crustaceans and echinoderms due to its thin shell. Therefore, intense activity by predators may limit A. senhousia's mat-forming abilities. In conclusion, further data to describe the distribution of A. senhousia's in the Solent are required.

Potential effects on European natural capital. Non-native species impact natural capital and thus
alter the value of ecosystems in terms of the ecosystem goods and services provided. Tables 2, 3, 4 and 5 provide summaries of potential impacts (both positive and negative) associated with A. senhousia on ecosystem services (addressing the categories of Provisioning, Regulating, Supporting and Cultural) and identifies key knowledge gaps which should be addressed in the short term as a priority.
Provisioning services. Arcuatula senhousia has been reported to reduce the growth rate and survivorship of commercially important clams by competing for space and food [69][70][71] and indirectly increasing predation 72 . In the Solent, oysters (O. edulis); clams; cockles and polychaetes for angling bait are commercially harvested from    75,76 . We only found one individual from Portsmouth Harbour growing within a bed of Zostera spp., although A. senhousia co-occurs with seagrasses in both its native and introduced ranges 3, 24,34,77,78 . Seagrass beds are biodiverse ecosystems providing a variety of ecosystem services across the world, such as carbon capture, coastal defence and the provision of nursery habitat for juvenile fish, including those of significant commercial value in Europe [79][80][81][82][83] . Since the late 1800s, seagrass beds have suffered from substantial degradation due to a host of biotic and abiotic factors (although some recent recovery has been reported) 84,85 . These degraded beds, and new beds transplanted for restoration schemes (for example, Project Seagrass 86 ), may be at risk, since A. senhousia mats have been found to inhibit rhizome growth in recovering populations with low plant density (in contrast, impacts of A. senhousia on established beds have been reported as small and non-consistent) 34 . Solent densities Table 2. A summary of the impacts of A. senhousia in relation to Provisioning ecosystem services. ( +) denotes a potentially positive impact, (-) denotes a potentially negative impact. Priority questions are those that should be addressed by researchers to generate a full risk assessment and management plan.  Table 3. A summary of the impacts of A. senhousia in relation to Regulating ecosystem services. ( +) denotes a potentially positive impact, (-) denotes a potentially negative impact. Priority questions are those that should be addressed by researchers to generate a full risk assessment and management plan.

A. senhousia impacts and observations + /− Supporting information + /− Priority questions
Waste (excess nutrients, toxic pollutants) remediation + Removes excess nitrogen and phosphorus from water 100  www.nature.com/scientificreports/ (290 m −2 ) may be currently too low to impact seagrass, compared to 15,000 m −2 in San Diego Bay -the site of the aforementioned seagrass study 34 . Whether higher densities form in the future will depend on a complex interplay of environmental conditions and biological factors. Within this study we found evidence for A. senhousia attachment to empty O. edulis shells, M. edulis shells and concrete tiles in the Hamble estuary. In a different study, A. senhousia was also found attached to cultured Crassostrea hongkongensis in Hong Kong 37 . The colonisation of locations in both fully saline and brackish European waters by A. senhousia could increase the cost of shellfish aquaculture via biofouling and directly compete with the commercial species for substrate and food. Biofouling has been estimated to be 20-30% of shellfish production costs, though this cost varies depending on the commercial species and the geographic location of the operation 87,88 . Disease introduction and hybridisation with commercial species are also possible outcomes that could have significant risks for the European aquaculture industry. For example, the cultivation of the nonnative Pacific oyster (Magallana gigas) in France since 1966 is likely to have contributed to the arrival and spread of gill disease to Portuguese oysters (Crassostrea angulata) 89 . Further, expanding populations of Mytilus trossulus in the UK, likely driven by commercial mussel growing activity, have been associated with the appearance of M. trossulus x M. edulis hybrids which are less valuable as a commercial species 90 . However, at the time of writing, investigations into potential disease spread from A. senhousia to other shellfish, or hybridisation between A. senhousia and other mussels could not be found. Nonetheless, A. senhousia may be a suitable host of a native generalist parasite, the pea crab Pinnotheres pisum, in the UK, considering that Pinnotheres novaezelandiae was found within A. senhousia in New Zealand 91 . Pinnotheres spp. are known to negatively impact the condition index, oxygen consumption and filtration rate of Mytilus spp. 92,93 .
Any non-native species is likely to have positive and negative effects on provisioning services and this is the case for A. senhousia. For example, it can be eaten by humans for food 22 or could be used to provide products to the pet trade 94 . Reusch and Williams 34 also found it could be beneficial to seagrasses by providing nutrients and Table 4. A summary of the impacts of A. senhousia in relation to Supporting ecosystem services. ( +) denotes a potentially positive impact, (-) denotes a potentially negative impact. Priority questions are those that should be addressed by researchers to generate a full risk assessment and management plan.  Table 5. A summary of the impacts of A. senhousia in relation to Cultural ecosystem services. ( +) denotes a potentially positive impact, (-) denotes a potentially negative impact. Priority questions are those that should be addressed by researchers to generate a full risk assessment and management plan. www.nature.com/scientificreports/ could even protect vulnerable habitats from erosion if it forms mats. Increases in habitat diversity through an increase in structural complexity from mats or aggregations of A. senhousia may provide significant benefits for other species and biodiversity more generally. Thus, any risk assessment needs to cover both potential negative and positive impacts so that informed management decisions can be made.

Cultural ecosystem services + /− A. senhousia impacts and observations + /− Supporting information + /− Priority questions
Regulating, supporting and cultural services. The densities currently reported are unlikely to have an influence on key regulating and supporting services at anything, but the very local scale. Nevertheless, the potential for nutrient bioremediation, carbon sequestration, water clarity improvements and habitat provision will grow if densities increase in combination with the spatial extent of the Solent's populations across the multiple habitats. The effects on cultural services, such as human health and recreation, are some of the most difficult to predict, but could have the most direct and widespread impact on people within the region and as well as the blue economy. Impact assessments and management plans for newly arrived species must be balanced by considering both negative and positive impacts, such as those in Tables 2, 3, 4 and 5, and accounting for shifting baselines (see discussion by Crooks 71 ). The imperative is to answer the key questions we have posed in Tables 2, 3, 4 and 5 about the effects (both positive and negative) and the subsequent risks to European habitats and coastal economies. This requires investment in monitoring, but also examination of the potential interactions between A. senhousia and key habitats and species. This two-pronged approach is essential for determining whether A. senhousia or other biotic and abiotic factors are responsible for ecosystem change 71 . Moreover, as previous invasion trajectories of A. senhousia are diverse, predicting the impacts on services (and any restoration efforts to improve colonised but protected habitats) will be challenging without context-relevant experimental data. For example, Mastrototaro et al. 10 found that a population in the Mediterranean had increased to densities of up to 3800 m −2 within two years of arriving. In contrast, the density of a population in Auckland, New Zealand declined by 60% in one year, decreasing from 16,000 m −2 to 5,500 m 262 . Large temporal variation in density is typical of an opportunistic species, with highly erratic population dynamics, increasing the risk of population extinctions as well as expansions 1,23,63 . The risk of rapid non-native species population expansion emphasises the need for prompt responses to new introductions. The delay between the earliest detection of A. senhousiain the UK (2011) and the first published report of its arrival (2017) suggests the need for improvement of the national invasive species reporting and response systems. Furthermore, there is a need to prioritise the identified impacts of A. senhousia so that management resources can be effectively allocated. This requires identification of the ecosystem service/s at risk (this study), assessment of the magnitude and scale of ecosystem service impacts, and ecosystem service valuation (ESV) 114 . ESV can be done in a variety of ways including the assignment of an economic monetary value (e.g. 115 ). For impacted ecosystem services which have a direct value (such as commercial shellfish stocks) ESV is relatively simple, but for others with an indirect value, such as bioremediation, the replacement cost valuation method can be used (e.g. 116,117 ). ESV methods are still very much open to discussion 118 .

Conclusion
Our study confirms that A. senhousia has been in the Solent for at least eight years, indicating stable, selfsustaining populations located on the periphery of the Greater North Sea ecoregion (and by extension Europe). We believe that A. senhousia is likely to spread further within this region. In fact, A. senhousia has already been reported from the Netherlands (in 2018), although it is not clear whether the 30 individuals collected represent an established population 119 . Where A. senhousia populations establish in the future will be dependent on a wide variety of factors, such as its genetic variation and phenotypic plasticity 120,121 , hydrodynamics 122 , propagule processes, and environmental conditions 123 . If the lineage in the Solent is one that is predisposed to colder water adaptation (Asif and Krug 61 suggested this as a reason for its ability to exist in more northerly regions within its introduced range), the colonisation of diverse waters of Europe could be eminently achievable. The presence of established, self-sustaining A. senhousia UK populations that can reproduce and colonise multiple habitat types, and whilst tolerating variable environmental conditions, highlights a potentially significant risk to the blue economy and natural capital within the Greater North Sea. We advocate that increased monitoring of this species is essential, especially in habitats of conservation and commercial importance. We also recommend the completion of a thorough and standardised risk assessment to aid awareness raising, inform policy and facilitate prioritisation of actions. Concurrently, determined efforts should be made to address the fundamental ecological and biological questions we have highlighted to confirm if A. senhousia will, soon be added to Europe's list of invasive non-native species.

Data Availability
All raw data can be made available upon request to the authors.