European Coasts Research Articles

Investigating Non-Native Ribbon Worm Cephalothrix simula as a Potential Source of Tetrodotoxin in British Bivalve Shellfish.

Tetrodotoxin (TTX) is a potent marine neurotoxin found in several phylogenetically diverse organisms, some of which are sought as seafood. Since 2015, TTX has been reported in bivalve shellfish from several estuarine locations along the Mediterranean and European Atlantic coasts, posing an emerging food safety concern. Although reports on spatial and temporal distribution have increased in recent years, processes leading to TTX accumulation in European bivalves are yet to be described. Here, we explored the hypothesis that the ribbon worm species Cephalothrix simula, known to contain high levels of TTX, could play a role in the trophic transfer of the toxin into shellfish. During a field study at a single location in southern England, we confirmed C. simula DNA in seawater adjacent to trestle-farmed Pacific oysters Magallana gigas (formerly Crassostrea gigas) with a history of TTX occurrence. C. simula DNA in seawater was significantly higher in June and July during the active phase of toxin accumulation compared to periods of either no or continually decreasing TTX concentrations in M. gigas. In addition, C. simula DNA was detected in oyster digestive glands collected on 15 June 2021, the day with the highest recorded C. simula DNA abundance in seawater. These findings show evidence of a relationship between C. simula and TTX occurrence, providing support for the hypothesis that bivalves may acquire TTX through filter-feeding on microscopic life forms of C. simula present in the water column at particular periods each year. Although further evidence is needed to confirm such feeding activity, this study significantly contributes to discussions about the biological source of TTX in European bivalve shellfish.

Multiple introduction events expand the range of the invasive brown alga Rugulopteryx okamurae to northern Spain.

The brown alga Rugulopteryx okamurae is one of the most recent and aggressive marine invaders known, with a non-native range that spans from the western Mediterranean to the southern Iberian Peninsula, and the European Macaronesian archipelagos. Here, we provide the first record of its presence at three disjunct sites from northern Spain, supported by morphological and molecular evidence. Rugulopteryx okamurae was discovered in the port of Bilbao in the northeastern Atlantic of Spain, more than 1200km away from the nearest invaded location previously known. It was also found at two separate sites in northwestern Spain: San Amaro, a small inlet at the entrance of the port of A Coruña, and Monteferro-Playa América, a site close to aquaculture facilities and the port of Vigo. In two locations, R. okamurae was found to cover up to 100% of the bottom, suggesting that it may behave as an aggressive invader in northern Spain. Our findings confirm the propensity of R. okamurae for long-distance dispersal and reveal that this invader is expanding its range northwards along the European Atlantic coast.

Photoperiod and temperature interactions drive the latitudinal distribution of Laminaria hyperborea (Laminariales, Phaeophyceae) under climate change.

Due to global rises in temperature, recent studies predict marine species shifting toward higher latitudes. We investigated the impact of interacting abiotic drivers on the distribution potential of the temperate kelp Laminaria hyperborea. The ecosystem engineering species is widespread along European coasts but has not yet been observed in the High Arctic, although it can survive several months of low temperatures and darkness. To investigate its ability to extend northward in future, we conducted a long-term multifactorial experiment with sporophytes from Porsangerfjorden, Norway-close to the species' documented northernmost distribution margin. The samples were exposed to three different photoperiods (PolarDay, LongDay, and PolarNight) at 0°C, 5°C, and 10°C for 3 months. Optimum quantum yield of photosynthesis (Fv/Fm), dry weight, pigments, phlorotannins, and storage carbohydrates were monitored. Both physiological and biochemical parameters revealed that L. hyperborea was strongly influenced by the different photoperiods and their interaction with temperature, while temperature alone exerted only minor effects. The Fv/Fm data were integrated into a species distribution model to project a possible northward expansion of L. hyperborea. The combination of extended day lengths and low temperatures appeared to be the limiting reason for northward spread of L. hyperborea until recently. However, with water temperatures reaching 10°C in summer, this kelp will be able to thrive also in the High Arctic. Moreover, no evidence of stress to Arctic winter warming was observed. Consequently, L. hyperborea has a high potential for spreading northward with further warming which may significantly affect the structure and function of Arctic ecosystems.

Simulating the aftermath of Northern European Enclosure Dam (NEED) break and flooding of European coast

Simulating the aftermath of Northern European Enclosure Dam (NEED) break and flooding of European coast

Functional traits of ecosystem engineers as predictors of associated fauna

The ongoing combination of global warming and increased anthropogenic pressure is causing latitudinal shifts in marine species, potentially impacting community composition, local richness, and marine trophic webs. This study investigates the factors influencing the distribution and diversity of intertidal seaweed and associated peracarid communities, including their functional traits, and explores various facets of beta diversity (taxonomic and functional). We hypothesize that: 1) abiotic factors such as temperature and anthropogenic pressure significantly influence seaweed distribution and diversity shifts, and 2) changes in seaweed functional diversity have an impact on the diversity and functioning of its associated peracarid communities. The sampling was conducted along a wide latitudinal gradient in the NE Atlantic (27°N - 65°N), encompassing three distinct ecoregions: Northern European coasts, the Iberian Peninsula, and Macaronesia. The identified seaweed and peracarid species were classified functionally, and taxonomic and functional diversity were analysed on a large geographic scale. The northern region exhibited large brown canopy seaweeds and epibiotic isopods, while Macaronesia featured small red, highly branched, and calcareous crust seaweeds with burrower and tube-building tanaids. The Iberian Peninsula acted as a transitional zone, showcasing a mix of green, red, and brown seaweeds, along with Amphipoda peracarids found across all ecoregions. Our findings underscore the impact of geographic distance on total beta diversity, revealing distinct seaweed and peracarid communities across spatial gradients. Environmental variables, particularly pH and maximum sea surface temperature, emerged as significant factors influencing beta diversity patterns of seaweeds, indicating the potential impact of acidification and heat waves on community composition. In addition, seaweed functional traits were shown to be significant in shaping the diversity and abundance of associated peracarid assemblages, impacting both taxonomic and functional beta diversity. These findings provide crucial insights into the factors influencing the biogeography and biodiversity dynamics of intertidal seaweeds and associated peracarids, offering essential implications for conservation and management strategies amid ongoing environmental changes.

Regional tidal modeling on the European coast using tide gauges and satellite altimetry

The study investigates sea-level measurements along the coastal area of Europe for the 60-year (1961–2020) time span. Linear and quadratic modeling of tide gauge measurements showed an almost positive rate of trend of sea-level rise (0.09 to 3.6 mm/yr) and low acceleration (−0.05 to 0.40 mm/yr2). A least-squares harmonic estimation tidal modeling was carried out to estimate frequency (cycles per day) for a certain period. The smaller and higher tidal frequencies of these stations indicate their stability in terms of their surface variation. We used the 1993–2020 satellite altimetry data from the nearest grid points of the tide gauge station. The correlation coefficient between observed and satellite altimetry (lowest 0.53 and highest 0.93) varies at each station. This happens because of many factors that can affect the large difference in the sea-level trend between the satellite-derived and tide gauge results. Finally, to implement a global reference system for physical heights, the offshore topographic slope direction and slope range with contour spacing from the sea to the associated coastline were analyzed using bathymetry data. The abrupt change in slope from the coastline toward the sea can be seen toward the east, west, and southeast on the European coast. This is also an important factor that affects the variation of sea level.

Thermal performance of native and invasive crab species: investigating the invasion potential of Hemigrapsus takanoi in southern European Carcinus maenas’ habitats

Invasive species’ plasticity and latitudinal spread have become an increasing matter of concern in rapidly changing climate. The brush-clawed shore crab (Hemigrapsus takanoi), native along northwestern Pacific shores, has established dense populations in northern European coasts. Detrimental effects in natura have been reported, such as displacement of native species and competition for shelter, rasing concerns regarding shellfish early life stages and food webs-posing a threat to wild stocks’ recruitment and aquaculture. The species has not yet been observed in southern European countries (e.g., Portugal), where niche habitats such as harbours, mudflats, and lagoons reach higher temperatures than the invaded northern range. The aims of this study were to determine H. takanoi potential to acclimate to a southern thermal range, assess thermal performance, and identify potential competitive advantages over the native shore crab Carcinus maenas. This was accomplished through individual exposure to one of three treatments, resembling a southern European thermal range (17 °C, 21 °C, or 25 °C), for 30 days. H. takanoi thrived in temperatures as high as 25 °C, a realistic thermal scenario in Óbidos Lagoon (the biggest Portuguese lagoon)—while the native shore crab C. maenas underperformed at that treatment, showing lower survival, growth, and feed intake than H. takanoi. These results suggest that H. takanoi may be able to colonise southern and warmer European locations and have higher plasticity than the native species already inhabiting areas close to its upper thermal limits. Under global warming scenarios, inferences of further latitudinal spread and increased invaded range may also be taken to the invaded northern European habitats.

Crafting markets of exclusion: Frontex and the border security sector

This article analyzes processes, networks and actors on markets for border control as they relate to the operational work and structures of the European Border and Coast Guard (Frontex). More specifically, based on data collection and semi-structured interviews with stakeholders, it contributes new research on migration industries and infrastructures, by examining how Frontex has been reconfigured towards a functionality of generating and developing mar-kets for border security, including surveillance deportations and infrastruc-tures through tenders, contracts and company relations. The article also exam-ines how this development has facilitated the creation of EUROSUR, and EU collaboration with Libyan authorities as well as the implementation of depor-tations from Greece and Turkey. It concludes with more general observations on the political economy of border control resulting from Frontex's crafting of border markets, and the implications it has for human rights and transparency.

The Evolution of the Principle of Shared Responsibility in the Area of European Integrated Border Management

The article includes a legal analysis of the principle of shared responsibility, which is the current basis for implementing European Integrated Border Management (EUIBM). In particular, the article analyzes the main instruments strengthening the institutional position of the European Union in border management, that is, the Frontex standing service and its powers to apply coercive measures and the obligation of Member States to cooperate with the agency. There are three stages in the evolution of the principle of shared responsibility. The first stage begins with the reform of the EUIBM system resulting from the then migration pressure, initiated by the European Commission’s communication of December 2015 and the European Border and Coast Guard (ESGiP) Regulation of 2016, replacing the original Frontex Regulation. The reform allowed for the adoption of the first binding definition of European integrated border management and introduced the principle of shared responsibility. The second stage is marked by the ESGiP reform of 2019, which significantly expanded Frontex’s competences by establishing the agency’s force corps. The third stage, carried out during the period of intensifying geopolitical changes in Europe and the world, begins with the adoption in 2023 of the first multi-annual program implementing and aimed at the effective implementation of EUIBM, the so-called “strategic policy.” The article puts forward the thesis that geopolitical conditions favoring the tendency to tighten operational cooperation have enabled the Union to build an EUIBM based on the principle of shared responsibility and a significant evolution of the principle itself. It seems that EUIBM in its current formula has exhausted the treaty possibilities and further expansion of the system should be preceded by a treaty reform.

Frontex: Transcending the EU Borders

ABSTRACT Does the work of Frontex transform borderlands? The article aims to explore the role of the European Border and Coast Guard Agency (Frontex) beyond the EU borders. Challenging the dominant portrayal of Frontex as a technical border control “tool” deployed at the EU borders, this article unpacks Frontex’s impact outside the EU geographic space. Applying the “cultures of border control” analytical framework and analyzing Frontex’s cooperation with third countries, it casts light on Frontex’s cultural impact in the Western Balkan region, using Albania and Serbia as case studies. The article argues that Frontex is expanding and diffusing the EU border control culture beyond the EU borders. This transcends Frontex’s institutional mandate set by its creators and the technical nature of cooperation agreed by its third partners. Actually, through its cultural impact, it both solidifies and expands its role and salience in border control, escaping the EU geographic limits.

Longevity of subtidal mussel beds (Mytilus edulis) in eutrophic coastal areas

Mussel populations (Mytilus edulis) around the coasts of Europe and the North Atlantic are often ephemeral and have notably experienced a large decline in abundance. Since 1993, annual blue mussel stock surveys have been carried out in the Limfjorden, Denmark. We used the stock survey data combined with electronic monitoring fishing data and Mechanistic Models for the Limfjorden, providing environmental data to investigate the impact of various stressors. Multiple factors were found to affect the longevity of subtidal mussel beds in Limfjorden. Predation by starfish, fishing activities, shell length of the mussels, amplitude in summer temperature and consecutive days of oxygen depletion decreased the longevity of the beds. Conversely, increased biomass, multiple cohorts and increasing water depth demonstrate stabilising effects. Water column stratification had both a negative and positive impact on bed longevity depending on the duration. These analyses can help inform environmental, conservation, and fisheries managers on the long-term trends of population dynamics and gain a deeper understanding of what factors can affect mussel bed longevity in the context of declining stocks.

Reconstructing dynamics of the Baltic Ice Stream Complex during deglaciation of the Last Scandinavian Ice Sheet

Abstract. Landforms left behind by the last Scandinavian Ice Sheet (SIS) offer an opportunity to investigate controls governing ice sheet dynamics. Terrestrial sectors of the ice sheet have received considerable attention from landform and stratigraphic investigations. In contrast, despite its geographical importance, the Baltic Sea remains poorly constrained due to limitations in bathymetric data. Both ice-sheet-scale investigations and regional studies at the southern periphery of the SIS have considered the Baltic depression to be a preferential route for ice flux towards the southern ice margin throughout the last glaciation. During the deglaciation the Baltic depression hosted the extensive Baltic Ice Lake, which likely exerted a considerable control on ice dynamics. Here we investigate the Baltic depression using newly available bathymetric data and peripheral topographic data. These data reveal an extensive landform suite stretching from Denmark in the west to Estonia in the east and from the southern European coast to the Åland Sea, comprising an area of 0.3 million km2. We use these landforms to reconstruct aspects of the ice dynamic history of the Baltic sector of the ice sheet. Landform evidence indicates a complex retreat pattern that changes from lobate ice margins with splaying lineations to parallel mega-scale glacial lineations (MSGLs) in the deeper depressions of the Baltic Basin. Ice margin still-stands on underlying geological structures indicate the likely importance of pinning points during deglaciation, resulting in a stepped retreat signal. Over the span of the study area we identify broad changes in the ice flow direction, ranging from SE–NW to N–S and then to NW–SE. MSGLs reveal distinct corridors of fast ice flow (ice streams) with widths of 30 km and up to 95 km in places, rather than the often-interpreted Baltic-wide (300 km) accelerated ice flow zone. These smaller ice streams are interpreted as having operated close behind the ice margin during late stages of deglaciation. Where previous ice-sheet-scale investigations inferred a single ice source, our mapping identifies flow and ice margin geometries from both Swedish and northern Bothnian sources. We anticipate that our landform mapping and interpretations may be used as a framework for more detailed empirical studies by identifying targets to acquire high-resolution bathymetry and sediment cores and also for comparison with numerical ice sheet modelling.

The geoeconomics of protecting profits from migrants in maritime distress

AbstractCommercial vessels play a significant role in rescuing migrants on the Mediterranean. There are physical and financial risks to performing these commercial rescues that are poorly understood and often ignored. For ship captains, migrants in maritime distress embody the close linkages between geoeconomic and geopolitical risk. I argue that the negotiation of migrant rescues by commercial shipping captains captures the ways in which geoeconomic and geopolitical security‐seeking discourses become entangled and contradictorily embodied in everyday life and suffering. The rationale and logic that underwrite the securitisation of the supply chain is the discursive site where geopolitical and geoeconomic strategy define the risks in maritime rescue space. This study extends from a conceptual framework that identifies the spatial structuring of an ongoing dialectic between geopolitical and geoeconomic strategy as a defining feature of geoeconomic analyses. The research is based on interviews with 24 maritime professionals and a series of freedom of information requests to the European Border and Coast Guard Agency (Frontex) for information about rescues involving commercial ships. This paper contributes new information on the human geography of maritime rescues that involve commercial ships and migrants in the Central Mediterranean.

When Nature Requires a Resource to Be Used-The Case of Callinectes sapidus: Distribution, Aggregation Patterns, and Spatial Structure in Northwest Europe, the Mediterranean Sea, and Adjacent Waters.

The Atlantic blue crab Callinectes sapidus, which is native to the western Atlantic coast and listed among the 100 most invasive alien species in the Mediterranean Sea, is attracting a great deal of interest because of its rapid colonisation of new areas, the significant increase in its population, and the impacts it may have on ecosystems and ecosystem services. Outside its natural distribution range, the species was first found on European Atlantic coasts in the early 1900s and was introduced into the Mediterranean Sea a few decades later, probably through ballast water. Currently, it is found in almost the entire Mediterranean Basin and is also expanding into the Black Sea and along the north African and Iberian Atlantic coasts. Based on a systematic review of C. sapidus occurrences, this study describes its distribution, aggregation patterns, and spatial structure in Northwest Europe, the Mediterranean Sea, and adjacent waters through a series of ecological indicators elaborated using GIS spatial-temporal statistics. The main results highlight that the species is expanding in the Mediterranean and adjacent waters, while in northern Europe, the population remains confined in some areas. Furthermore, the main species detection methods are analysed, finding that traps and nets are the most frequently used methods, and management suggestions are provided.

The Effect of the 18.6‐Year Lunar Nodal Cycle on Steric Sea Level Changes

AbstractWe show that steric sea‐level varies with a period of 18.6 years along the western European coast. We hypothesize that this variation originates from the modulation of semidiurnal tides by the lunar nodal cycle and associated changes in ocean mixing. Accounting for the steric sea level changes in the upper 400 m of the ocean solves the discrepancy between the nodal cycle in mean sea level observed by tide gauges and the theoretical equilibrium nodal tide. Namely, by combining the equilibrium tide with the nodal modulation of steric sea level, we close the gap with the observations. This result supports earlier findings that the observed phase and amplitude of the 18.6‐year cycle do not always correspond to the equilibrium nodal tide.

Frontex as a hub for surveillance and data sharing: Challenges for data protection and privacy rights

The European Border and Coast Guard Agency, more commonly known as Frontex, was established in 2004 with “a view to improving the integrated management of the external borders of the Member States of the European Union.” It was tasked with the responsibility of providing technical support and expertise to Member States in the management of borders. Over the years its mandate has increased considerably through amendments to its legislative framework. This expansion has taken place against a background of serious allegations concerning Frontex's role in violating the fundamental rights of asylum seekers through its involvement in pushback operations – the practice of stopping asylum-seekers and migrants in need of protection at or before they reach the European Union's external border. While Frontex's complicity in pushbacks has been widely examined by academics, its transformation into a major surveillance and data processing hub and its compliance (or lack thereof) with the fundamental rights to privacy and protection of personal data have received limited academic attention.This paper traces the evolution of Frontex over the years and fundamental rights implications of the transformation of its role.

European Atlantic deciduous forests are more resilient to fires than Pinus and Eucalyptus plantations

In this study, we assess how ecosystem type affects the short-term post-fire resilience of fire-prone forests, which are the predominant ecosystems in South West (SW) Europe. For this purpose, we focused on three forest systems, widespread across the European Atlantic coast, which are highly susceptible to fires at their SW distribution boundary, namely the European thermophilous Atlantic deciduous forests, dominated by Quercus robur L., and two types of forest plantations widespread globally, Eucalyptus globulus Labill. and Pinus pinaster Aiton plantations. At the community structure level, resilience to fire was measured based on changes in cover of each vertical stratum. At the functional level, resilience to fire was measured based on replacements of (both morphological and regenerative) plant functional types in the overstory and understory layers. The study variables were vertical structure and abundance of plant functional types in both overstory and understory layers. Vertical structure was assessed by defining nine vertical strata and estimating the cover in each vegetation stratum. All species identified in the overstory and understory layers were functionally classified according to their morphological attributes and regenerative traits pertaining to fire. One year after fire, we observed considerable differences in the structure between the three forest communities studied, particularly in the overstory layer, because most of the vertical strata were well recovered in the oak forest, partially recovered in the E. globulus plantation, and completely lost in the P. pinaster plantation. In contrast to the Atlantic deciduous forest and eucalyptus plantation, the pine plantation displayed distinct changes in its distribution of plant functional types. The pine plantation showed extremely low resilience to fire because the overstory layer decreased considerably, and the understory layer experienced substantial changes in the distribution of morphological and regenerative plant functional types. Fire favoured certain species which prevailed over others and, while grasses, herbaceous and spiny shrubs increased, the abundance of most morphologically functional plant types disappeared or decreased. These changes in the structure of forest communities and in the distribution of plant functional types have important implications for ecosystem functioning and services provision. Therefore, this study may serve as a foundation for devising forest management measures aimed at increasing ecosystem resilience to fire.

Impact of the Gulf Stream front on atmospheric rivers and Rossby wave train in the North Atlantic

The Gulf Stream (GS) ocean front exhibits intense ocean–atmosphere interaction in winter, which has a significant impact on the genesis and development of extratropical cyclones in the North Atlantic. The atmospheric rivers (ARs), closely related with the cyclones, transport substantial moisture from the North Atlantic towards the Western European coast. While the influence of the GS front on extratropical cyclones has been extensively studied, its effect on ARs remains unclear. In this study, two sets of ensemble experiments are conducted using a high-resolution global Community Atmosphere Model forced with or without the GS sea surface temperature front. Our findings reveal that the inclusion of the GS front leads to approximately 25% enhancement of water vapor transport and precipitation associated with ARs in the GS region, attributed to changes in both AR frequency and intensity. Furthermore, this leads to a more pronounced downstream response in Western Europe, characterized by up to 60% (40%) precipitation increases (reductions) around Spain (Norway) for the most extreme events (exceeding 90 mm/day). The influence of the GS front on ARs is mediated by both thermodynamic and dynamic factors. The thermodynamic aspect involves an overall increase of water vapor in both the GS region and Western Europe, promoting AR genesis. The dynamic aspect encompasses changes in storm tracks and Rossby wave train, contributing to downstream AR shift. Importantly, we find the co-occurrence of ARs and the GS front is crucial for inducing deep ascending motion and heating above the GS front, which perturbs the deep troposphere and triggers upper-level Rossby wave response. These findings provide a further understanding of the complex interaction between the oceanic front in the western boundary current regions and extratropical weather systems and the associated dynamics behind them.

Distribution of the critically endangered fan mussel Pinna nobilis population in the Çanakkale Strait and Marmara Sea

This study was conducted with the primary objective of determining the presence of both healthy and impacted Pinna nobilis populations along the European coasts of the Sea of Marmara, followed by the identification of P. nobilis abundance and survival rates in the region encompassing the Çanakkale Strait and the southern coasts of the Sea of Marmara. Underwater surveys were randomly conducted at 19 distinct stations, including 8 stations along the European coasts of the Sea of Marmara, 7 stations along the southern coasts of the Sea of Marmara, and 6 stations within the Çanakkale Strait. SCUBA diving equipment was utilized to record information on habitat structure, water temperature, depth, and visibility at each station. The transect length during underwater surveys and the number of transects at each station were determined based on the condition of the seabed and the size of the area, respectively. Throughout the study period (September 2021 and October 2023), water temperature fluctuated between 17.5°C and 26.6°C. At the end of the study, a total of 395 individuals (147 live, 248 dead) were observed, with live individuals exhibiting total lengths ranging from 16.4 cm to 50.9 cm. This study represents the first investigation into the spatial distribution of P. nobilis along the European coast of the Marmara Sea. The study contributes significantly to enhancing our understanding of the ecology of P. nobilis populations in both the Sea of Marmara and the Çanakkale Strait. Additionally, recommendations for the rehabilitation of impacted populations and the conservation of healthy populations have been provided for decision-makers and fisheries managers.

Unravelling the distribution of three Ammonia species (Foraminifera, Rhizaria) in French Atlantic Coast estuaries using morphological and metabarcoding approaches

Assessing the distribution of species in natural environments is essential for their use in environmental surveys. Here, we investigate the distribution of three pseudo-cryptic species formerly lumped in the morphospecies Ammonia tepida (Cushman, 1926), commonly found on estuarine mudflats along the European coasts: Ammonia veneta Schultze, 1854 (T1), Ammonia aberdoveyensis Haynes, 1973 (T2) and Ammonia confertitesta Zheng, 1978 (T6). We studied their distribution at 51 sites located in seven estuaries of the French North Atlantic coast (Elorn, Aulne, Odet, Crac'h, Auray, Vilaine, Vie), using both morphological and molecular identification methods. Ammonia veneta was detected by both approaches at most of the stations. While A. aberdoveyensis was frequently identified by the morphological method but not detected with metabarcoding, the presence of A. confertitesta in the eDNA data often contrasted with its absence in the morphological analysis. The absence of A. aberdoveyensis in eDNA of sites where it was identified morphologically could be the consequence of its relative scarcity, and eventually a patchy distribution. Concerning A. confertitesta, we hypothesise that these contradictory results can be explained by the supposedly invasive character of this species. Despite the widespread presence of A. confertitesta genetic material (including adults, juveniles and propagules), a mature population has not yet fully developed everywhere. The seven investigated estuaries seem to represent different stages of replacement of the autochthonous species A. veneta and A. aberdoveyensis by A. confertitesta. Our study demonstrates that the combination of visual observations and molecular approaches is ideal for monitoring the progressive spreading of exotic species.