Broad monitoring of NIS across regions, habitats, and taxa
In recent years, genetic methods have gained attention in the field of marine conservation and management (Darling and Blum, 2007; Westfall et al., 2020) and DNA metabarcoding has been promoted as a tool to monitor biological invasions (Comtet et al., 2015). While earlier surveillance efforts for aquatic NIS have primarily monitored one or a few species and have been limited geographically (Ficetola et al., 2008; Mahon et al., 2013; Tréguier et al., 2014), the methods used in this study allow for a broad and continuous screening of NIS across the pan-European regional seas, with sample locations in various habitats, and detecting a wide range of eukaryotic taxa.
During this study, 63 non-indigenous taxa were detected from 1828 identified species across 112 ARMS-MBON sampling events in 19 observatories across 14 countries. Most of these NIS were previously known in the areas where they have been detected.
The most widespread NIS throughout the dataset were Amphibalanus improvisus, Bonnemaisonia hamifera, Bugula neritina, Balanus trigonus, Acartia tonsa and Hydroides elegans. This was an expected result as all these species are well-known marine NIS and successful invaders on European coasts. Information about their respective known ranges and impacts can be found in Supplementary Text S4. The presence of these species across our dataset shows that the chosen approach in this study allowed for the clear detection of well-known invaders of European waters and therefore can be used to monitor their potential expansion in the future. Also, this study showed that a genetic monitoring network such as ARMS-MBON, when performing continuous deployments over long periods of time, does detect well-established NIS in an area.
However, some potential NIS detected in the samples did not appear in our curated data sets. The reason for this is that some taxa, despite being known NIS in Europe, are not registered in WRiMS for the respective study area. As an example, Juxtacribrilina mutabilis (which appeared with its current unaccepted name Cribrilina mutabilis in our data set), a recently described bryozoan from Japan (Ito et al., 2015), is not registered in WRiMS for any European marine region but only for the Pacific and Atlantic coasts of the US. However, it was sampled in the observatories Koster and Swedish West Coast (Sweden), Laeso (Denmark), Svalbard (Norway), Vigo (Spain) and Plymouth (United Kingdom) during this study. This species has shown an unusually fast range expansion within the last two decades (Martaeng et al., 2023), probably because, like many other invaders, this species can attach to different substrate (including ships’ hulls) and be spread across long distances (Dick et al., 2020).
Bougainvillia mucus is a similar case: it is a potential new invader in the Red Sea, where we detected it with 25 reads. It did not appear in our curated NIS data set because it is not listed in WRiMS for any of the selected areas either, as it originates from Europe and has not been previously recorded in the Red Sea. Only manual checking of the overall species list alongside literature search allowed for the identification of its introduction status in the Red Sea which underlines the drawbacks when relying on databases such as WRiMS.
Early warning potential
The methods used to detect NIS in our dataset relied on WRiMS and were designed to minimise false positives and negatives. While this increased the certainty of our results, this also limited the possibilities to detect newly introduced species mainly because the screening was limited to 824 species registered in WRiMS for the larger marine regions our study sites were located in. Despite these limitations, and since we targeted a large geographic area, we encountered a few cases of potential new introductions.
Eucheilota menoni was detected on the north coast of the Adriatic Sea, in Ravenna (Supplementary Figure S9). The respective mOTU was composed of one ASV that was matched to E. menoni with 100% similarity on BOLD (GenBank: 100% query cover and 100% percentage identity). The following match was with Lovenella assimilis, with a 99.68% similarity on BOLD (GenBank: 100% query cover and 99.68% percentage identity). Brylinski et al., (2016) summarised the debate about whether these two species could represent a single species, noting that the criteria for morphological differentiation are still under discussion. Both species, originating from the Indo-Pacific, have never been found in the Adriatic before. We only detected E. menoni in our study as it is registered in WRiMS for both the Bay of Biscay and the Belgian coast where some of our study locations were located at. However, we found no occurrences of this species on ARMS from observatories in these two regions. It is also interesting to note that no occurrences of this taxon were recorded in GBIF for the Belgian coast. The first record in Europe for E. menoni dates from 2009, in the Bay of Biscay (Altuna, 2009). In 2010, L. assimilis was detected in the same area (Altuna and De Okendo, 2010), which was a first record of this species outside of the Indo-Pacific Ocean. In 2016, specimens resembling L. assimilis have been reported for the first time in the eastern English Channel and the southern bight of the North Sea. Morphological analyses showed differences between the two species while genetic analyses using 18S, ITS and mitochondrial COI and 16S rDNA (16S), showed high genetic similarities to available sequences in DNA repositories attributed to E. menoni (Brylinski et al., 2016). For COI, the only reliable L. assimilis COI partial sequence was almost identical to the E. menoni COI sequence (only 1 bp difference). For 16S, the sequences the authors obtained from their specimens were 99% identical to E. menoni sequences. For 18S, their sequences were 100% identical to the only E. menoni sequences available on GenBank. Although this could indicate that these two species are in fact one, the authors highlight that these data should be carefully approached, as some sequences on GenBank do not have a valid morphological description of the specimen used. It is possible that some sequences available in GenBank of E. menoni are based on misidentified specimens of L. assimilis. Therefore, the mOTU found in our study is considered to be E. menoni, but this result should be further investigated. Considering the fouling behaviour of this species, and its ability to easily reproduce asexually, it was expected to be found in the Mediterranean at some point after its discovery on the Iberian Peninsula (Altuna, 2009). In this study, the relatively low read number of this occurrence could point to a recent introduction and a currently low abundance of this species in the area.
Potential new invaders were also detected in the Red Sea. Bugula neritina, a colonial bryozoan originating from the Mediterranean Sea, and which is widespread in Europe (Ryland et al., 2011), is one example. No previous records of this species in the Red Sea were found in GBIF (Supplementary Figure S6) or in the Ocean Biodiversity Information System, OBIS (Intergovernmental Oceanographic Commission of UNESCO, 2024), nor in WoRMS or in other scientific literature. However, it appeared in our data set because Bugula neritina is recorded in WRiMS for four of our selected areas (“Spanish part of the Bay of Biscay”, “Spanish part of the North Atlantic Ocean”, “Swedish part of the Skagerrak'' and “North Sea”). It was detected with 15 reads during one sampling event in the Red Sea, which could suggest a recent introduction. Similarly, Bougainvillia muscus, which we detected manually and not based on WRiMS records (see above), is also a potential new invader in the Red Sea. This species is a hydrozoan native and commonly reported in the Mediterranean Sea, the North Sea and English Channel (Lee II and Reusser, 2012), which was detected with a total of 25 reads in Eilat, Israel, in the COI dataset (Supplementary Figure S10). The presence of B. neritina and B. muscus in the Red Sea might be due to a potential reverse Lessepsian migration through the Suez Canal, so-called anti-Lessepsian migration (Bos et al., 2020). Few Mediterranean species can migrate southwards via this path due to the prevailing south-to-north flow of the canal, but the Suez Canal hosts heavy ship traffic that has been identified as a vector for biological invasions in the Mediterranean (Bereza et al., 2020) and that can be responsible for these anti-Lessepsian movements. Additionally, the Red Sea is characterised by higher salinity, fewer nutrients, and a notably richer diversity of life forms compared to the eastern Mediterranean, which can impede the arrival of these potential invaders (Elsaeed et al., 2021). Therefore, potential anti-Lessepsian migrants should be closely monitored and researched to better understand their migration patterns, ecological impact, and adaptation strategies in new environments, particularly in the context of changing marine conditions and human-induced environmental shifts.
Apionsoma (Apionsoma) misakianum, originating in the Indo-Pacific, was detected in Eilat, Red Sea (part of its native range), in Getxo, Spain, in the Gulf of Piran, Slovenia, and in Roscoff, France. It had previously been detected and considered non-indigenous in the Aegean Sea in 2007(Açik, 2008) and in other Mediterranean areas according to SeaLifeBase (Palomares and Pauly, 2024), but in GBIF no record of this species can be found further north than the Red Sea (Supplementary Figure S5). Our results suggest a potential range extension of this species outside of the Mediterranean Sea.
Fenestrulina delicia, a fouling bryozoan, showed 8 reads in Crete, Greece, in the COI dataset. This species is known on the European Atlantic coasts from France to Norway (Supplementary Figure S7) and could have been recently introduced in the Mediterranean (De Blauwe et al., 2014).
Ostreopsis ovata, an epiphytic, toxic dinoflagellate, coming from tropical regions was found with 8 reads in the 18S dataset in the observatory located in Koster in Sweden (Supplementary Figure S8). (Granéli et al., 2011) concluded that current global warming can influence the geographical expansion to higher latitudes and biomass accumulation by blooms of O. ovata. It was previously known in the Mediterranean, on the Atlantic coasts of France and the Iberian Peninsula (Accoroni et al., 2024; Chomérat et al., 2022; David et al., 2013; Fabri-Ruiz et al., 2024).
Herdmania momus, a known invader in the Mediterranean Sea from the Indo-Pacific Ocean and a known case of Lessepsian migration (Evans et al., 2013; Gewing et al., 2014; Shenkar and Loya, 2008), occurred with 6 reads in Limfjord, Denmark (Fig. 4). This species appears to be adapted to tropical climates; however, according to Gewing et al., (2019), the invasive population in the Mediterranean demonstrated significantly greater survivability under cold conditions than the native population from the Red Sea.
It is important to highlight that these results should be carefully interpreted as the read numbers these species were detected with in the respective areas are relatively low. They might point towards early introductions, spontaneous detections (as opposed to an established and growing population), but they might also be artefacts of sequencing or bioinformatic processing. Future investigations of ARMS-MBON data will increase the confidence of these observations.
Mapping potential range shifts
By comparing the occurrences from our dataset with the extent of those recorded in GBIF, among the latter being many records of species already found outside their native range, we identified discrepancies that may indicate new range expansions and recent introduction events. The geographic location of new detections and distance to known occurrences could potentially indicate the cause of the range shift e.g., global warming (i.e., species detected at moderate distance further north than their known occurrence) or shipping (i.e.,. species from Europe found in distant locations such as the Red Sea).
Scaling up the integration of recent sightings with established extent of occurrence data from databases like GBIF can be a useful tool to identify dynamics of range shifts, vectors of introduction, and factors influencing habitat suitability for NIS. This can also enable predictions of species' responses to various environmental conditions. As ARMS-MBON continues to generate more data, and as ecological data from around the world become increasingly shared (Tenopir et al., 2020) and Findable, Accessible, Interoperable, and Reusable (FAIR) (Tanhua et al., 2019), such methods could provide a robust framework for studying shifts in species distribution both in Europe and globally. Additionally, coupling these methods with species distribution models and/or artificial intelligence algorithms to efficiently identify migrations patterns can facilitate the detection of new NIS in an area or region.
Identification of potential NIS hotspots
We did not detect a significantly higher number of NIS in samples from marinas and harbours compared to samples from other locations (e.g., marine protected areas, areas with low human influence, etc.). Marinas and harbours are often described as NIS hotspots as they are subject to increased anthropogenic impacts such as ballast water discharge (Bailey, 2015) and “transport” of fouling biota (Bailey et al., 2020; Chan et al., 2022; Georgiades et al., 2021), which are common vectors for the introduction of NIS. Marinas and harbours are known to serve as gateways for international and local vessels, which inadvertently transport organisms across biogeographical boundaries. The frequent movement and docking of boats create a dynamic environment that facilitates the establishment and spread of NIS (Ashton et al., 2022). Moreover, the infrastructure of marinas and harbours often provides suitable habitats for NIS to thrive in, including sheltered waters and hard substrates for attachment. However, NIS can also migrate due to climate-change driven vectors such as changes in water temperature and salinity (Floerl et al., 2013; Occhipinti-Ambrogi, 2021). Hence, locations impacted by maritime traffic may not be the only places with high prevalence of NIS which could explain our results.
Additionally, investigating data sets with standardised sampling and sequencing effort allowed for a robust detection of areas with relatively high NIS counts (i.e., potential hotspots): the north of the Adriatic Sea (Ravenna and Gulf of Piran) and the English Channel appeared to be the locations with the most NIS detected across the comparable data sets. These observatories are located near or in marinas/ports, which could indicate a tendency of higher NIS prevalence in these types of monitoring sites. Overall, these results show that large-scale observatory networks such as ARMS-MBON can facilitate the identification of NIS hotspots. As more data continues to be collected through ARMS-MBON, this enables further investigations in this direction.
Methodological challenges
This study demonstrates the potential of our chosen methods for monitoring NIS across extensive geographical areas like Europe. We advocate for the increased use of DNA metabarcoding to detect and track NIS, but also for the application of this method at a large geographic scale, to have a better understanding of species movements beyond the boundaries of countries or single marine regions.
However, we also acknowledge various technical and practical challenges associated with this approach. They can concern different aspects:
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the importance of standardisation when working with a larger monitoring network,
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the accuracy of taxonomic identification through DNA metabarcoding,
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the curation and updating of databases recording introduced species,
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the sensitivity of species detection.
We provide specific points of attention and suggestions for addressing these topics in future research in Supplementary Text S5.