In recognition of the global scale of biological invasions (Sardain et al. 2019; Bailey et al. 2020) and the impacts they have (Anton et al. 2019; Dueñas et al. 2021; Cuthbert et al. 2022), there is increasing emphasis on the prevention and management of invasive alien species. Target 6 of the Kunming-Montreal Global Biodiversity Framework (CBD/COP/DEC/15/4) directly addresses this imperative. To support progress towards this ambitious target and comply with national legislative directives, comprehensive measures of spatial and temporal trends in alien species occurrences are needed (Lehtiniemi et al. 2015; McGeoch et al. 2023). Such data, obtained through consistent monitoring and surveillance (Loureiro et al. 2021), underpins risk assessments (Kumschick et al. 2020), management feasibility assessments (Booy et al. 2020), and contingency planning (Booy et al. 2017; Padayachee et al. 2019).

Despite their importance, invasion records and long-term survey data are largely incomplete in the marine realm due to challenges with monitoring in highly connected environments, limited search efforts, insufficient methodological standardization, ambiguous invasion histories, and declines in taxonomic expertise (Ojaveer et al. 2014; Bailey et al. 2020; Fowler et al. 2025). These gaps are particularly pronounced for smaller-bodied taxa (Galil et al. 2018) and understudied groups (e.g. Ascidians (Zhan et al. 2015) and Anthozoans (Glon et al. 2020)). The absence of such baseline occurrence data is especially problematic in the marine realm, where invasive species management is uniquely challenging. When a marine invader goes undetected in a region and becomes established, eradication becomes exceedingly challenging and is seldom feasible (Guastella et al. 2019; Simberloff 2020; Ribeiro et al. 2023). In such cases, containment of the invader and the identification of high-risk areas where proactive management might still be feasible, may be the only viable options (Giakoumi et al. 2019).

For these reasons, once a marine invader is identified within a country, understanding the extent of the invasion (e.g. Mabin et al. 2017; Bezuidenhout and Robinson 2020) is an essential first step for management intervention. Anticipating spread, particularly to nearby vulnerable areas, is the next step. Species distribution models are increasingly used to forecast suitable regions for marine invasions (e.g. Goldsmit et al. 2020; Blanco et al. 2021; Ohanna et al. 2025). These models can support the creation of watchlists and contingency plans (Jarnevich et al. 2023; Clarke et al. 2025), ultimately reducing invasion-related costs by guiding proactive management (Diagne et al. 2021; Cuthbert et al. 2022).

The alien anemone Cereus pedunculatus Pennant, 1777 is established and naturalized in Langebaan Lagoon marine protected area (MPA) on the west coast of South Africa (Robinson and Swart 2015; Fig. 1) and exemplifies the challenges posed by limited and outdated baseline data. This European species is native to northern Europe and the Mediterranean Sea (Schmidt 1972; Manuel 1981) and was first reported from Langebaan Lagoon in 2001 as Sagartia (now Cylista) ornata (Acuña et al. 2004), although recent work has clarified its identity as Cereus pedunculatus (Andersen et al. in press). The distribution of C. pedunculatus in Langebaan Lagoon changed considerably during the first 12 years after it was first detected (Robinson and Swart 2015). However, no subsequent monitoring has been conducted. This lack of up-to-date information limits our understanding of its invasion dynamics in this important MPA and hinders compliance with South Africa’s National Environmental Management: Biodiversity Act 2004 (NEMBA Act No. 10 of 2004), which requires regular reporting on the status of biological invasions.

Historically, C. pedunculatus was thought to be restricted to Langebaan Lagoon (Robinson and Swart 2015), due to the deep waters and narrow intertidal zones of Saldanha Bay and the exposed nature of the open coast (Griffiths et al. 2010; Assis et al. 2015). Yet, in January 2025, a population of this anemone was detected over 650 km away in Knysna Estuary on the South African south coast (van Blerk unpublished data). Knysna Estuary is of particular conservation importance as it supports several at-risk species (e.g. the range-restricted endemic limpet Siphonaria compressa Allanson, 1958 (Angel et al. 2006), the endangered seahorse Hippocampus capensis Boulenger, 1900 (Lockyear et al. 2006), and the locally endangered seagrass Zostera capensis Setchell, 1933 (Adams 2016)). The estuary also shares faunal and environmental similarities with Langebaan Lagoon (Angel et al. 2006). This raises concerns about the potential vulnerability of other sheltered areas, such as Berg River Estuary, which is even closer to Langebaan Lagoon.

This study used the invasion of C. pedunculatus in South Africa as an example of maintaining baseline data on a marine alien species and assessing its potential for spread. Specifically, we pursued two complementary aims: (1) to update and extend the long-term dataset for C. pedunculatus in Langebaan Lagoon MPA by conducting the first surveys of its distribution and abundance since 2013; and (2) to identify potential invasion hotspots at national and global scales using ensemble-based species distribution modelling. Integrating field surveys with distribution modelling enhanced our current understanding of the C. pedunculatus invasion in South Africa and its potential for spread, elucidating priority areas where proactive management measures should be focused.

Monitoring the abundance and distribution of alien species is an important prerequisite for evidence-based management (Giakoumi et al. 2019) and for fulfilling national and international regulatory obligations (Lehtiniemi et al. 2015). Yet many marine alien species go unmonitored after they are first reported (e.g. alien anemones (Gimenez and Brante 2021; Gimenez et al. 2022)). The present study provides an updated assessment of the abundance and distribution of the alien anemone Cereus pedunculatus in Langebaan Lagoon marine protected area (MPA), contributing to a long-term dataset for the species and supporting compliance with reporting requirements (NEMBA Act No. 10 of 2004). Surveys suggest that C. pedunculatus populations are persistent and widespread across the lagoon, as the anemone was detected in six previously uninvaded areas and has extended into the high- and low-shore zones for the first time. Particularly concerning are the high anemone abundances recorded in Zostera capensis beds, given the regionally endangered status of this seagrass (Adams 2016). Of the 31 highly suitable sites detected by the species distribution model between the Namibian border and East London, four are located within MPAs. The knowledge that these sites, and several other regions identified across the globe, are vulnerable to C. pedunculatus can support focused surveillance efforts and trigger proactive management measures at these locations.

Introductions of C. pedunculatus to South Africa and other highly suitable regions, such as Denmark and Norway, Alaska, Australia, New Zealand, and Argentina, may be facilitated by two key pathways. The first is commercial shipping, particularly via sea-chests (i.e. cavities within the hulls of ships through which seawater is taken up for use). Although C. pedunculatus has occasionally been recorded on artificial hard substrata in its native range (Lenzi et al. 2009), its absence from such habitats in South Africa suggests that hull fouling is not a likely mechanism. In contrast, sea-chests have been known to provide refuge for fouling organisms in transit, including several alien anemones (Coutts and Dodgshun 2007; Frey et al. 2014). The reproductive strategies of C. pedunculatus, which include planktonic planula larvae in its native range (Rossi 1975) and the asexual release of live young (Bocharova and Kozevich 2011; Andersen et al. in press), are conducive to the uptake and release of viable offspring by sea-chests. Additionally, the recognised tolerance of this anemone to anoxic conditions (Riedel et al. 2008) supports the plausibility of survival over long-distance translocation in sea-chests. Given the proximity of the Port of Saldanha Bay to Langebaan Lagoon, it is possible that C. pedunculatus was introduced via shipping activity in the bay (e.g. iron ore export (Henrico and Bezuidenhout 2020)) and subsequently transported into the lagoon by tidal currents, as has been reported for other alien taxa (e.g. Ackland et al. 2025a).

The second possible pathway of introduction for C. pedunculatus is via oyster mariculture. South Africa has a long history of importing oyster spat from Chile and Europe for cultivation (Keightley et al. 2015). This practice has been linked to several species introductions across South Africa, including in Saldanha Bay and Knysna Estuary (Haupt et al. 2010a). In Europe, range expansions of C. pedunculatus have been linked to shellfish imports (e.g. in the Oosterschelde estuary, Netherlands (den Hartog and Ates 2011)). Given that South Africa has received oyster spat from the native range of the anemone (i.e. France (Haupt et al. 2010a)), this pathway provides a clear link to its presence in South Africa. Saldanha Bay is the focus of South African mariculture (Probyn et al. 2015, 2023), with oyster cultivation dating back to 1985 (Haupt et al. 2010b). An experimental oyster farm was run in Langebaan Lagoon for a short time before the MPA was established (Haupt et al. 2010b). Oyster cultivation in Knysna Estuary dates back to 1973 (Haupt et al. 2010b) but ceased in the early 2010s (Keightley et al. 2015). Given that there are no published records of C. pedunculatus in Knysna Estuary before early 2025 (van Blerk unpublished data), despite numerous benthic surveys (e.g. Allanson et al. 2000; Barnes 2021), it is unlikely that C. pedunculatus was introduced to Knysna before Langebaan Lagoon. However, it is unclear as to whether the presence of the anemone in Knysna is the result of a secondary introduction or spread from Langebaan Lagoon (over 650 km away). Surveys of highly suitable areas with past mariculture operations (i.e. Kleinsee (Haupt et al. 2010a) and Swartkops Estuary in Algoa Bay (Haupt et al. 2010b)) may help elucidate the current distribution and invasion history of the anemone within the country. Given the known prevalence of the species in brackish and polluted lagoons in its native range (e.g. Cognetti and Maltagliati 2000), surveys should not be limited to pristine sites.

Considering that the anemone is not tolerant to turbulent conditions (Schmidt et al. 1972), like those observed along South Africa’s wave-exposed coastline, natural dispersal from Langebaan and Knysna is unlikely. However, both domestic oyster spat transport (Haupt et al. 2010a) and recreational activities may facilitate more localized spread of C. pedunculatus between suitable sites. Yachts are documented mechanisms for the dispersal of marine alien species in South Africa (Peters et al. 2017). Although C. pedunculatus is unlikely to foul vessel hulls, juveniles may be transported in bilge water, which is periodically discharged at new locations, including harbours and estuaries (Fletcher et al. 2020). Another plausible mechanism is the translocation of live bait species, a recognized pathway for the unintentional dispersal of associated marine organisms (Sá et al. 2017; Smith et al. 2020). In both Langebaan Lagoon and Knysna Estuary, recreational fishers frequently collect sand and mud prawns (Nel and Branch 2014; Simon et al. 2019). As a sediment-dwelling species, C. pedunculatus may be transported alongside prawns and unintentionally released into new habitats. Inspecting bilge water, fishing gear, and bait aboard recreational vessels in Langebaan Lagoon and Knysna Estuary may provide insights into the intra-regional spread of this anemone.

Given that the species distribution model found much of the South African west and south coasts suitable for C. pedunculatus, it may already be present at several of the sheltered sites identified near Langebaan and Knysna that have not yet been surveyed. As such, highly suitable sites like the Berg River Estuary, St Helena Bay Harbour, Hoedjiesbaai, and Grotto Bay (near Langebaan), as well as Goukamma MPA and the site east of the Knysna Estuary mouth, should be prioritised for surveys of C. pedunculatus. Likewise, management agencies for the four MPAs should consider surveying for this species and developing contingency plans (sensuBooy et al. 2017) to support effective responses to future incursions at these ecologically important sites. Uninvaded but suitable sites near oyster mariculture operations (e.g. Kleinsee and Kings Beach), commercial ports (e.g. Santos Bay and Kings Beach), or recreational vessels traffic (e.g. North of Baboon Point and Kabeljousriviermond) should be closely monitored for future introductions of C. pedunculatus so that management action can be applied upon first detection of the species. As for sites that are already invaded, understanding the extent of the invasion is an essential first step for management (Giakoumi et al. 2019). Given that this information is unknown for Knysna Estuary, distribution and abundance surveys are necessary to determine whether the species has become established and if management interventions are still feasible. Considering that much of this estuary contains suitable habitat for the anemone (i.e. mudflats, seagrass beds, and cordgrass (Claassens et al. 2020)), it is likely that C. pedunculatus will spread within the estuary if it has not already done so.

Looking beyond South Africa, this study identified several highly suitable regions across the globe from where C. pedunculatus is not yet known. Monitoring programs in these regions should include C. pedunculatus on their watchlists. To avoid possible impacts associated with an invasion, priority should be placed on critical habitats, such as seagrass beds, within ecologically, culturally, or economically significant areas (i.e. MPAs and World Heritage Sites). Interestingly, northwestern Africa is also highly suitable for the species and lacks occurrence records despite its vicinity to more well-studied regions where the anemone is known (i.e. The Canary Islands and Morocco). Given the relatively low survey effort in northwestern Africa (Ackland et al. 2025b), it is possible that the anemone is present there but has not yet been detected. If C. pedunculatus is present, this region may either represent part of its native range or a range expansion.

While this study provides a crucial update to the long-term dataset for C. pedunculatus in Langebaan Lagoon MPA and highlights its potential for establishment in other parts of South Africa and various regions globally, the source of the invasion and the relationship between the Langebaan Lagoon and Knysna Estuary populations remain unknown. A population genomics approach could offer insights into connectivity between these locations and the native range helping to inform effective management going forward. Collectively this knowledge supports prioritisation of suitable sites for targeted monitoring and proactive management, providing an opportunity to institute preventative measures to manage pathways to these sites (Padayachee et al. 2019). Such a two-pronged approach will minimise the probability of spread and the potential for establishment of new populations, ultimately mitigating impacts that may be associated with this alien anemone.

MNA: conceptualization, methodology, investigation, data curation, formal analysis, writing - original draft, review and editing; GS: methodology; writing - review and editing; TBR: conceptualization, methodology, writing - review and editing, supervision.

The authors have declared that no competing interests exist.

No ethical approval was required for this study. Fieldwork was conducted under research permits CRC/2024-2025/011--2024/V1 and RES2025/67.

This research was funded by the Centre for Invasion Biology at Stellenbosch University.

This project would not have been possible without the support of SANParks West Coast National Park staff in the field. We also thank Zeané Nel for her assistance with field surveys, Daniel van Blerk for prompting the detection of Cereus pedunculatus in Knysna, and Megan van der Berg for her guidance with species distribution modelling. We are grateful to the anonymous reviewer whose comments helped improve this manuscript.

All datasets and associated code have been uploaded to Zenodo and are available at https://doi.org/10.5281/zenodo.17097997.