This study documents the first occurrence and rapid expansion of the solitary ascidian Cnemidocarpa irene in natural marine habitats of Tenerife (Canary Islands). Native to the Indo-Pacific, C. irene had previously been introduced to the Caribbean, Brazil, and Cape Verde. It was first observed in Tenerife in 2020, though retrospective records through citizen science tools date its presence back to 2018. A total of 74 sightings along the island’s coasts were reported between 2018 and 2024, when it reached densities of ca. 2 individuals/aggregates per square metre in the initial introduction area. Thus, the species is undergoing a clear proliferation and a spatial expansion. Morphological and genetic analyses confirmed the identity of C. irene and its phylogenetic placement, closely related to other Cnemidocarpa and related genera such as Asterocarpa. This species shows concerning invasive characteristics, such as a fast expansion, abundance in natural habitats, and aggregative behaviour, suggesting potential threats to native biota. Due to its limited natural dispersal capacity, the introduction of C. irene to Tenerife is attributed to anthropogenic vectors, particularly oil platforms arriving at major Canary Island ports. The proximity of the initial records to port areas supports this hypothesis. Given the potential species’ ecological risks, the authors recommend close monitoring, manual removal where feasible, and strengthened involvement of citizen science. This case highlights the vulnerability of oceanic islands to marine biological invasions and the importance of ports and marinas as critical entry points, underscoring the need for proactive surveillance and early intervention strategies.

Biological invasion, citizen science, DNA barcoding, taxonomy, tunicate

Non-indigenous species (NIS) are a major cause of ecological change in marine communities and one of the main threats to global biodiversity (Dukes and Mooney 1999; Bax et al. 2003; McGeoch et al. 2010). Assessing the initial stages of any introduction is key to allow prompt management and to ascertain the introduction dynamics and forecast potential spread. However, this information is lacking for most known introductions and subsequent invasions in the marine environment.

Oceanic islands in particular are well known models for biogeography and ecology studies. They are particularly sensitive to invasions by non-native species due to their high degree of endemism (Sax and Gaines 2008). While terrestrial introductions in insular systems have been extensively studied as models in invasion science (e.g., Simberloff 1995; Lenzner et al. 2020), much less is known on the effects of marine invasions on islands, with Pacific archipelagos being the most studied so far (e.g., Paulay et al. 2002; Inglis et al. 2006; Carlton and Eldredge 2015).

The Macaronesian region in the eastern Atlantic (including Azores, Madeira, Canary Islands and Cabo Verde archipelagos) has been the subject of some studies of marine NIS (e.g., Canning-Clode et al. 2013; Chainho et al. 2015; Pajuelo et al. 2016). In a recent review of Macaronesian marine non-native species (Castro et al. 2022), the Canary Islands have been signalled as a hotspot for introductions compared to nearby archipelagos, and this is attributable to the intense shipping activity, touristic pressure involving a dense network of marinas, oil drill settings, and aquaculture activities, among others. Over 110 marine NIS have been reported in the Canary Islands (https://www.biodiversidadcanarias.es/exos, accessed 21 December 2024).

Among marine invasive organisms, ascidians are an important group that includes species of major concern (Lambert 2001, 2007), some of which have worldwide colonisation ranges (Lambert 2007; Zhan et al. 2015). They are sessile marine filter feeders with some characteristics suitable for successful colonisation: rapid growth rate and short time to maturity (Rius et al. 2009; Pineda et al. 2013; Casso et al. 2018), physical and chemical defences (Stoecker 1980; López-Legentil et al. 2006), and the production of a large number of short-lived non-feeding planktonic larvae (Lambert 2001). In the Canary Islands, the presence of nearly 50 new ascidians has been recorded over the past 10 years (https://www.biodiversidadcanarias.es/exos; Martín-Solà et al. 2022).

Invasive ascidians are commonly found on artificial substrates (Tyrrell and Byers 2007) which makes them one of the main fouling groups in marinas, aquaculture facilities and other human-built structures (Airoldi et al. 2015). There are, however, some cases of ascidian invasions of natural benthic communities, where they can aggressively overgrow epibenthic communities and cover large areas of the seafloor. Some prominent instances are Didemnum vexillum in the NE American coast (Bullard et al. 2007; Kaplan et al. 2018), Pyura praeputialis in Chile (Castilla et al. 2004), or Distaplia stylifera in Baja California (Moreno-Dávila et al. 2023).

We describe here the recent arrival and expansion of a solitary ascidian previously unreported in the Canary Islands. This species is identified as Cnemidocarpa irene (Hartmeyer, 1906), an Indo-Pacific species that has been first introduced in the Caribbean, from where it expanded its range to Brazil and Cape Verde islands. The species can thrive in natural habitats and therefore poses a worrisome threat to native biota, warranting urgent monitoring.

The specimens studied match previous descriptions of Cnemidocarpa irene, in particular the detailed ones of Monniot (1983, as C. areolata), Nishikawa (1991), Rocha et al. (2012), and Monniot (2018). It should be noted that the synonymy of the species is complicated, as discussed by Monniot and Monniot (1984) and Nishikawa (1991). C. irene is a species found from shallow waters to deep sea (Monniot 2021), and is a cause of concern for its invasive potential (Locke 2009; Monniot 2018). Punta Cangrejo, the locality of the first sightings and where the highest densities of the species have been found, is located a few kilometres south of the main port of Santa Cruz, in an area with predominantly southward currents. Thus, it is likely that the species first entered the island via the commercial hub of its capital.

Although detailed morphological examination is essential for the reliable identification of ascidian species, it is not feasible for all sightings, particularly those reported through citizen science platforms that rely primarily on photographs. Therefore, diagnostic characters based on external features are desirable. In C. irene, aside from overall shape and coloration, the inner siphonal pattern can serve as a useful diagnostic trait, as the marbled appearance and coloration of the siphons are distinctive. Many Styelidae and Pyuridae, groups that include similarly sized solitary ascidians, exhibit a marked pattern of longitudinal bands in the siphons. A potential source of confusion is Pyura herdmani, which has similarly marbled inner siphonal linings, occurs in NW Africa, and has recently been introduced to Europe (Rius et al. 2024). However, individuals of this species are much larger (up to 30 cm) than those of C. irene. For this and other solitary ascidian species, it is strongly recommended that citizen science practitioners obtain underwater pictures of individuals with their siphons wide open to capture the characteristic colour patterns.

Cnemidocarpa irene was described from Japan (Hartmeyer 1906) and is common in the Indo-West Pacific region (Kott 1985, as C. areolata, Rho and Park 1998; Monniot and Monniot 2003; Locke 2009; Lambert et al. 2021), but in the western Atlantic it had only been reported for the Caribbean (Monniot 1983, as C. areolata; Monniot and Monniot 1984, as C. valborg). Monniot (1983) indicated that it had likely been introduced in the Caribbean, and the species has been recently reported as introduced to Brazilian waters (Rocha et al. 2012, 2024; Dias et al. 2013; Granthom-Costa et al. 2025).

In the eastern Atlantic it was first reported in Cape Verde in 1990 by Monniot and Monniot (1994), albeit these authors noted also that the species C. maroccana may be instead C. irene, in which case there would also be a report in Morocco, pushing back in time the presence of the species in the eastern Atlantic. The type specimen being lost, this fact remains unverifiable (Monniot and Monniot 1994).

The genetic results show that the sequences obtained formed a robust clade, which in turn groups with that of C. verrucosa and Asterocarpa humilis. The latter was initially placed in the genus Cnemidocarpa, until the new genus Asterocarpa was described (Brewin 1946). Both genera are closely related (Kott 1985), and are distinguished on the basis of a different gonad arrangement, albeit this separation is not accepted by some authors (Monniot et al. 2001). Our genetic results did not support the establishment of a separate Asterocarpa genus. The closest clade to the Cnemidocarpa + Asterocarpa one comprises the species of Styela, a genus closely related to Cnemidocarpa also on morphological grounds (Kott 1985). On the other hand, the nature of the rather divergent sequence of C. finmarkiensis surely deserves more studies with taxonomically verified material and more sequences. Albeit COI lacks resolution to establish a reliable phylogeny, the results obtained with this dataset confirm the placement of our sequences in the genus Cnemidocarpa and agree in some aspects (such as the polyphyly of Polyandrocarpa and Polycarpa) with more detailed phylogenomic analyses (Alié et al. 2018).

Given the reduced natural dispersal abilities of the short-lived larval stage of ascidians (Svane and Young 1989; Lambert 2001), the arrival of C. irene to the Canary Islands is attributable to anthropogenic transport. There are several potential mechanisms, from plastic trash rafting to oil platforms that can move entire benthic and fouling communities. However, oil platforms have been recently highlighted to be one of the most important introduction vectors in the Canary Islands to explain the massive exotic fish species arrivals of the last two decades, which has coincided in time with an increase of oil platform arrivals (Pajuelo et al. 2016; Falcón et al. 2023) to Santa Cruz in Tenerife and Las Palmas in Gran Canaria, the main harbours of these islands. The exotic communities mainly come from tropical regions of the eastern Atlantic and the Gulf of Guinea. However, Canary Islands also receive platforms from western tropical Atlantic regions such as Brazil, the Caribbean. Finally, a few rigs have been transported from the Indo-Pacific region. These movements imply that large ports, particularly those receiving oil platforms, are hotspots for non-indigenous species and, at the same time, priority areas for the early detection and management prior to the spread of potential marine biological invasions (Lehtiniemi et al. 2015; Romeo et al. 2015; Tempesti et al. 2020). The presence of C. irene was traced back to 2018, and in the shallow natural environment. These initial records and the majority of C. irene sightings reported on the RedPROMAR platform were concentrated in the northeastern part of the island (RedPROMAR 2025), near the port area of Santa Cruz de Tenerife, the main maritime hub of the island. After the identification of C. irene, its presence has also been noted in harbours of the island (Martín-Solà, pers obs), where it had likely been previously overlooked, so an initial date of introduction in harbours cannot be determined at present.

As the introduction is recent, it is not possible to ascertain its effects on local communities. While aggregates formed by C. irene may provide substrate for other species, they can also compete for food and space with the native biota and have detrimental effects, especially if they reach high abundances as the increasing trend observed suggests. Given the current extent of C. irene proliferation in Tenerife, eradication of the species is no longer a realistic goal; it may have been feasible only at the onset of its introduction (likely in 2018 or shortly before). At present, mitigation measures such as manual removal are only practical when newly colonizing individuals are detected early upon arrival at a locality. To be able to identify these early stages of introduction, a close monitoring is necessary, particularly in and near harbours and marinas. The help of local citizens and diving clubs can prove invaluable in this respect. This case highlights the vulnerability of oceanic islands to marine biological invasions, underscoring the need for proactive surveillance and early intervention strategies.

This research was partially financed by project BlueDNA (PID2023-146307OB), funded by the Spanish Ministry of Science, Innovation, and Universities (MICIU/AEI/10.13039/501100011033) and by ERDF/EU.

All authors, study conceptualization and design; LMA, JCH, MMS, data collection; LMA, MMS, underwater photographs; JCH, LMA, MMS, time series analyses; XT, morphology and genetic analyses, original draft; JCH, XT, funding; All authors, draft review and editing. We also acknowledge the contributions of two anonymous reviewers.

The authors acknowledge the collaboration of all anonymous citizens that participated in the RedPROMAR initiative. We are also grateful to the comments by two anonymous reviewers that improved our manuscript.