The Mediterranean Sea hosts over 80 introduced macrophyte species (Boudouresque and Verlaque 2002), so it is considered a hotspot for marine alien species (Rilov and Galil 2009). At least one-ninth of them are invasive, affecting food provision for human consumption, natural cycling of nutrients, diversity and complexity of assemblages and cultural services such as recreational uses (Katsanevakis et al. 2014). The introduced species Caulerpa cylindracea Sonder has shown notorious effects (Montefalcone et al. 2015). Its vegetative propagation by fragmented thalli allows it to attach itself to anchors and disperse by drifting. Marinas and harbours are the putative focus from which it starts to expand locally (Verlaque et al. 2003, Cantasano et al. 2017), as has been observed for its conspecific invader Caulerpa taxifolia (Vahl) Agardh (Boudouresque et al. 1995). Moreover, the invasiveness from harbours and marinas to the neighbouring urban shores could be facilitated by the usually higher values of some human-induced stressors such as eutrophication and sedimentation, which also affect the native recipient assemblages (Connell et al. 2008), generating synergistic effects between the invader and the stressors mentioned above (Bulleri et al. 2010, Gennaro and Piazzi 2011, Airoldi et al. 2015). The invasion of C. cylindracea has serious effects on recipient communities, but assemblages with large canopy-forming species seem to dampen the effects of the invasion (Ceccherelli et al. 2002). Thus, the impact on recipient communities could be related to their state of health, degraded ones being more prone to invasion because of the increased supply of limiting resources such as nutrients (due to eutrophication), or light and space when recipient canopy-forming species decrease in cover, allowing the spread of the exotic ones (Huenneke et al. 1990, Marín-Guirao et al. 2015). Many studies have detected that unhealthy native communities usually show higher cover values of C. cylindracea than healthy ones with a good status of canopy-formers, either involving seaweeds and seagrasses (Ruitton et al. 2005, Ceccherelli et al. 2014, Bulleri et al. 2017) or gorgonians (Ponti et al. 2018). Furthermore, the homogenization effect on community composition and structure caused by the invasion could involve other processes such as impaired fish recruitment (Cheminée et al. 2016).

In the eastern Iberian Peninsula, C. cylindracea recently spreads over the coastal area between Catalonia (García et al. 2015) and Andalusia (Altamirano et al. 2014), where it shows a similar distribution pattern to that reported in other Mediterranean regions (Verlaque et al. 2003, Klein and Verlaque 2008). Thus, it thrives in shallow subtidal waters and in deep subtidal habitats beyond 30 m depth. It is more abundant on margins of Posidonia meadows or on dead Posidonia matte but also colonizes maerl bottoms and photophilous assemblages (Ruiz et al. 2011, Enguix et al. 2014, García et al. 2015).

The detection of C. cylindracea in vermetid platforms of the southeastern Iberian Peninsula (Pena-Martín et al. 2003) and its high cover values in some locations could shed light on the drivers fostering or hampering its invasiveness.

Vermetid platforms constitute a distinctive Mediterranean habitat, yet it has been highly neglected to the present day (Milazzo et al. 2016). These shallow systems are present in southeastern Iberian littoral shores (Templado et al. 2016) and other warm Mediterranean areas and subtropical seas (Laborel and Kempf 1965, Safriel 1975). Vermetid platforms are well developed in limestone rocks, and their assemblages show a distinctive zonal pattern. Thus, in non-degraded situations, an outer vermetid rim is followed by fucoid algae forming a continuous belt on relatively exposed shores. In less exposed situations, such as in the inner parts of such platforms, low stands of smaller algae flourish and include dictyotacean algae as an important group. Encrusting corallines with vermetids also dominate the innermost part. Vermetid platforms harbour many Mediterranean endemisms and contribute to the increase in regional diversity as a result of the environmental gradient generated over the platform (Milazzo et al. 2016). The shallowness of these platforms entails that these systems are subjected to many anthropogenic stressors (Ballesteros et al. 2007). Thus, the vermetid platforms in the eastern Mediterranean have been severely impaired in a few decades (Galil 2013, Badreddine et al. 2019). Putative reported drivers of this impoverishment are the rise in sea level, global warming, trampling, acidification and all factors linked to human activities, including water deterioration (Milazzo et al. 2004, 2014, Badreddine et al. 2019). As for bioinvasions, little direct information has been reported despite their likely effects on these platforms. The invasive red sea mussel Brachydontes pharaonis (Fisher, 1870) is colonizing the vermetid platforms in the eastern Mediterranean, spreading towards the west (Milazzo et al. 2016). This invasive species was considered an inferior competitor to the native ones, but by increasing the pool of recruits, it became dominant at the expense of other indigenous sessile species (Rilov et al. 2004, Didham et al. 2007). Little is known about the effects of C. cylindracea on these shallow systems. In vermetid platforms with undegraded macroalgal communities, C. cylindracea has been mainly reported in a patchy distribution within the vermetid platform, in most cases with low cover values. The alga grew intermingled with the thalli of other macroalgae without any clear relationship between the abundance values of C. cylindracea and those of the dominant macroalgae (Ramos-Esplà et al. 2008, Balistreri and Mannino, 2017). However, in the south of Alicante Bay, we detected high cover values of C. cylindracea on degraded vermetid platforms distinguished by their degraded macroalgal composition, which is attributable to the eutrophication of Alicante Bay (Aranda et al. 1994, Aguilar 2009). The C. cylindracea population of degraded vermetid platforms contrasts with the low cover values observed in the nearby location of Cabo de las Huertas, though both locations happen to be the first ones where this invasive species was detected in Alicante province 15 years ago (Pena-Martín et al. 2003, Terradas-Fernández et al. 2018).

In order to understand whether the ecological status of vermetid platforms could be involved in the abundance of C. cylindracea, an annual survey was carried out in both locations, comparing the abundance of the macroalgal species forming overstory with the abundance of C. cylindracea. Environmental variables informative of anthropogenic affectation (sediment cover and nutrient concentration) were also measured. Hypotheses that could explain any differences regarding the cover of C. cylindracea and the ecological status of the recipient macroalgal communities were discussed.

In this study, the main differences in the composition of assemblages between locations are even greater than those within them along the entire platform width. Local drivers acting at the degraded location could explain the sharp differences found between locations. However, the specific causes driving a community shift in urban areas are still an open issue (Benedetti-Cecchi et al. 2001, Bertocci et al. 2017, Mancuso et al. 2018) and are far beyond the scope of this study. Even so, the correlation between urbanization and the replacement of fucoid algae and other phaeophycean groups by E. elongata is well known in the Mediterranean Sea (Pinedo et al. 2007, Mangialajo et al. 2008, Pinedo and Ballesteros 2019), pointing to eutrophication and sedimentation among the most essential drivers of change in anthropogenic-affected habitats (Munda 1993, Airoldi 2003, Pinedo et al. 2015). Moreover, these factors could interact synergistically, favouring the growth of turf algae that hampers the recruitment of other algal species (Gorgula and Connell 2004).

Being correlative, the results of this study do not reveal any cause-effect relationship between the low ecological status of the degraded location and its values in nutrient concentration and sediment cover. Furthermore, the high variability found leads to difficulties in interpreting the role of these variables at both locations, though at the degraded location there is a slight trend towards higher values of sediment cover and nutrient concentration.

The higher algal cover values found in the outer zones of each location could easily be related to the fact that their productivity potential (Steneck and Dethier 1994) is higher than that of the inner ones (Terradas-Fernández et al. 2018) and to the lower effect of low sea level events (pers. obs.). There is a clear homogenization of assemblages at the degraded location, where E. elongata determines its annual dynamics. The decline episodes in these articulated algae could be related to natural stressors such as low sea level events, high light intensity and high temperatures, all of which could act together, intensifying this decline. These episodes have also been reported, especially during summer months, in other very shallow systems dominated by articulated corallines (Benedetti-Cecchi and Cinelli 1994, Guerra-García et al. 2011, Bertocci et al. 2012). On the other hand, at the non-degraded location, the greater diversity in species and their temporal replacement keep a high overstory cover in the outer margin zones throughout the year.

The lack of replacement of articulated corallines by other overstory species could act as a key point in determining the abundance of C. cylindracea at the degraded location. Thus, the low ecological status of the overstory could act as potential driver of the invasion (Piazzi et al. 2016 and references therein). This spread of C. cylindracea could be fostered in warmer months, when the decline of E. elongata coincides with the optimal growth temperature for C. cylindracea (Samperio-Ramos et al. 2015). Then, presumably, C. cylindracea could take advantage of both the free space generated by E. elongata decay and the “thermal opportunity window”. However, some competitive interaction between E. elongata and C. cylindracea cannot be ruled out. The lack of detection of C. cylindracea populations beyond the studied platforms suggests that the growth of C. cylindracea is due to a persistent resting stage rather than to propagule-recruitment processes (Uyà et al. 2017), taking into account the unlikely sexual reproduction for C. cylindracea (Varela-Álvarez et al. 2012).

The higher stress occurring in the inner zones may hamper the invasiveness of C. cylindracea towards the inner parts of the platform. However, the high cover of C. cylindracea observed in the inner degraded zone in October 2016, coinciding with the low overstory cover, could be the result of high sea level episodes improving the growth of C. cylindracea. This scenario is likely to occur in late summer and autumn months, when the mean sea level is usually higher in the souteastern Iberian Peninsula (Soto 1987, Adloff et al. 2018).

All in all, in this study our results indicate that C. cylindracea acts rather like an opportunistic species, taking advantage of the less persistent overstory at the affected location. Though competitive interactions cannot be ruled out, factors affecting the recipient degraded overstory seem to better explain the decay of this overstory than interactive processes with C. cylindracea. This decline, when abiotic conditions are optimal for C. cylindracea growth, would be harnessed by this alien species, in line with Bulleri et al. (2010). Thus, C. cylindracea would rather behave as a passenger of change on these degraded platforms (MacDougall and Turkington 2005). However, stressors such as sedimentation and eutrophication could also foster C. cylindracea establishment (Piazzi et al. 2005, Gennaro and Piazzi 2011), and once established, C. cylindracea could enhance sediment accumulation, acting as a driver (Bulleri et al. 2010). Thus, the slightly higher values of both stressors at the degraded location makes their role as enhancers of C. cylindracea growth feasible. Owing to the high level of homogenization of the assemblages from the degraded and urbanized Mediterranean littoral fringe, the establishment and enhancement of C. cylindracea growth on other degraded Mediterranean vermetid platforms is plausible.

Further studies, especially long-term manipulative and correlative ones, are required to understand better how the ecological status of vermetid platforms affects the invasiveness of C. cylindracea. Possible future scenarios linked to global change are of particular concern: among others, the higher extremeness trend of disturbances such as storms and low sea level episodes, which could affect the overstory cover of the vermetid platforms (Sanz-Lázaro 2016, Zamir et al. 2018).