Studies of biofouling communities in Argentinian harbours began in the 1960s and have provided a fairly complete picture of the fouling communities from Buenos Aires Province and North Patagonia (Bastida 1971, 1973, Bastida et al. 1980, 1997, Martin et al. 2000). These studies have mainly dealt with faunal inventories, settlement cycles of species, trophic relationships, epibiosis, resistance of organisms to control biofouling substances and other general ecological topics (Bastida et al. 1976, 1980). They helped to clarify the mechanisms of colonization of these communities, as well as the processes of ecological succession on artificial substrates in marine and estuarine environments. More recently, studies were carried out in central Patagonia, specifically in the port of Comodoro Rivadavia (Rico and López Gappa 2006). However, studies of biofouling communities are virtually non-existent in southern Patagonia.

One of the objectives of studies on biofouling communities is to attempt to solve problems related to the bio-deterioration of man-made structures in the marine environment caused by organisms. These include docks, boats, facilities such as refrigeration plants and power stations that use seawater as a coolant, and underwater archaeological sites. Knowledge of the composition and succession cycle of fouling communities allows different strategies to be developed to avoid the settlement of marine organisms on artificial surfaces, or to control excessive development of communities (Bastida et al. 2004, Dürr and Thomason 2010). On the other hand, studies of fouling communities in harbours that are situated within, or adjacent to, reserve areas often have other objectives, usually related to the conservation of biodiversity. Knowledge of fouling communities settled in relatively pristine areas can establish a baseline for monitoring the impact on marine biodiversity caused by urban, port and industrial activities and detecting entry and establishment of invasive alien species related to shipping traffic (Lewis and Coutts 2010). This is the case of Bahía San Julián, which is part of a protected area within which intertidal communities have recently been studied, whereas the subtidal community is practically unknown. Knowledge of the community is particularly important in this case, as in 2013 local port activity began to recover slightly and fishing boats from harbours located northwards of the locality began to operate.

The sea anemone Metridium senile is a cosmopolitan species, predominantly found subtidally and frequently reported in biofouling communities of cold-temperate areas in the Northern Hemisphere. It often acts as a structuring species of the community by affecting the early stages of ecological succession (Nelson and Craig 2011). Riemann-Zürneck (1975) and Zamponi and Acuña (1991) reported this species in different parts of the Argentinean Continental Shelf at depths of 95-120 m. Subsequently, the species was also reported from the port of Cape Town in South Africa (Griffiths et al. 1996, Acuña and Griffiths 2004), where it was possibly introduced by shipping traffic from European ports. It is important to note that currently there is an ongoing study of different populations of M. senile worldwide to find out whether they are different species or subspecies (Häussermann and Försterra 2009).

In Argentina, the importance of this species in the benthic community of coastal southern Patagonia remains unknown but few studies have been done on biofouling in the region. The main goals of this study were to provide information on a fouling community dominated by M. senile in the harbour of Bahía San Julián, and to analyse processes of epibiosis that this anemone exerts as a strategy of colonization leading to its dominance in the community. This study also aims to provide a baseline regarding the characterization of the fouling community of Bahía San Julián, depending on the possible revival of the local fishing port and the potential risk of introduction of exotic marine species.

The fouling community from Bahía San Julián showed similar traits in its composition to those found in natural rocky substrates in the region, with a high abundance of mussels in lower intertidal levels, from where they were progressively replaced by other species in the subtidal. In the Ría Deseado (200 km N of Bahía San Julián), the association of Perumytilus purpuratus, Mytilus edulis platensis and Aulacomya atra atra dominates and characterizes the community in the lower intertidal level, mainly in the vertical rocky substrates inside the ría. In the shallow subtidal, the community of Ría Deseado is characterized by a decrease in the abundances of P. purpuratus and M. edulis platensis and a predominance of A. atra atra and Polyzoa opuntia, accompanied by Paramolgula gregaria, Crepidula dilatata dilatata (=Crepipatella dilatata), Photinula caerulescens, Balanus psittacus (=Austromegabalanus psittacus), Aerothyris venosa (=Magellania venosa) among the most conspicuous species (Ringuelet et al. 1962, Otaegui and Zaixso 1974, Zaixso and Pastor 1977). Despite the similarities found in the community of vertical hard substrates between the two sites, the absence of the sea anemone M. senile in works of the 1960s, 1970s and early 1980s in the Ría Deseado is striking. Indeed the very low abundance of Actiniaria as a group in the subtidal of Ría Deseado (Callebaut Cardu and Borzone 1979) is very different from the situation found in the present study, in which Actiniaria, represented only by M. senile, was a dominant group. Moreover, López Gappa et al. (1982) recorded the following actiniarians among fauna associated with Macrocystis pyrifera holdfast in Ría Deseado: Isoedwardsia nidarosiensis, Aiptasiamorpha minima, Acontiophorum mortenseni and Isanthus capensis. All of these showed very low abundance in the community.

The only exotic species of the fouling community of Bahía San Julián identified was the gammarid amphipod Monocorophium insidiosum, which is characterized as invasive on the coast of Argentina. This species, usually seen in high densities in biofouling of harbours, is widely distributed in the world, being transported mainly by human action. In Argentina, it was detected in the harbour-fouling community of Mar del Plata in the 1970s, when it was identified as Corophium comparable with Corophium insidiosum. Subsequently, it was reported for other harbours of Buenos Aires Province and the Patagonian coast (Bastida et al. 1980, Alonso de Pina 1997, Martin et al. 2000, Orensanz et al. 2002).

The virtual absence of exotic species in fouling and natural hard substrate communities of Bahía San Julián is in marked contrast with fouling from other harbours of the region, such as Puerto Madryn, Puerto Deseado and Comodoro Rivadavia (Fig. 1), which have high shipping activity. In these harbours several invasive alien species have been reported in recent years, such as Undaria pinnatifida, Carcinus maenas, Ascidiella aspersa, Balanus glandula and Cryptosula pallasiana (Piriz and Casas 1994, Orensanz et al. 2002, Hidalgo et al. 2005, Cuevas et al. 2006, Rico and López Gappa 2006, Martin and Cuevas 2006, Schwindt 2007, Martin and Bastida 2008). However, none has been found so far in Bahía San Julián. The low shipping activity in the local harbour, which has been practically inactive for several years, could be responsible for the absence of exotic species common in other harbours of the region. The absence of these alien species is consistent with the assumption that the dispersion along the Patagonian coast is fundamentally linked to commercial shipping traffic (commercial fishing and transport) and the passive spread by currents has limited scattering power due to the large extension of the coast. Thus, this study provides interesting information on this problem and provided a baseline for Bahía San Julián, especially taking into account the possible reactivation of port activities in the locality.

A particular feature of the fouling community of Bahía San Julián is the dominance of the sea anemone Metridium senile at all levels sampled. This funding has not been reported so far in the intertidal or subtidal natural hard substrates from the region. However, M. senile is currently a common species in hard substrate communities from the open sea around Bahía San Julián, along with other species of sea anemones, such as Antholoba achates and Bunodactis octoradiata (Garese et al. 2014). This sea anemone forms aggregates in free lower intertidal substrates, usually in tidal pools and on horizontal and vertical walls, and to a lesser extent as an epibiont (Acuña et al. 2011). This suggests that protected conditions in the inner of Bahía San Julián and the existence of a limited, but stable artificial substrate generate a suitable environment for successfully settlement of a sea anemone population, reaching high abundances and dominating the community. A similar phenomenon was observed in harbour biofouling of cold-temperate areas in the Northern Hemisphere (Nelson and Craig 2011).

It is important to note that M. senile is also currently an abundant species within the Ría Deseado, where it mainly inhabits the lower intertidal and shallow subtidal rocky substrate (Acuña et al. 2011). In that location, it can also be observed in intertidal shores composed of fragments of rock and mud inside the port and surrounding areas, and as an epibiont of the seaweeds Undaria pinnatifida, Macrocystis pyrifera and Lessonia sp. (Martin unpublished data). However, as mentioned above, the species was not cited in previous works in that locality in the 1960s 1970s and early 1980s (Ringuelet et al. 1962, Zaixso 1975, Zaixso and Pastor 1977, Callebaut Cardu and Borzone 1979, López Gappa et al. 1982), which suggests that its entry to the Ría Deseado could have occurred from the mid-1980s or 1990s, when commercial shipping traffic and fishing activity started and intensified in that locality. It is also important to note that the existence of this species could not be confirmed along the Chilean coast, despite extensive samplings conducted in the last decade (Häussermann and Försterra 2005, Häussermann 2006).

At San Julián harbour, shipping activity reached its peak during the 1990s, when fishing vessels operated from Argentinian harbours located northwards (e.g. P. Madryn and C. Rivadavia), and from some from foreign harbours. During the 1990s, for example, vessels from Spain and Cape Town (South Africa), where M. senile has been recorded as an invasive alien species since 1995 (Griffiths et al. 1996), were recorded as entering Bahía San Julián. Given the cosmopolitan nature of M. senile, its high abundance at present, and the fact that it has not been cited in previous works on the region, we recommend granting this anemone the status of an alien invasive species in Argentina, or at least the rank of a cryptogenic species: one for which it is not possible to demonstrate whether it is native or introduced (Carlton 1996).

There are several factors that make M. senile the dominant species of the biofouling community of San Julián harbour, one of which is its great ability to bind to different substrates, including other sessile organisms or structures derived from them, as in the case of polychaete tubes. The analysis of epibiosis processes shows that the biological substrates commonly used by M. senile are molluscs, in particular the mussel A. atra atra, the gastropod Crepipatella dilatata, and the tunicate Paramolgula gregaria. However, this would not be a result of active selection of these biological substrates. The significant correlation observed between the abundance of substrate-species and the number of individuals of such species with epibiosis indicates that the frequency with which a given organism is used as a substrate for M. senile is determined largely by its abundance. Furthermore, statistical analysis makes it clear that there is no difference between the relative abundance of substrate-species and their frequency of use (%F), indicating that there is no selection of them by the anemone. Neither do the substrate-species exhibit effective mechanisms to prevent or inhibit the binding of the anemone, although, for example, cleaning behaviour to avoid epibiosis was described for Mytilus edulis by Theisen (1972) and Bruzzone (1982). Moreover, analysis of the attachment intensity and average number of epibionts per individual suggests that the number of anemones settled on a substrate species is also linked to the size of the substrate and the surface available for epibiosis. Thus, although the sea squirt P. gregaria is not an abundant species in the community, it is one of the most intensively used substrates in the subtidal levels, due to the large size of its specimens and hence the large surface available for settlement of the sea anemones. A. psittacus, was found to have a negative selection as a substrate at Level 1 because, despite its relatively high abundance in the community, the epibiosis frequency was very low on this organism, possibly because they were of very small size at this level (mean =13.64 mm, sd=5.76 mm basal diameter), providing a very small attachment surface. This was not the case at Level 2 where, though the abundance of barnacle is much lower than that at Level 1, the specimens were significantly larger (mean =43.48 mm, sd=3.68 mm basal diameter), offering a wider attachment surface for M. senile. In the latter case, no differences between the relative abundance of the barnacle in the community and the epibiosis frequency on the barnacle were observed.

We conclude that for M. senile the frequency and intensity of epibiosis is primarily determined by abundance and size of each substrate-species available. This underlines the high plasticity of the anemone to attach on several substrates, increasing their characteristic capacity of colonization. Thus, epibiosis emerges as a strategy of colonization when hard substrate availability is scarce, as is common for many benthic organisms (Wahl 1997), and it is particularly important for M. senile inside Bahía San Julián.

The high ability for epibiosis reinforces the potential of colonization of M. senile and, with other biological characteristics, allows us to understand its great adaptation to harbour environments and its cosmopolitan distribution. These features include the clonal mode of reproduction, sustained mainly by pedal laceration. This mode of asexual reproduction is typical of subtidal anemones and, according to the hypothesis of Sibly and Calow (1982), would be selected to promote growth in favourable environments such as the subtidal, while longitudinal fission would be the dominant mode in less favourable environments for growth, such as the intertidal zone. In the Northern Hemisphere (Gulf of Maine, USA), similar results for the same sea anemone were observed, where it was dominant in a community of subtidal rocky substrate and prevailed upon ascidians, bryozoans, sponges, coralline algae and other anthozoans (Sebens 1986). The aforementioned author highlights the rapid growth of the anemone and its dense aggregation, which excludes even colonial ascidians, as mechanisms that determine the competitive success of this species. It should also be noted that this anemone can develop tentacles with aggressive feature (catch tentacles), that may prevent or limit the settlement of other organisms, especially genetically different clones of the same species. In dense populations of M. senile, where essentially all space is occupied, the catch tentacles are lost because of “habituation” or “induced tolerance” to neighbours (Shick 1991).

In some subtidal communities in the Northern Hemisphere, mussels provide an important secondary substrate for populations of M. senile, showing coverage of up to 100% (i.e. Kaplan 1984). However, the mobility of mussel patch aggregates can frequently result in the death of their epibionts, which can be buried or located in unfavourable positions incompatible with life. Thus, the relative instability of the biological substrate, such as mussels, stimulates locomotion of the anemone causing the laceration of the pedal disc and the clonal reproduction of individuals, as observed by Kenneth et al. (1995). Moreover, experiments carried out by Nelson and Craig (2011) on artificial substrates show that the movements of M. senile kill new recruits from other fouling organisms by asphyxiation (smothering), possibly due to stress conditions generated by the low concentration of dissolved oxygen below the pedal disc. This process would allow M. senile to eliminate potential space competitors of surrounding substrate and continue dominating the space inside the fouling community. The dominance of M. senile observed in the fouling community of Bahía San Julián may then be explained by the effect on new recruits demonstrated by Nelson and Craig (2011) and also by its ability to settle on a wide range of biological substrates.