RESULTS AND DISCUSSION

All specimens have a globular and irregular shape with the siphons, more or less prominent (Figure 1); the colour is brown, and some individuals have small epibionts (i.e. rhodophytes, foraminifers and serpulids) on the tunic. The smallest specimen (12 mm long) was found in Alsancak Harbour (January); and the sizes of the Tunisian specimens ranged between 15 and 35 mm (July). The siphonal spines (Figure 2) are curved (‘shark-fin like', Kott, Reference Kott1985) and about 50 µm in length. The gonads of the smaller specimen were not fully developed; whereas the specimens from the Gulf of Gabès had well-developed gonads.

Fig. 1.

Microcosmus exasperatus from south of Kerkennah (Tunisia) sampled by dredge, 23–25 m depth on dead Posidonia oceanica matte.

Fig. 2.

Siphonal spines of Microcosmus exasperatus from Alsancak Harbour. Scale bars: 50 µm.

The individuals were not aggregated, and only small groups of 3–5 specimens were observed on dead Posidonia oceanica rhizomes. Some of the specimens were sampled on artificial substrata (concrete piers and harbours) in shallow waters (0–10 m depth), others on benthic habitats drastically influenced by trawling (Posidonia meadows and muddy detritic bottoms). In the degraded P. oceanica meadows (9–25 m depth), some shoots have still alive leafs, but the majority are dead matte with the alien bivalve Pinctada radiata (Leach, 1814). In muddy detritic bottoms (34–36 m depth), the ochrophyte Arthrocladia villosa (Hudson) Duby, 1830 and some rhodoliths are present. In the previous studies (Méliane, Reference Méliane2002; Bitar et al., Reference Bitar, Ocaña and Ramos-Esplá2007; Shenkar, Reference Shenkar2008; Izquierdo-Muñoz et al., Reference Izquierdo-Muñoz, Díaz-Valdés and Ramos-Esplá2009; Shenkar & Loya, Reference Shenkar and Loya2009), specimens of Microcosmus exasperatus were also collected mainly on artificial substrates (concrete reefs, piers, harbours and plastic nets).

This opportunistic species colonized altered or degraded communities, often forming monospecific aggregations (Rius et al., Reference Rius, Pineda and Turón2009). Although, M. exasperatus was observed in natural shallow rocky habitats in Israel (Shenkar & Loya, Reference Shenkar and Loya2009), the community was impoverished with the dominance of the calcareous red algae (Amphiroa, Jania, Corallina, Lithophyllum and Neogoniolithon), due to the high herbivorous pressure (Siganus spp., Sparisoma cretense (Linnaeus, 1758), Conomurex persicus (Swainson, 1821)). The area was also degraded by bottom trawling.

Microscomus exasperatus, was first recorded from Djerba Island (south of Tunisia) in 1998 (Méliane, Reference Méliane2002), and then it was observed on the coasts of Lebanon and Israel (Bitar et al., Reference Bitar, Ocaña and Ramos-Esplá2007; Turón et al., Reference Turón, Nishikawa and Rius2007; Shenkar, Reference Shenkar2008; Izquierdo-Muñoz et al., Reference Izquierdo-Muñoz, Díaz-Valdés and Ramos-Esplá2009; Shenkar & Loya, Reference Shenkar and Loya2009) in the eastern Mediterranean (Table 2). With regards to M. squamiger, it was first reported from northern Tunisia (identified as M. exasperatus) in 1962 (Monniot, Reference Monniot1981). Afterwards, M. squamiger was observed around the western Mediterranean and the Atlantic adjacent waters (Turón, Reference Turón1987; Ramos-Esplá, Reference Ramos-Esplá1988; Ramos-Esplá et al., Reference Ramos-Esplá, Buencuerpo, Vázquez and Lafargue1993; Naranjo, Reference Naranjo1995; Mastrototaro & Dappiano, Reference Mastrototaro and Dappiano2005; Turón et al., Reference Turón, Nishikawa and Rius2007), and the northern part of the Ionian Sea (Monniot, Reference Monniot1981; Mastrototaro et al., Reference Mastrototaro, Petrocelli, Cecere and Matarrese2004; Izquierdo-Muñoz et al., Reference Izquierdo-Muñoz, Díaz-Valdés and Ramos-Esplá2009) (Figure 3).

Fig. 3.

Geographical distribution of the Microcosmus squamiger (triangles) and M. exasperatus (rhombus) in the Mediterranean Sea, with the first record/year of the species (from different sources; see text).

Table 2.

Records of Microcosmus exasperatus from the Mediterranean Sea.

The distribution of M. exasperatus in the eastern Mediterranean basin (Figure 3) appears to be discontinuous, occurring in three areas (Gulf of Gabès, Levantine coasts and eastern Aegean Sea). The occurrence of this species along the Mediterranean coast of Israel and southern Lebanon (airport piers of Beirut) is more or less continuous (Shenkar & Loya, Reference Shenkar and Loya2009), but not in northern Lebanon, as this species has not been observed in natural habitats (e.g. Ras Chekka and Anfeth; A.A.R.E., personal observation). On the other hand, it seems to be isolated at the location in the eastern Aegean Sea (Izmir Bay), as it was not found in previous studies performed along the Turkish coast (Çinar et al., Reference Çinar, Bilecenoglu, Öztürk and Can2006, Reference Çinar, Bilecenoglu, Öztürk, Katağan, Yokeş, Aysel, Dağlı, Açık, Özcan and Erdoğan2011).

At present, no M. exasperatus record has been made from the northern-central Ionian and western Aegean areas (Italy, Greece, Malta and eastern Tunisia). The introduction vector for the spread of this species within the Mediterranean Sea could have been by the maritime traffic through commercial harbours between the Gulf of Gabès and Izmir Bay (Skhira and Zarzis in Tunisia and Alsancak in Turkey). It is the first time that an alien ascidian (M. exasperatus) has been reported from the Turkish coast of the Aegean Sea. In the previous studies (Çinar et al., Reference Çinar, Bilecenoglu, Öztürk and Can2006, Reference Çinar, Bilecenoglu, Öztürk, Katağan, Yokeş, Aysel, Dağlı, Açık, Özcan and Erdoğan2011), three alien ascidians (Phallusia nigra Savigny, 1816, Symplegma brakenhielmi (Michaelsen, 1904) and Herdmania momus (Savigny, 1816)) were reported from the Levantine coast of Turkey. Kondilatos et al. (Reference Kondilatos, Corsini-Foka and Pancucci-Papadopoulou2010) and Thessalou et al. (Reference Thessalou, Aydogan, Bekas, Bilge, Boyaci, Brunelli, Circosta, Crocetta, Durucan, Erdem, Ergolavou, Filiz, Fois, Gouva, Kapiris, Katsanevakis, Kljajic, Konstantinidis, Konstantinou, Koutsogiannopoulos, Lamon, Macic, Mazzette, Meloni, Mureddu, Paschos, Perdikaris, Piras, Poursanidis, Ramos-Esplá, Rosso, Sordino, Sperone, Sterioti, Taskin, Toscano, Tripepi, Tsiakkiros and Zenetos2012) also observed P. nigra on the south-eastern coast of Rhodes and the Peristera Island in Greece, respectively.

Comparing the present distributions of the two alien Microcosmus in the Mediterranean Sea, it appears that their geographical areas are not overlapping and the colonization of M. exasperatus seems to be more recent than M. squamiger (Figure 3). Microcosmus squamiger occurs in the western Mediterranean and the northern part of the Ionian Sea, whereas M. exasperatus has become well established in the eastern Mediterranean basin (Levantine Sea) and the Gulf of Gabès (southern Ionian Sea). These findings might indicate that two different routes of colonizations are possible: one from the Atlantic through the Strait of Gibraltar (for M. squamiger); and another from the Indo-Pacific region through the Suez Canal (for M. exasperatus). The historical record might also suggest that the initial establishments of both species took place in the central Mediterranean, and that they have successfully expanded their distributional ranges to the western and eastern parts of the Mediterranean. This kind of spread might have been mediated by maritime traffic and/or current regimes in the area. Furthermore, different ecological conditions between the eastern and western Mediterranean basins (Pérès, Reference Pérès1967) could have affected their distributional patterns.

No discernible negative effect of M. exasperatus on the ecosystem and economy has been reported from the area so far. It is possible that this species could create problems in prevailing ecosystems and man-made structures, like other invasive ascidian species (see Lambert, Reference Lambert2007). However, the ecological and distributional features of this species in the area should be investigated and monitored to assess its possible impacts on the Mediterranean ecosystems.