Recent non-indigenous ascidians in the Mediterranean Sea

A revision of the non-indigenous ascidians in the Mediterranean Sea has been carried out, from published and unpublished records. The records considered include the last 50 years, which encompasses the period after the Pérès inventory (in 1958). Our aim is to analyze the ancient and recent records with comments about their validity and possible introduction vectors.


Introduction
The Mediterranean Sea, due to strong temperature gradients (spatially: from Alborán to Levantine Seas; and temporally: tropical in summer and temperate in winter) represents a suitable area for the introduction of species from different origins.In addition, the Mediterranean is particularly exposed to biological invasions through shipping from maritime traffic (Zibrowius 2002) and the opening of the Suez Canal (Por 1978;Galil 2000).Indeed, the Mediterranean has been progressively invaded by nonindigenous species (NIS) (Zibrowius 1992;Zenetos et al. 2005;Streftaris et al 2005;Galil 2007).
Ascidians represent one of the main biofouling taxons, particularly on ship hulls (Millar 1969;Monniot et al. 1985;Lambert 2002).They colonise all types of hard substrata, both natural and artificial (Lambert 2007), mainly in environments characterized by lowdiversity fauna (e.g.estuaries, lagoons, harbors, shellfish farms, eutrophic habitats).The last inventory of littoral ascidians (< 200m depth) in the Mediterranean Sea was published over 50 years ago by Pérès (1958a).From that time new non-indigenous ascidians have become established in some sectors of the Mediterranean, through the Strait of Gibraltar and Suez Canal, or introduced by shellfish culture.In this review, we examine the geographical distribution of these species, citation of source, NIS category and possible vector of introduction.Further, we critically evaluate the validity of the citation.

Methods
Following Williamson and Fitter (1996) and Boudouresque and Verlaque (2002), a NIS must fulfil the following criteria: i) be present in a new area not previously reported; ii) the extension of its range is linked, directly or indirectly, to human activity; iii) there is a geographical discontinuity between its native area and the new area (i.e.remote dispersal); and iv) the new generations of the species are born in situ without human assistance, thus constituting self-sustaining populations (i.e.naturalized species).Pérès (1958aPérès ( , 1967) ) compiled a list of ascidians (132 species) in the Mediterranean Sea, which represented the first complete inventory of this taxon in the area.In the present review, we analyse the new records of NIS ascidians in the Mediterranean, excluding the Black Sea, including published and unpublished records (updated May 2008).We have added information, mainly from the Eastern basin (e.g., Israel, Lebanon, Cyprus, Malta, eastern Tunisia) and, in particular, from the PORTAL project (http://www.ciesm.org/marine/programs/portal;Izquierdo-Muñoz et al. 2006).The NIS have been grouped into the six categories proposed by Zenetos et al. (2005): established, casual, questionable, cryptogenic, excluded and invasive.

Results
Since 1958, ten NIS tunicates have become established and colonized some sectors of the Mediterranean (Figure 1, Annex 1).Some species have been reported only from specific habitat types rather than in the Mediterranean Sea proper.These include Polyandrocarpa zorritensis (Van Name, 1931), found in delta/estuarine or degraded habitats (Brunetti 1981;Turón and Perera 1988;Brunetti and Mastrototaro 2004); Botrylloides violaceus Oka, 1927 in the Venetian lagoon (Zaniolo et al. 1998;Occhipinti-Ambrogi 2000); and Styela clava Herdman, 1881, recorded recently in the Thau lagoon in France (Davis and Davis 2008).These last two species, whose origin is Japan, probably have been introduced by shell-fish cultures.Mastrototaro and Brunetti (2006).
According to Zenetos et al. (2005), most ascidians introduced in the Mediterranean prior to 1958 (Pérès 1958a) must be considered as cryptogenic species.These species have not definite evidence of their native or introduced status or, else, the probable introduction occurred 'in early times'.

Discussion
Although ship-mediated introductions have been only incidentally and infrequently documented in the Mediterranean (Zibrowius 2002), the majority of NIS ascidians have probably been transported in the adult stage by shipping (across the Straits of Gibraltar and Suez Canal) attached on hulls.Botrylloides violaceus and Styela clava seem to have been recently introduced by shellfish culture (Occhipinti-Ambrogi 2000; Davis and Davis 2008).The introduction of NIS has increased in the last years (Zibrowius 1992;Zenetos et al. 2005;Streftaris et al. 2005), enhanced by the progressive tropicalization of the Mediterranean.Thus, it is highly probable that future invaders will be arriving from tropical regions (Francour et al. 1994;Boero 2002;Occhipinti-Ambrogi 2007).This is corroborated by the spread of some recent NIS ascidians with tropical affinities (Rhodosoma turcicum, Phallusia nigra, Herdmania momus, Microcosmus exasperatus and M. squamiger) in the Mediterranean.
Taxonomy and identification have been major limiting factors in the ability to detect NIS ascidians in the area.This is compounded by the lack of updated local fauna inventories and the lack of taxonomic expertise and knowledge in some Mediterranean areas (i.e., North Africa and Levantine Sea).Several NIS ascidians now known from the Mediterranean Sea were probably initially misidentified; Cystodytes philippinesis was reported as C. dellechiajei, Perophora multiclathrata as P viridis and Symplegma brakenhielmi as S. viride.Ecteinascidia moorei (cited at Alexandria by Harant 1939) has become synonymized as E. turbinata.A revision of the citations of these species would show a greater range of expansion.

Figure 1 .
Figure 1.Location of the recent non-indigenous ascidians in the Mediterranean.The families are indicated by symbols: • Polycitoridae ▲ Perophoridae o Corellidae ■ Ascidiidae x Styelidae ♦ Pyuridae Rhodosoma verecundum is in reality R. callense (Lacaze-Duthiers 1865).Distaplia bermudensis Van Name 1902, identified only by Pérès in Iviza (1957), has been recently recorded in South Italy by in Port Said(Egypt).Recent records of non-indigenous ascidians in the Mediterranean Sea.Year of first record, author and year of the report, the NIS category, vector and point of entry of introduction, habitats and locations colonized in the Mediterranean Sea and some observations are indicated.(RS)Red Sea and (SG) Strait of Gibraltar.Numbers for locations are shown in Figure1