Alien crabs in the Mediterranean Sea: current status and perspectives

Dimitris Klaoudatos1, Kostas Kapiris2

Hellenic Centre for Marine Research, Institute of Marine Biological Resources, 47th km Athens-Sounio, Mavro Lithari P.O. Box 712, 19013, Anavissos, Attica, Greecee-mail: 1 dklaoudatos@hcmr.gr, 2 kkapir@hcmr.gr

AbstractBiological invasions are recognized worldwide as an important element of global change. The Mediterranean Sea a semi-enclosed, deep, oligotrophic basin is one of the seas most affected by biological invasions in terms of duration of invader’s presence, number of alien species detected and the unprecedented rate of introduction.

The opening of the Suez Canal in 1869 had serious zoogeographically and ecologically affected both Red Sea and the Mediterranean Sea, with different faunistic and hydrographic attributes, tropical Indo-Pacific origin the first, temperate the second one. The impacts of invasive species on their new environment include the restructuring of established food webs, competition with native organisms, alteration of gene pool, extinctions, and introduction of new diseases.

The total number of the alien species in the Mediterranean (2012) was 986 species (775 in the eastern Mediterranean, 249 in the central Mediterranean, 190 in the Adriatic Sea and 308 in the western Mediterranean). The native range of the alien species in the Mediterranean is most commonly the Indo-Pacific Ocean (40%), the Indian Ocean (17%), the Red Sea (11%) and pantropical (9%). The majority of aliens in the easternmost Mediterranean have entered through the Suez Canal (68%, 15% vessel-transported, 2% mariculture), whereas mariculture (43%) and vessels (37%) are the main means of introduction in the western Mediterranean.

The alien decapod fauna in the Mediterranean has lessepsian and Atlantic origin. Approximately 77% of all alien Mediterranean decapod species have an Indo-Pacific/Indian/Red Sea origin, while only 23% are from the Atlantic. The biggest number of the alien decapods (82%) is located in the eastern part of the Mediterranean in comparison to the western one. The increasing number of the alien species in the last years could be attributed mainly to the increasing passage of vessels through the Suez Canal, the development of mariculture and other anthropogenic pressures.

A number of 39 alien Brachyura species of Red Sea/Indo-Pacific origin, belonging to 19 families, have been recorded in the Mediterranean Sea, mainly in the eastern part. In this area, the pathway of introduction of decapods species is the Suez Canal, but penetration has been either unintentional (Lessepsian migration) or ship-mediated. The families Portunidae, Pilumnidae and Leucosiidae show the highest number of Red Sea/ Indo-Pacific aliens, while the remaining families are represented by only one or two species.

The most known invader crabs in the Mediterranean are: Portunus pelagicus and Percnon gibbesi which are the most recent and successful invaders in the Mediterranean Sea. The blue crab Callinectes sapidus was transported into the

Corresponding author

Mediterranean in ballast tanks from the north-east coast of the USA and has been widely recorded in different Mediterranean regions.

The negative effects of alien species in fishery, in tourism, in local economy, in human health, in other socio-economic activities and, mainly, in the biodiversity have triggered the social and scientific media to take some precautions against the dispersal of aliens among regions or among localities within a particular region.(4,000-35,000 Words)

IntroductionThe Mediterranean Sea: Description of the environmentThe Mediterranean Sea is unique, being a semi-enclosed marginal Sea, with a narrow connection with the Atlantic Ocean through the Strait of Gibraltar, the manmade connection to the Red Sea via the Suez Canal and the narrow Bosphorus Strait connecting it to the smaller enclosed Black Sea (Lascaratos et al., 1999; Turley, 1999). The Strait of Gibraltar restricts the exchange of Atlantic and Mediterranean waters, which has an important role to play in the circulation and productivity of the Mediterranean Sea. It contains some of the most extreme oligotrophic waters in the world (Dugdale and Wilkerson, 1988).

The general thermohaline circulation of the Mediterranean is determined by the flux of incoming Atlantic water by the Gibraltar Straits and by the sinking of waters formed at the three coldest areas of the sea: the Gulf of Lions, the northern Adriatic and the North Aegean Sea. Over the past three decades, this general pattern has been disrupted in several ways (Lejeusne et al., 2010) (Fig. 1).

Figure 1. Schematic representation of the thermohaline circulation in the Mediterranean Sea. (Lascaratos et al., 1999).

The negative freshwater and heat budgets of the Mediterranean basin drive its lagoonal (anti-estuarine) thermohaline circulation and determine its hydrological

characteristics. (Lascaratos et al., 1999). Low salinity Atlantic water (AW) that enters in the upper layer of the Gibraltar Strait is transformed to saline Mediterranean water that subsequently exits into the Atlantic via the lower layer. Although the surface AW progressively loses its characteristics through mixing and evaporation during its travel to the east, (salinity rising from 36.15 at Gibraltar Strait, to 38.6 in the eastern Levantine Basin according to Lacombe and Tchernia, 1972; Ozsoy et al., 1989), the transformation to intermediate and deep water occurs in selected areas within the basin (Wust, 1961). In these areas, favourable oceanic conditions and extreme air–sea interaction processes lead to the downward mixing extending deep into the water column.

Warm surface Atlantic water, already stripped of much of its nutrients by phytoplankton growth in the surface of the Atlantic, flows through the narrow Strait of Gibraltar and returns some 80–100 years later, having circulated the Mediterranean basin in an anticlockwise direction (Fig. 2).

Figure 2. A schematic summary of the major current and gyre systems of the Mediterranean Sea and their seasonal variability. Thick line = winter circulation; thin line = summer circulation. A: Algerian current and eddies; B: Branches of the Ionian stream; C: Tyrrhenian cyclonic current; D: summer antyciclone in the eastern Tyrrhenian Sea; E: Ligurian-Provenc¸al current; F: Lions gyre; G: Syrte anticyclone; H: mid-Mediterranean jet; I: Shikmona and Mersa-Matruth gyres system; J: Cilician and Asia Minor current; K: Rhodes gyre; L: Iera-Petra gyre; M: western Cretan gyre; N: Pelops gyre; O: Ionian cyclonic current; P: southern Adriatic gyre; Q: eastern Adriatic coastal current; R: western Adriatic coastal current; S: western Ionian gyre. Modified after Pinardi and Masetti (2000). (Bianchi, 2007).

During its passage eastward, its nutrients are decreased even more by phytoplankton (Bethoux et al., 1997) while climatic factors such as evaporation have resulted in its salinity increasing by up to 10% (Milliman et al., 1992). The water flowing out of the Mediterranean, the Mediterranean Deep Water (MDW) is therefore denser and flows below the incoming lighter Atlantic water. The Mediterranean Sea displays a specific hydrology, with well-identified water masses in each sub-basin and at different depths. One peculiarity is a homogeneous deep-water layer below 250 m that does not get colder than 12–13 8 °C (Lejeusne et al., 2010).

The Mediterranean is divided into the four subregions described under the Marine Strategy Framework Directive (MSFD), namely: (i) the Western Mediterranean Sea (WMED); (ii) the Central Mediterranean Sea (CMED); (iii) the

Adriatic Sea (ADRIA); and (iv) the Eastern Mediterranean Sea (EMED) (Zenetos et al., 2012).

The Western Mediterranean Sea occupies a key position because it receives the influx of surface waters from the Atlantic, through the Strait of Gibraltar. It is further compartmentalized into fairly isolated sub-basins with different climatic and hydrologic conditions. These sub-basins have a different biogeographic character, which may affect invasion and settlement of aliens. The Alboran Sea, situated immediately east of Gibraltar, exhibits stronger Atlantic affinities, due to the continued penetration of Atlantic flora and fauna with the incoming influx of water (Harmelin and D'hondt, 1993).

The bulk of the Central Mediterranean Sea is represented by the Ionian Sea, the least known of all the Mediterranean sub-basins (Zenetos et al., 1997). The Ionian is connected to the Western Mediterranean Sea through the narrow Strait of Messina, a micro-sector that harbours a wealth of biogeographic peculiarities, including Pliocene Atlantic remnants and local endemisms (Fredj and Giaccone, 1995), and the larger Strait of Sicily, the meeting point of native Western and Eastern Mediterranean species (Bianchi, 2007), as well as of aliens of either Atlantic or Indo-Pacific origin (Coll et al., 2010).

The Adriatic Sea is a rather unique and differentiated area within the Mediterranean, with a strong contrast between the predominantly linear sandy shores along the western (Italian) side, and the opposite complex coasts of the eastern side (Slovenia, Croatia, Montenegro and Albania) forming a maze of islands and inlets with rocky shores. The hydrographic conditions are also peculiar, with very low winter temperatures in the northern part, which is also quite shallow (40 m depth), and very hot summers in the southern part, which is much deeper. All these features lead to differentiation between the northern and southern Adriatic areas.

The Eastern Mediterranean Sea includes two major bodies of water: the Levant Sea and the Aegean Sea, together with the smaller Sea of Marmara, which connects it to the Black Sea. The Levant Sea is warmer than the rest of the Mediterranean and harbours a significant number of circumtropical species. Atlantic-Mediterranean elements and Mediterranean endemics are comparatively scarce (Morri et al., 2009).

Since the construction of the Suez Canal, the Levant Sea is experiencing an important influx of Red Sea species. Por (1990) defined the geographical limits to the expansion of Red Sea immigrants in the Mediterranean as the ‘Anti-Psara line’ to the north (Anti-Psara being an island in the Aegean) and the Strait of Sicily to the west: these boundaries match the 15 °C surface isotherm for February (Bianchi, 2007).

The Sea of Marmara exhibits peculiar hydrological conditions, with low salinity waters coming from the Black Sea stratifying over saline waters of Mediterranean origin on the bottom (Unluata et al., 1990). This hydrological regime should facilitate the diffusion of Black Sea species into the Northern Aegean rather than vice-versa, but our knowledge on the exchanges between the two areas is limited and their biotic affinity is low (Koukouras et al., 2001). In recent times, climatic change favoured an increase of biotic penetration from the Sea of Marmara into the Black Sea, which therefore has been undergoing a process of ‘Mediterranization’ (Tokarev and Shulman, 2007).

Alien Species: General remarks. Pathways of introduction and vectors of dispersalAccording to Zenetos et al. (2012) the definition of an alien, non-indigenous, exotic, non-native or allochthonous species is defined by its presence in the wild, through introduction outside its natural range and beyond its natural dispersal potential. In the last decade, the establishment in marine ecosystems of invasive alien species (i.e., non-indigenous species having an adverse effect on biological diversity, ecosystem functioning, socioeconomic values and/or human health in invaded regions: Olenin et al., 2011) has rapidly become a central environmental issue (Ruiz et al., 2000; Grosholz, 2002; Occhipinti- Ambrogi, 2007; Galil et al., 2009; Walther et al., 2009; Occhipinti- Ambrogi and Galil, 2010).

Marine alien species are a component of global change in all marine coastal ecosystems. The Mediterranean Sea is today one of the areas worldwide most severely affected by biological invasions, in terms of detected number of alien species and rate of introduction (Raitsos et al., 2010; Occhipinti-Ambrogi et al., 2011; Zenetos et al., 2012).

The most `typical' Mediterranean flora and fauna obviously occur in the central parts of this sea, and especially in the western basin. The Alboran Sea, located immediately east of Gibraltar, exhibits stronger Atlantic affinities, due to the continued penetration of Atlantic flora and fauna with the incoming flux of water (Harmelin and d'Hont, 1993). The Alboran basin, at the entrance to the Mediterranean, acts as a buffer reducing gene flow (Lejeusne et al., 2010). On the contrary, the Levant Sea is experiencing an important influx of Red Sea species after the opening of the Suez Canal a phenomenon known as `Lessepsian migration' in recognition of Ferdinand de Lesseps, the French diplomat who promoted the cut of the Canal (Galil, 1993). Lessepsian species now acclimated in the Mediterranean include algae, a sea grass, various invertebrates and fish (Golani, 1998); they are so abundant that the south-eastern Mediterranean Sea has been proposed as a separate biogeographic province (Por,1999). The rate of Lessepsian migration has been increasing particularly in the last decade. This is partly attributed to the continued enlargement of the Suez Canal. According to Rilov and Galil (2009) this is the main cause of the apparent acceleration in the rate of Lessepsian invasion over the last five decades.

A significant number of Indo-Pacific species reaches the Western Mediterranean, which is enriched by ship-transferred species of Pacific origin mostly among macrophytes (Hilgen and Langereis, 1993). The Strait of Gibraltar is essentially different from the Suez Canal as a potential pathway for alien species. It constitutes an ancient waterway, believed to have originated 5.33 million years ago (Hilgen and Langereis, 1993), compared to the 142 years of the Suez Canal. Therefore, the current distribution of Atlantic species, tropical or not, with part of their range in the Mediterranean is the result of a natural process over a long time; these species do not in any case qualify as aliens, even if their discovery in the Alboran Sea comes later than their first description in the Atlantic. Additionally, the Atlantic coast of Morocco is swept by a prevalently southward oceanic circulation that prevents many potential newcomers to approach the Strait of Gibraltar.

More than half (54%) of the marine non indigenous species in the Mediterranean Sea were probably introduced by corridors (mainly Suez) (Fig 3). Shipping is the second most common pathway of introduction, followed by

aquaculture and aquarium trade. The Suez Canal, as a pathway of non indigenous species, is believed to be responsible for the introduction of 493 alien species into the Mediterranean; approximately 11% being invasive (55 species) with, only 270 of these species are definitely classified as Lessepsian immigrants. Of these 270 Lessepsian immigrants, 71 consist of casual records while 175 are successfully established, 126 out of them (including 17 invasive ones) are limited to the Eastern Mediterranean Sea, whereas the others are progressively spreading in the neighbouring Marine Strategy Framework Directive subregions (Zenetos et al., 2012).

Figure 3. Geography of the Mediterranean Sea with the main routes of species range expansion. Bold capital abbreviations correspond to the main Mediterranean subregions (ALB: Alboran Sea; NWM: North Western Mediterranean; TYR: Tyrrhenian Sea; ADR: Adriatic Sea; ION: Ionian Sea; AEG: Aegean Sea; LEV: Levantine Basin) and adjacent seas (ATL: Atlantic Ocean; BLA: Black Sea; RED: Red Sea). Italic abbreviations correspond to some remarkable Mediterranean locations (Gib: Gibraltar Straits; GoL: Gulf of Lions; Sue: Suez Canal). Temperatures correspond to winter–summer mean sea-surface temperatures. Arrows represent main routes of species range expansion according to their origin: Mediterranean natives (orange), Atlantic migrants (green) and Lessepsian migrants (red). (Lejeusne et al., 2010).

Shipping is blamed directly for the introduction of 12 species only, whereas it is assumed to be the only pathway of introduction (via ballasts or fouling) of further 300 species. In addition, for approximately 100 species shipping counts as a parallel possible pathway along with the Suez Canal or aquaculture. (Zenetos et al., 2012).Lessepsian species decline westwards, while the reverse pattern is evident for ship-mediated species and for those introduced with aquaculture. There is an increasing trend in new introductions via the Suez Canal and via shipping (Zenetos et al., 2012).

Increase in trade, tourism and maritime activities have provided new and enhanced pathways for the spread of marine non indigenous species through shipping. Shipping has been reported to be responsible for the introduction (either among hull fouling or in ballast waters) of 54 NIS until 1950. The current rate (based on the last decade) of ship-mediated non indigenous species in the Mediterranean is one new species every six weeks. (Zenetos et al., 2012). In the Western Mediterranean Sea, shipping remains the most prominent pathway of introductions.

The increasing importance of non indigenous species is particularly evident for the Central Mediterranean Sea, which separates the western from the eastern sectors of the basin. In this subregion shipping is the main pathway that accounts for the introduction of most species. (Zenetos et al., 2012).

The observed increased trend in new introductions by shipping is not expected to halt unless effective measures are taken. Trends in new introductions of alien species by shipping are expected to decrease only when the ‘International Convention for the Control and Management of Ships’ Ballast Water and Sediments’ (BWM Convention) becomes legally binding, by substantially reducing the transfer of marine species via ballast water. Nevertheless, introductions by hull-fouling will remain. (Zenetos et al., 2012).

Introduction of non indigenous species through aquaculture is apparently slowing down. In the last decade, aquaculture has been responsible for 14 new non indigenous species in the Mediterranean vs 18 species in the previous two decades 1981-1990 and 1991-2000, but new non indigenous species continue to appear in the vicinity of oyster farms (M. Verlaque and F. Mineur, unpubl. data).

While maritime traffic and other human activities such as aquaculture are important vectors for the introduction of alien species worldwide (Ruiz et al., 2000), in the Mediterranean they are not the main reasons responsible for the large differences observed among the four basins. In the Eastern Mediterranean, the human intervention responsible for most of the aliens is the reestablishment of the connection with the Indo-Pacific through the Suez Canal (1869), rather than the actual transfer of the invaders. In addition, with the present climate change (Belkin, 2009), the tropical features and temperature of the waters are increasing more quickly in the Eastern Mediterranean, implying dramatic modifications of the biota (Por 2009, 2010). As a consequence, Indo-Pacific species (regardless of the mode introduction) have found optimal environment for settlement in the Eastern Mediterranean.

On the contrary, the Adriatic Sea, which is topographically a dead end, comprises the area with the lowest number of aliens, receiving them among those already established in the Eastern Mediterranean and Central Mediterranean that spread northwards, or among those introduced via shipping or aquaculture in hot spot areas such as the Venice Lagoon (Occhipinti-Ambrogi et al., 2010).

The list of exotic animals and plants that invaded the Mediterranean is getting longer every day (Zibrowius, 1991; Ribera and Boudouresque, 1995). Besides the afore-mentioned Lessepsian migrations, species are intentionally or accidentally introduced into the Mediterranean via ship fouling, ballast waters, aquaculture, trade of living bait, wrapping of fresh seafood with living algae, aquariology, and even scientific research.

The principal pathway of crustacean introduction varies according to the subregion. In the Eastern Mediterranean Sea almost 80% are derived from the Indo-Paciic through the Suez Canal, although in some cases these inputs can be dual (corridors and shipping, either in ballast water or among hull fouling) or even caused by aquaculture. In the Western Mediterranean Sea the situation is different, as a considerable proportion of non indigenous species (between 57% and 71%) has been introduced by shipping, 24% to 33% used corridors as a primary pathway (Suez Canal, and in a few cases inland canals), and only 10% to 14% can be linked to

aquaculture. The increase of maritime traffic is an important pathway for introduction and dispersal of alien decapod species, since larvae can survive long periods in ballast water (Mizzan, 1999; Occhipinti Ambrogi, 2000). The presence of non indigenous species populations in some Mediterranean areas can also be related to their trade: Necora puber and Paralithodes camtschaticus (Faccia et al., 2009) are quite frequently found alive in the markets.

Larval crab stages (zooea, megalopae) have been found in ship’s ballast but are by no means common, and crabs are rarely on hulls. Adult crabs have been found in bottom sediments in ballast tanks and in sea chests and other areas not routinely affected by ballast water management (Grosholz, 2011). Once introduced to a new continental margin, crabs may frequently expand their range through dispersal of planktonic larval stages by advection of ocean currents. Several introduced crab species have been rapidly dispersed by ocean currents along a coastline following an initial human-mediated introduction to a new continental region.

Many crab species have the tendency to expand their native range significantly into areas that are considerably outside of their typical range. This is partially caused by their long planktonic development periods, during which developing larvae may be carried many hundreds of miles by ocean currents. Among the species that best exemplify this pattern are the swimming crabs of the genus Callinectes, which include the commercial blue crab Callinectes sapidus.

Once introduced to a new region, many crabs can rapidly expand their range along the coastline. These dispersal events include some of the fastest range expansions recorded for any introduced species. Among the most rapid range expansions is that of the European green crab, which spread along the western coast of the United States at rates of 200 km per year. Also, the crab Hemigrapsus sanguineus experienced rapid range expansion on the eastern coast of the United States. In Europe, Eriocheir sinensis also spread rapidly along the coast and throughout many river systems during the early twentieth century (Grosholz, 2011). Rapid spread is by no means the rule or even the norm for the same species. For instance, Carcinus maenas has spread very little in southern Australia and Tasmania, and the rates of spread in eastern North America were very minimal for long periods.

The sequential stages of an invasion process according to (Walther et al., 2009), start from the introduction of a few precursor individuals, which only temporarily occur in a site during short favourable climatic periods or are spatially restricted to favourable micro-habitats (Fig. 4). Continued climatic warming might then prolong the duration of these occasional occurrences of initial introductions, increase their frequency or enlarge the range and area of suitable habitats, making it more likely for these species to persist, to occur more frequently and to develop larger populations. With further global warming, alien species originating from warmer regions could build up numerically and spatially larger populations that might spread to wider areas.

Figure 4. Influence of climate change on all the sequential transitions of a successful invasion process. (Walther et al., 2009).

Effects of climateThe Mediterranean Sea is land-locked and its dynamics are mainly linked to climate. Its geochemistry depends on marine dynamics, on Atlantic input and on atmospheric and terrestrial sources of matter of natural or anthropogenic origin. From a geographic point of view, with a mean depth of about 1500 m and a coastal zone (O-200 m depth) covering about 20% of the area, it cannot be considered as a coastal sea (Bethoux and Gentili, 1996). The evolutions in physical and chemical characteristics occurring in offshore waters as well as in coastal biological species showed that the Mediterranean Sea reacts rapidly to environmental changes. These evolutions may be used as signatures and modelling constraints of changes that occurred in the coastal area. Examples of ecosystem evolutions may be found in the Eastern Basin where more than 350 species have immigrated (Por, 1990; Galil, 1993). This event was coupled with the increase of Red Sea inflow (after the deepening and widening of the Suez Canal and the vanishing of salinity effect along the Bitter Lakes) and the disappearance of Nile low salinity water along the Egyptian coast consecutive to the High Dam closing in 1964.

From 1985 to 2006 the temperature in the upper layer of the Mediterranean Sea has been increasing at an average rate of 0.03°C year-1 for the Western Mediterranean Sea and 0.05°C year-1 for the Eastern Mediterranean Sea (Nykjaer, 2009). Abrupt rising temperature since the end of the 1990s has modified the potential thermal habitat available for warm-water species, facilitating their settlement at an unexpectedly rapid rate, and it has been shown that the introduction of tropical alien species has been exacerbated by the warming of the Eastern Mediterranean Sea (Raitsos et al., 2010).

The rate of Lessepsian non indigenous species extending their distribution in the Adriatic Sea has doubled the last two decades. It seems that the changes in the patterns of water exchange between the Adriatic Sea and the Mediterranean as well as a rise in the eastern Adriatic Sea surface temperatures in 1985-1987 and 1990-1995 are correlated with the occurrence of Indo-Pacific species, some of them for

the first time, others expanding their distribution from the neighbouring sub regions where they are already established. (Zenetos et al., 2012).

Some authors proposing that the Mediterranean Sea is heading towards ‘tropicalisation’ (Bianchi, 2007). The use of this term might appear exaggerated in view of the data currently available, but a ‘meridionalisation’ of the Mediterranean (a definite augmentation of the proportion of thermophilic species in the Mediterranean biota) seems a more realistic description of changes to come (Lejeusne et al., 2010). Climatic models (Parry, 2000) further predict that the Mediterranean basin will be one of the regions most affected by the ongoing warming trend and by an increase in extreme events.

Climatic fluctuations exert an overriding role on the marine biota (Cushing and Dickson, 1976; Southward and Boalch, 1994; Wilkinson and Buddemeier, 1994; Southward et al., 1995; Bianchi, 1997). Biodiversity is affected by a combination of: (i) a direct effect on the organisms (temperature causes changes in survival, reproductive success, dispersal pattern and behaviour); (ii) effects mediated by biotic interactions (conferral of competitive advantage to one of a pair of overlapping species); and (iii) indirect effects through ocean currents. Sanford (1999) showed that small changes in climate may generate large changes in marine communities through regulation of keystone predation. Petchey et al. (1999) demonstrated that environmental warming alters food-web structure and function of aquatic ecosystems. There is some evidence that Mediterranean biodiversity patterns are presently facing changes that can be related to increasing seawater temperature (Francour et al., 1994).

A direct consequence of warming is a simultaneous increase in the abundance of thermotolerant species and the disappearance reduction of ‘cold’ stenothermal species. Such changes occur as shifts in distribution ranges and/or population dynamics, and were detected as early as the 1980s (Francour et al., 1994). Although seawater warming probably affects the entire Mediterranean (Rixen et al., 2005; Moron, 2003), range shifts have mainly been reported in north western Mediterranean taxa; this is either due to the higher proportion of cold stenotherm species in the north western Mediterranean or to observation bias, or a mixture of both. Considering the north western Mediterranean only, one of the coldest areas in the Mediterranean, tens of significant range expansions of species of warm water affinity have been recorded, two-thirds of which correspond to mobile species (UNEP-MAP-RAC/SPA, 2008).

Mapping the surface isotherms of the Mediterranean Sea, averaged over a century of records and therefore representing the climatology of the basin (Brasseur et al., 1996), shows that the isotherm of 15°C for February (the coldest month in the year) crosses the Straits of Sicily, splits the Ionian Sea into a north-western and a south-eastern part, and finally separates the Peloponnese from the Aegean Sea (Fig. 5). According to Bianchi (2007) the February 15°C surface isotherm follows quite closely all the biogeographic boundaries between the western and eastern Mediterranean, possibly explaining the similarity between the Aegean Sea biota to that of the western Mediterranean (both basins laying mostly to the north of the February 15°C surface isotherm) than to that of the Levant Sea (which remains to the south of that isotherm).

Figure 5. Surface isotherms of February (traced every 0.25°C) of the Mediterranean Sea (climatological means from the historical data set 1906–1995). The 14°C and the 15°C isotherms are highlighted by a thicker tract (Bianchi, 2007).

Climate change combines with Atlantic influx, lessepsian migration and the introduction of exotic species by humans to favour the occurrence and establishment of warm-water species, whether exotic or native, in the Mediterranean Sea. Climate change has been suggested for the expansion of the biogeographical range of benthic and nektobenthic marine species, as recorded in the last decade in the western Mediterranean (Francour et al., 1994; Vacchi et al., 1999; Bianchi and Morri, 2000; Laubier et al., 2004) as well as in other areas (Bianchi, 1997).

The consequences of climate-mediated biological invasions are far-reaching and more controversial than those of past invasions not affected by climate change, where species typically originate from habitats with similar climatic conditions (D’Antonio and Meyerson, 2002; Kowarik, 2003).

Mediterranean marine ecosystems are certainly heading towards a climate-induced revolution in their functioning. Their resilience to such changes remains to be determined since other disturbances (biotic and abiotic) combine and interact. For example, the arrival of new species in these ecosystems (through natural range shifts or human-induced introductions) might be a major disruptive force for ecosystem functioning. The arrival of new key species, sometimes acting as ecosystem engineers, could alter competition patterns between native species, and/or transform current ecosystems into new ones.

The effects that climate change will have on this region should serve as an example of what could happen globally. Among the uncertainties, it is not known whether all parts of the Mediterranean will be equally affected by global change. Different climatic conditions are involved in the Mediterranean basin, so responses are expected to vary.

Impact of Invasive speciesMan is greatly altering marine biodiversity in many ways, ranging from the overexploitation of biological resources and habitat modification to the introduction of exotic species (Cognetti and Curini-Galletti, 1993; Castilla, 1999; Connell and Glasby, 1999; Leppakoski et al., 1999). Habitat modification and introduction of

exotic species combine to cause loss of biodiversity through biotic homogenization: endemic species unable to tolerate the alteration of the ecosystem (the `losers') will get extinct, exotic species transported by man (the `winners') will expand their geographic range (McKinney, 1999).

According to Zenetos et al. (2012) the definition of an alien, non-indigenous, exotic, non-native or allochthonous species is defined by its presence in the wild, through introduction outside its natural range and beyond its natural dispersal potential. In the last decade, the establishment in marine ecosystems of invasive alien species (i.e., non-indigenous species having an adverse effect on biological diversity, ecosystem functioning, socioeconomic values and/or human health in invaded regions: Olenin et al., 2011) has rapidly become a central environmental issue (Ruiz et al., 2000; Grosholz, 2002; Occhipinti- Ambrogi, 2007; Galil et al., 2009; Walther et al., 2009; Occhipinti- Ambrogi and Galil, 2010).

With few exceptions, the ecological impact of invasive alien species on the native Mediterranean biota is poorly known (Zibrowius, 1991; Boudouresque, 2004), though it is believed that keystone invasive species may cause major shifts in community composition. The impacts of invasive alien crustaceans often reflect in changes in the trophic structure of native communities, and, in turn, on energy flows through the ecosystem (Hanfling et al., 2011; Mancinelli et al., 2013).

Impact of invasive species has been defined by Parker et al. (1999), who also discussed a variety of measures of impact. In the marine environment, Ruiz et al. (1999) also describe impact by alien species and interactions with other stress factors. The impact of these species on native communities has been evaluated in many localities all over the world leading to the concept of biotic pollution. This is especially evident in the Mediterranean Sea (Occhipinti-Ambrogi, 2007). Similarly Elliot (2003) observes that there are many aspects in which introduced marine organisms can be regarded as being no different from chemical pollutants and encourages the use of the term biological pollution.

Indo-Pacific species (of warm water affinity), established in different phases after the opening of the Suez canal, have caused changes in the Levantine part of the Mediterranean far beyond recorded impacts in other marine ecosystems. Nearly half of the fish catches along the Israeli coast consist of Indo-Pacific species (Goren and Galil, 2005). The process has accelerated in recent years, with increasing records of newly discovered Indo-Pacific species and expansion towards other areas of the Eastern (Galil and Zenetos, 2002) and Western Mediterranean (Harmelin-Vivien et al., 2005; CIESM, 2005). The unabated influx of the Indo-Pacific biota is rooted in the continuous enlargement of the Suez Canal that has altered its hydrography and hydrology, and enhanced its potential as a ‘‘corridor’’ allowing ever greater numbers of organisms through.

The most notorious and best studied invasive species in the Mediterranean are the coenocytic chlorophytes: Caulerpa taxifolia (Meinesz et al., 2002), and Caulerpa racemosa (Verlaque et al., 2004). Other studies traced the impacts of invasive aliens that entered the Mediterranean from the Red Sea through the Suez Canal (Por, 1978; Golani, 1998; Galil, 2000, 2006; Goren and Galil, 2005). It had been suggested that most invasive aliens, are part of a synergetic complex of drivers where ‘‘habitat disturbance frequently increases the impacts of invasive species’’ (Didham et al., 2005; MacDougall and Turkington, 2005).

The successful invasion of a biological community appears to be the result of the relationship between native species richness and alien species ability to colonize new habitats (Bulleri et al., 2008). This concept implies that habitats with high levels of diversity are difficult to invade. In contrast, species-poor communities, or stressed ecosystems, are arguably more prone to invasion, primarily due to lack of biotic resistance (Occhipinti-Ambrogi and Savini, 2003).

Alien species can be viewed as drivers and passengers of change in biological communities (MacDougall and Turkington, 2005; Didham et al., 2005). Many invasive species exert strong impacts on invaded communities and ecosystems (Vila et al., 2009) and transform ecosystem properties (Richardson et al., 2000), which inevitably leads to changes in biological communities. Acclimated exotic species may replace indigenous species, by altering trophic webs and interspecific relationships inducing profound modifications in the original ecosystems (Dukes and Mooney, 1999). Resident species can become increasingly poorly adapted to the local environment, which will then provide opportunities for newcomers that are better adapted and, thus, more competitive under the new conditions.

Combinations of the invasion of alien species and climate change have resulted in the reorganization of marine ecosystems, as shown for example in the Atlantic waters off the coast of the USA (Stachowicz et al., 2002), Europe (Boelens et al., 2005) and in the Mediterranean Sea (Occhipinti-Ambrogi, 2007). In a changing world, it will be increasingly difficult to evaluate the impacts of alien species and it is likely that the increasing presence of ‘new’ species and the decline of ‘old’ ones will change succession patterns and ecosystem functioning (Harrington et al., 1999; McNeely, 2001).

On the other hand, beneficial aspects of introductions are claimed, since intentionally introduced species have significantly contributed to aquaculture production (FAO DIAS, 1998), as well as fisheries and angling (Minchin and Rosenthal, 2002). Unintentionally introduced species, such as the Indo-Pacific species which entered the Mediterranean through the Suez Canal, have become locally of commercial importance (Golani and Ben Tuvia, 1995); the Mediterranean mussel Mytilus galloprovincialis, accidentally introduced to the West coast of South Africa in the mid 1970s, was deliberately introduced to the South coast for mariculture purposes, despite the fact that it had become invasive, outcompeting local mussels (Branch and Steffani, 2004).

However according to Galil (2007) the optimistic view on the effects of invasion by Tortonese (1973) was wrong. It seems that the establishment of alien biota, and the concurrent adverse changes in the native communities, are part of a catastrophic anthropogenic ecosystem shift in the Mediterranean Sea.

Introduced crabs constitute some of the best examples of introduced marine and estuarine species that have had significant impacts on coastal habitats and economies. Among the best studied is the European green crab Carcinus maenas, for which ecological and economic impacts have been demonstrated on several coasts. Green crabs contributed substantially to the demise of the commercial soft-shell clam fishery in the northeastern United States during the 1940s and 1950s. There have also been long-term changes in the benthic communities in bays and estuaries in central California because of green crab predation on small native crabs and

clams. Green crabs have also had long-term impacts on shell shape and morphology of herbivorous snails in New England (Grosholz, 2011).

The Chinese mitten crab Eriocheir sinensis is another species for which impacts have been quantified fairly extensively. In Europe during the early twentieth century, population explosions resulted in extensive efforts to mitigate their impacts in rivers, canals, and associated municipal facilities (Grosholz, 2011). It caused damages to river banks by burrowing and considerable damage to fisheries by consuming netted fish and by cutting nets (CIESM, 2014). In the San Francisco Bay following population outbreaks in the late 1990s, substantial investments by state and federal agencies were required to prevent clogging of water pumps and associated fish salvage facilities. Major losses were experienced by commercial and sport fishing interests as well in central California.

Among the regions that could potentially provide the source for introduced crabs, the most common source region is overwhelmingly the Indo-Pacific region. Every introduction is strongly influenced by the level of taxonomic effort in addition to characteristics of propagule pressure habitat matching between source and recipient communities, and other general processes known to influence species introductions have clearly influenced patterns of crab introductions (Grosholz, 2011). Particular recipient regions in warm temperate and subtropical areas such as the Mediterranean region have the greatest number of established introduced crab species.

Newcomers (whether natural or introduced) can trigger major changes in ecosystem functioning. Τhe same ecosystems are increasingly exposed to pollution, overfishing, and to alterations in the normal patterns of temperature and several other physical–chemical factors associated with temperature, such as sea level changes and acidification. If pollution, mass mortalities and biological invasions are also taken into account, the effects on ecosystem functioning are likely to be dramatic. Only a multidisciplinary approach can tackle such a complex problem: linking functional ecology with invasion biology and macrophysiology.

Legislation in EEAlthough the European states have a comprehensive regulatory framework to protect economic interests against diseases and pests, these are often inadequate to safeguard against species that threaten native biodiversity. Moreover, the regulatory system pertains to pathogens while large sized species that may have considerable impact on health or the economy are not considered to date.

Changes in European reference regulations testify this growing concern: in the Water Framework Directive (WFD; 2000/60/EC) alien species were not included among the ecological quality indicators for coastal habitats (EU, 2000); conversely, in the Marine Strategy Framework Directive (MSFD; 2008/56/EC), and, more recently, in the Biodiversity Strategy to 2020 [2011/2307(INI)] IAS have been explicitly recognized as a biological pressure (MSFD descriptor D2), whose magnitude and functional effects need to be estimated for an integrated assessment of the ecological status of marine ecosystems (EU, 2010; Borja et al., 2010).

The ‘‘Jakarta Mandate on Marine and Coastal Biological Diversity’’, adopted by the Parties to the ‘‘Convention on Biological Diversity’’ (CBD), cites ‘‘invasion of exotic species’’ as one of the five main categories of the anthropogenic impact on

marine and coastal biota (www.biodiv.org). Marine invasions are recognized as imperilling global ‘‘biodiversity, marine industries (including fishing and tourism) and human health’’ (Bax et al., 2003).

The Conference of Parties to the Convention on Biological Diversity called upon governments to act ‘to prevent the introduction of, control or eradicate those alien species which threaten ecosystems, habitats or species’ (CBD 1992, Article 8h), but preventing alien species introductions is a task which needs scientific, administrative and political coordination at the regional, if not international, level. The results thus far have been disappointing. The International Maritime Organization (IMO) and the shipping industry have concentrated their attention on ways to provide a uniform international instrument to regulate ballast water management, since ballast-mediated bio invasions have caused significant economic and environmental losses. The International Convention on the Control and Management of Ships Ballast Water and Sediments, a potentially significant environmental achievement, was adopted by a Diplomatic Conference in 2004. To enter into force the convention should be ratified by 30 States, representing 35% of world merchant shipping tonnage (www.IMO.org).

The past decade saw rapid growth in mariculture in the Mediterranean. However, the Mediterranean countries have not developed the comprehensive legal and institutional systems that are capable of responding effectively to the introduction of alien species for use in mariculture. Though legislation on introduction and transfers of alien species exists in some countries, in practice the administrative measures to control the deliberate importation of alien and limit their dispersal are still rudimentary and an effective policy of prevention is hardly enforced (Occhipinti-Ambrogi, 2002). Key industry groups, governmental bodies, and even local environmental groups have a poor appreciation of the magnitude of the problem. As a consequence, too often, responses are insufficient, late and ineffective.

The Aquaculture Regulation -Council Regulation (EC) No 708/2007 of 11 June 2007- concerning use of aliens and locally absent species in aquaculture - established a framework with which to assess and minimize the possible impacts of aliens and locally absent species used in aquaculture, including procedures for risk assessment, to ensure adequate protection of aquatic habitats from non-native species. However, since 2008 the whole European shellfish aquaculture is affected by severe and repetitive oyster mortalities and massive imports of non-European livestock are again being considered as a solution for the crisis, despite the risks of accidental NIS introduction associated with these imports. (Zenetos et al., 2012).

The peri-Mediterranean countries possess a valuable instrument were they ready to tackle marine bioinvasions. The Barcelona Convention (1976) and its relevant protocols, initially aimed at reducing pollution, has been updated with the adoption of new protocols. The Protocol concerning Specially Protected Areas (SPA), that had been adopted in 1982 and came into force in 1986, prohibits ‘the introduction of exotic species’ (Article 7e). In 2003 the Mediterranean Action Plan (MAP), United Nations Environment Programme (UNEP), drafted an ‘Action Plan concerning species introductions and invasive species in the Mediterranean Sea’ (UNEP(DEC)MED WG.232/6). In 2005, the Regional Activity Centre for Specially Protected Areas (RAC/SPA) convened a meeting of scientists to identify the most

important taxonomic and geographic data gaps concerning Mediterranean alien biota, and to propose guidelines for the prevention of biodiversity loss caused by vessel and mariculture-introduced alien species. These are meant to assist the Contracting Parties to the Barcelona Convention in implementing the SPA protocol (Barcelona, 1995). The desired goal would be the creation of a new Protocol specifically aimed at preventing further loss of biological diversity due to the deleterious effects of the intentional and unintentional introductions of alien invasive biota in the Mediterranean.

Current status of the Mediterranean alien biotaAlthough the Mediterranean Sea is only 0.82% in surface area and 0.32% in volume of the world ocean (Defant, 1961), the marine organisms which dwell in this sea represent 4-18% of the world marine species, depending on the phylum taken into consideration (Lejeusne et al., 2010). The dominant group among alien species in the Mediterranean sea is Mollusca (with 215 species), followed by Crustacea (159) and Polychaeta (132) (Zenetos et al., 2012) (Fig. 6).

One reason for the high number of species present in the Mediterranean Sea might be its tormented geological history, which has led to high rates of environmental change and speciation. Another reason is the variety of climatic and hydrologic situations to be found in this sea, leading to the co-occurrence of cold, temperate and subtropical biota (Bianchi and Morri, 2000). Finally, the rate of endemism i.e. the number of species living exclusively in the Mediterranean is relatively high, averaging more than one-quarter of the whole Mediterranean biota (Tortonese, 1985; 1992; Giaccone, 1999; Lejeusne et al., 2010).

Figure 6. Contribution of marine alien taxa in the Mediterranean Sea (Zenetos et al., 2012).

The high marine biodiversity could be the result of its geological history (Maldonado, 1985; Stanley and Wezel, 1985; Ruffel, 1997), which has led to a rate of

environmental change, and hence species occurrence, with few equals in the world. Another reason is the present-day variety of climatic and hydrologic situations to be found within the Mediterranean, leading to the occurrence in this sea of both temperate and subtropical biota (Sara, 1985).

The Mediterranean marine biota has historically been divided into three major biogeographical provinces (the western and eastern basins, and the Adriatic Sea), each subdivided along latitudinal patterns (Fredj, 1972). Species of subtropical origin are therefore more abundant in southern parts of these provinces whereas temperate species dominate the northern parts. According to Bianchi and Morri (2000) there are 10 distinct biogeographic sectors within the Mediterranean, namely: (A) Alboran Sea; (B) Algeria and southern Spain; (C) Balearic Sea to Tyrrhenian Sea; (D) Gulf of Lyon and Ligurian Sea; (E) North Adriatic; (F) Central Adriatic; (G) South Adriatic; (H) North Aegean; (I) Ionian Sea and South Aegean; (J) Gulf of Gabes to Levant Sea.

The marine biota of the Mediterranean are comprised by several biogeographic categories: (i) temperate Atlantic-Mediterranean species; (ii) cosmopolitan species; (iii) endemic elements; (iv) subtropical Atlantic species; (v) boreal Atlantic species; (vi) Red Sea migrants (especially into the Levant Sea); (vii) eastern Atlantic migrants (especially into the Alboran Sea). (Bianchi and Morri, 2000).

Most of the introduced biota in the Mediterranean originates from the Red Sea (i.e. Lessepsian migrants; about 67%), with an additional set of species (about 7%) from other tropical areas. The bulk of the species introduced into the Mediterranean are of tropical origin. They have long been confined to the easternmost Levantine shores, but the warming of the Mediterranean favours their spread (Occhipinti-Ambrogi, 2007). They are now rapidly progressing westwards and northwards, through the entire eastern basin, with some now reaching the Adriatic and the western basin. Although it is difficult to disentangle natural range expansion through time from climate-induced effects, particularly in regions lacking systematic field monitoring and temperature records, it seems that the last 20 years have seen an accelerated rate of westward migration of Lessepsian species (Galil and Zenetos, 2002).

The eastern Mediterranean is an important source of thermophilic species expanding their ranges in the north-western Mediterranean and the Adriatic. In addition to native Mediterranean species, a particular case of distribution range expansion is that of introduced species. An introduced species is defined here as a species that (a) colonises a new area, (b) displays a geographical discontinuity with its native area, (c) has human activities as a primary vector of range extension, and (d) can reproduce in situ without human assistance. It is termed invasive when it acts as key or engineer species within the recipient ecosystem (Lejeusne et al., 2010). The Mediterranean is one of the areas worldwide most severely hit, with about 600 introduced species (Boudouresque et al., 2005; Galil, 2008). Their number has more or less doubled every 20 years since the beginning of the 20th century (Galil, 2008; Ribera and Boudouresque, 1995), but this increase does not seem to be related to climate change.

According to the most recent data (Zenetos et al., 2012) the total number of non-indigenous species present in the Mediterranean is 986 (775 in the eastern

Mediterranean, 249 in the central Mediterranean, 190 in the Adriatic Sea and 308 in the western Mediterranean) (Fig. 7).

Figure 7. Number of non indigenous species introduced per decade, according to major groups, in the Mediterranean Sea (Zenetos et al., 2012).

Crustacean alien species in the Mediterranean SeaOnly 37 alien crustaceans were known before 1950s in the Mediterranean. Between the 1950s and the 2000s, further 122 species were recorded, which represents an increase of 327%. The highest increase in decapod non indigenous species occurs in the Eastern Mediterranean Sea (Zenetos et al., 2012).

The highest proportion (78%) of crustacean non indigenous species occurs in the Eastern Mediterranean Sea, being 28% in the Western Mediterranean Sea, 23% in the Central Mediterranean Sea, and 14% in the Adriatic Sea (Zenetos et al., 2012) (Fig. 8). The rate of increase of alien crustacean species in the Mediterranean Sea is currently 3-4 species per year. Approximately 60% of the known alien brachyuran species worldwide have been reported from the Mediterranean, which makes it the richest sea in terms of invasive crabs (Brockerhoff and McLay 2011).

The status of some species with an eastern Atlantic origin in the Western Mediterranean Sea is difficult to determine, mainly because of intense fishing and transport activities that occur between Africa and Spain or France (Luque et al., 2012). Furthermore, the Strait of Gibraltar is a boundary more or less permeable to Atlantic species that naturally increase their distribution range. Finally, there is a limited knowledge on the biodiversity from North African littoral. Therefore a number of species reported in the Western Mediterranean Sea considered to have expanded their geographic range, could in fact have been introduced by man. (Zenetos et al., 2012).

Figure 8. Number of marine alien species per major groups in the Marine Strategy Framework Directive subregions of the Mediterranean Sea (Zenetos et al., 2012).

The principal pathway of crustacean introduction varies according to the subregion. In the Eastern Mediterranean Sea almost 80% are derived from the Indo-Paciic through the Suez Canal, although in some cases these inputs can be dual (corridors and shipping, either in ballast water or among hull fouling) or even caused by aquaculture. In the Western Mediterranean Sea the situation is different, as a considerable proportion of non indigenous species (between 57% and 71%) has been introduced by shipping, 24% to 33% used corridors as a primary pathway (Suez Canal, and in a few cases inland canals), and only 10% to 14% can be linked to aquaculture. The increase of maritime traffic is an important pathway for introduction and dispersal of alien decapod species, since larvae can survive long periods in ballast water (Mizzan, 1999; Occhipinti Ambrogi, 2000). The presence of non indigenous species populations in some Mediterranean areas can also be related to their trade: Necora puber and Paralithodes camtschaticus (Faccia et al., 2009) are quite frequently found alive in the markets.

Larval crab stages (zooea, megalopae) have been found in ship’s ballast but are by no means common, and crabs are rarely on hulls. Adult crabs have been found in bottom sediments in ballast tanks and in sea chests and other areas not routinely affected by ballast water management (Grosholz, 2011). Once introduced to a new continental margin, crabs may frequently expand their range through dispersal of planktonic larval stages by advection of ocean currents. Several introduced crab species have been rapidly dispersed by ocean currents along a coastline following an initial human-mediated introduction to a new continental region.

Many crab species have the tendency to expand their native range significantly into areas that are considerably outside of their typical range. This is partially caused by their long planktonic development periods, during which developing larvae may be carried many hundreds of miles by ocean currents. Among the species that best exemplify this pattern are the swimming crabs of the genus Callinectes, which include the commercial blue crab Callinectes sapidus.

Once introduced to a new region, many crabs can rapidly expand their range along the coastline. These dispersal events include some of the fastest range expansions recorded for any introduced species. Among the most rapid range

expansions is that of the European green crab, which spread along the western coast of the United States at rates of 200 km per year. Also, the crab Hemigrapsus sanguineus experienced rapid range expansion on the eastern coast of the United States. In Europe, Eriocheir sinensis also spread rapidly along the coast and throughout many river systems during the early twentieth century (Grosholz, 2011). Rapid spread is by no means the rule or even the norm for the same species. For instance, Carcinus maenas has spread very little in southern Australia and Tasmania, and the rates of spread in eastern North America were very minimal for long periods.

Review on the geographical distribution, abundance and biological data of ecologically important alien crabs found in the Mediterranean basinA number of 40 alien Brachyura species of Red Sea/Indo-Pacific origin, belonging to 19 families, have been recorded so far in the Mediterranean Sea (Galil, 2011; Zenetos et al., 2012; Corsini-Foka et al., 2013; Karhan et al., 2013; Galil and Mendelson, 2013; Zaouali et al., 2013). The families Portunidae, Pilumnidae and Leucosiidae show the highest number of Red Sea/ Indo-Pacific aliens, with 11, 5 and 3 species respectively, while the remaining families are represented by only one or two species.

Up to date, most Indo-Pacific decapod species introduced to the Aegean waters (21 species) are concentrated along the coasts of the south-eastern corner of the basin, a marine environment particularly suitable for the establishment of warm-water alien species (Pancucci-Papadopoulou et al., 2012). In particular, all Indo-Pacific alien Brachyura reported from the waters around Rhodes have been introduced via the Suez Canal (12 species) (Corsini-Foka and Pancucci-Papadopoulou, 2012; Corsini-Foka et al., 2013).

The most known invader crabs in the Mediterranean are Portunus pelagicus and Percnon gibbesi which are the most recent and successful invaders in the Mediterranean Sea and the blue crab Callinectes sapidus.

Portunus pelagicusThe blue crab Portunus pelagicus, also known as the flower crab, blue swimmer crab, blue manna crab or sand crab (Fig. 9) is abundant in the Indic and West Pacific Oceans: From Japan, and Philippines troughout Southeast and East Asia, to Indonesia, the East of Australia, and Fidji Islands, and westward to the Red Sea and East Africa. The geographical distribution of the blue swimmer crab P. pelagicus comprises the Red Sea and the Indo-West Pacific Ocean. It was first recorded in the Mediterranean Sea at Port Said, Egypt, in 1898 (Galil and Zenetos, 2002) and, later, at Palestine (Fox, 1924), Mediterranean coasts of Turkey, Lebanon, Syria (Holthuis and Gottlieb, 1958; Kocatas, 1981), Cyprus (Demetropoulos and Neocleous, 1969) and Italy, eastern Sicily (Crocetta, 2006; Ariani and Serra, 1969; Zibrowius, 1991; Galil et al., 2002). Recently it has been collected in the Tyrrhenian Sea and in the Aegean Sea, Turkey (Yokes et al., 2007; Corsini-Foca et al., 2004) (Fig. 10). Global warming is expected to favour this tropical species (Galil, 2007). Found primarily within estuaries and inshore coastal waters on sandy or muddy substrate, intertidal to 55 m praying on slow-moving invertebrates and scavenging (Lai et al., 2010). Commercially valuable since the early 1920s in the Levant basin CIESM, 2014).

Figure 9. Male Portunus pelagicus.(http://en.wikipedia.org/wiki/File:Portunus_pelagicus_male.jpg)

Figure 10. Distribution of Portunus pelagicus in Mediterranean Sea (CIESM, 2014).

Callinectes sapidusThe Atlantic blue crab Callinectes sapidus Rathbun, 1896 (Decapoda, Brachyura, Portunidae) (Fig. 11) originates from western Atlantic and has extended its distribution in most European coastal waters (Fig. 12) and in Japan; transporting in ballast water is considered the most probable vector (Nehring, 2011). In the Mediterranean, C. sapidus was first recorded in the Northern Adriatic Sea (Giordani Soika, 1951); to date, it results almost ubiquitous (Eleftheriou et al., 2011; Galil, 2011) and is considered an invasive species (Streftaris and Zenetos, 2006; Duruer et al., 2008). Carrozzo et al. (2014) suggested that the established population of Callinectes sapidus in the Torre Colimena basin, located along the Ionian coasts of the Salento peninsula, may occupy a predatory position in the food chain, exerting a potentially high ecological impact on the coastal habitat.

The tolerance to extreme variations in water conditions (the species is euryhaline and eurythermal, inhabiting estuaries and lagoons from Argentina to

Nova Scotia: McMillen-Jackson and Bert, 2004 and literature cited), high fecundity (females produce 2 to 8 million eggs per spawn: Jivoff et al., 2007), large body size (individuals may attain a maximum carapace width of 225 mm and a wet weight of 550 g: Millikin and Williams, 1984) coupled with aggressive behaviour (Reichmuth et al., 2011; Weis, 2010), are considered key ecological and biological determinants of its invasion success (Nehring, 2011). Noticeably, with the only exception of the Aegean Sea (Pancucci-Papadopoulou et al., 2005; Tureli Bilen et al., 2011) the majority of the reports in Mediterranean and, in general, European waters refers to episodic catches, limited in the number of collected specimens and the temporal period considered (Dulcic et al., 2010; Nehring, 2011).

Figure 11. Female of Callinectes sapidus caught in German Weser estuary on 20th of July 2007. Pphotos by Jörg Albersmeyer. (Nehring, 2012)

In its native habitats, Callinectes sapidus has omnivorous, opportunistic trophic habits, feeding on plants, invertebrates, conspecifics and carcasses (Dittel et al., 2006; Seitz et al., 2011) and represents an important functional component of benthic food webs (Baird and Ulanowicz, 1989; Dittel et al., 2000).

Figure 12. Distribution of Callinectes sapidus in Mediterranean Sea (CIESM, 2014).

Competition with other crabs has been implied as an impact for Mediterranean populations (Gennaio et al., 2006). It is also known to feed on fish caught in nets and to damage fishing gear (Främmande Arter, 2006). The occurrence of Callinectes sapidus could be an important harmful factor in the human health system as well as in the tourism sector because blue crabs have been implicated as carriers of strains of the bacterium Vibrio cholerae which are responsible for outbreaks of human cholera (Hill et al. 1989).

There have been other claims of established populations of Callinectes sapidus in the western and central Mediterranean Sea (Gennaio et al., 2006; Beqiraj and Kashta, 2010; Dulcic et al., 2011; Castejón and Guerao, 2013) based on occasional collections of adult specimens and ovigerous females. Taking into account the current increase in the temperature of Mediterranean, and, in general, European coastal waters (Lejeusne et al., 2010; Lima and Wethey, 2012), climate change can be envisioned as one of the main determinants of the invasion success of Callinectes sapidus, past and future (Hines et al., 2010). To date, biological invasions and global warming are no longer considered independent in their effects on the biodiversity and functioning of natural ecosystems (Walther et al., 2009); large-scale studies, encompassing environments with a diverse range of seasonal variability in water temperature, will provide useful information on climate-related mechanisms regulating the occurrence of this IAS in invaded habitats.

Callinectes sapidus has been included among the 100 worst IAS in the Mediterranean Sea (Streftaris and Zenetos, 2006); however, with the exception of the eastern sectors of the basin, the actual spatial-temporal characteristics of blue crab populations in non native environments have been poorly investigated, while their functional importance and ecological impact on autochthonous communities are to date virtually unexplored.

Xanthias lamarckiiXanthias lamarkii is the most common species of the genus Xanthias (Serène, 1984) (Fig. 13), with a distribution extending in the Indo-West Pacific Ocean, from Eastern Africa (South Africa), Mauritius, Réunion, Madagascar, Europa Isl., Comoros (Anjouan, Mayotte) to Australia, Japan, Hawaii, and French Polynesia, including Wallis and Futuna Islands (Serène, 1984; Davie, 2002, 2012; Legall and Poupin, 2013). It is a shallow waters crab (0-100 m of depth), commonly observed in depths of 0 to 6 m, on hard bottom (rock and rubbles) (Legall and Poupin, 2013). Xanthias lamarckii was located in south-eastern Aegean Sea. The occurrence of this Indo-West Pacific species was reported for the first time in the Mediterranean waters and exhibits the on-going process of biological invasion of the basin (Corsini-Foka et al., 2013).

Figure 13. The female specimen of Xanthias lamarckii from Chtenia, Rhodes (carapace length 11.3 mm, carapace width 17.8 mm), just fixed in ethanol (A, dorsal view; B, ventral view, C, frontal view) and the external face of the right chela (length 9.9 mm), showing the three granular furrows (D). White bar 10 mm. (Corsini-Foka et al., 2013).

Actaea savigniiThe recently discovered Actaea savignii (Milne Edwards, 1834) off Israel and Turkey (Karhan et al., 2013) (Fig. 14) is the second alien xanthid crab recorded from the Mediterranean except Atergatis roseus (Rüppell, 1830) a species first collected off Israel in 1961 and now common along the Levantine coast (Galil, 2011). The recent record of the Levantine populations of Actaea savignii is a testament to the on-going Erythraean invasion of the Mediterranean Sea.

Figure 14. Actaea savignii (H. Milne Edwards, 1834) A, C, D, male, cw 18.3 mm, cl 14 mm; B, female, cw 12.8 mm, cl 9.6 mm; A, B, live specimens; C, D, male specimen as above, freshly preserved; A – C, dorsal view; D, frontal view. Photographs by S.Ü. Karhan. (Karhan et al., 2013).

Matuta victorOf the 34 Indo-Pacific alien decapod crustaceans species recorded off the Israeli coast, 4 are known from single specimens and 5 more are rarely found, but the remainder have established large populations and some are more prominent in the Levant than in their native habitats in the Red Sea.

Matutidae is a family of crabs, sometimes called moon crabs, adapted for swimming or digging. Moon crabs are carnivorous and facultative scavengers, their diet composed primarily of crustaceans and molluscs, with smaller individuals feeding on smaller, softer-shelled species, whereas large size classes prey on shelled sessile or slow-moving species such as anomurans, bivalves and gastropods (Perez and Bellwood 1988). As a predator of slow-moving benthic invertebrates, Matuta victor (Fig. 15) may influence the abundance and distribution of its prey items was it to achieve numerical abundance in Levantine sandy shores. (Galil and Mendelson, 2013).

Figure 15. Matuta victor (Galil and Mendelson, 2013).

Percnon gibbesiIn the Western Mediterranean Sea the only crab species markedly invasive is Percnon gibbesi (Fig. 16), which shows a rapid expansion of its geographical distribution range, e.g. in the east Spanish coast (Deudero et al., 2005): first recorded from the Balearic Archipelago in 1999 (Garcia and Reviriego, 2000), it established populations since 2002 in Barcelona, 2003 in Alicante and Murcia, 2006 in Almeria. This crab species is also well established in the Central Mediterranean Sea (e.g. Relini et al., 2002; Sciberras and Schembri, 2008; Pipitone et al., 2001; Elkrwe et al., 2008) and in the eastern part of the Mediterranean also, like in Greece, Turkey, Syria, Cyprus, Israel, Egypt (Cannicci et al., 2006; Katsanevakis et al., 2011; Konstantinou and Kapiris, 2012 in Thessalou-Legaki et al., 2012) (Fig. 17). It lives under boulders, or in narrow crevices, where it seeks shelter when threatened in depths down to 30 m. It is active during daytime with feeding peaks at sunset. It is strictly herbivorous, using its small claws to pick off the fine film of algal growth.

Figure 16. Percnon gibbesi. Male specimen, collected near Al Haniyah, 29 July 2007. Photograph by H. M. Elkrwe. (Elkrwe et al., 2008).

Two vectors of introduction of P. gibbesi have been speculated, either through shipping (Mori and Vacchi, 2002; Galil et al., 2002) or by larval drift by the Atlantic surface current that enters the Mediterranean Sea through the strait of Gibraltar

(Pipitone et al., 2001; Abelló et al., 2003). A third possible vector that may not be excluded is aquarium trade, as for other Mediterranean aliens (Jousson et al., 1998; Padilla & Williams, 2004). Abelló et al. (2003) suggested larvae of Percnon gibbesi have entered the Mediterranean with the Atlantic currents. These currents sweep east wards along the southern Mediterranean coast. Similarly, Pipitone et al. (2001) proposed that the mechanism for the introduction and spread of Percnon gibbesi in the Mediterranean are larval transport by surface currents.

Figure 17. Distribution of Percnon gibbesi in Mediterranean Sea (CIESM, 2014).

Only three crabs (Callinectes sapidus, Dyspanopeus sayi and Rhithropanopeus harrisii) are definitely established in the Adriatic Sea and all three originate from the Atlantic coast of USA. The spreading of the two panopeid crabs in the Adriatic Sea in recent years (Froglia and Speranza, 1993; Mizzan and Zanella, 1996; Onofri et al., 2008) has been facilitated by the development of mussel aquaculture both in lagoons and open sea, with transfer of mussels’ seed and half grown mussels among aquaculture plants.

Dyspanopeus sayiDyspanopeus sayi (Smith, 1869) is a species of mud crab that is native to the Atlantic coast of North America (Fig. 18). It has become established in the Mediterranean Sea since the 1970s (Fig. 19). It can reach a carapace width of 20 mm, and has black tips to its unequal claws. It feeds on bivalves and barnacles, and is in turn eaten by predators including the Atlantic blue crab, Callinectes sapidus. Dyspanopeus sayi lives predominantly on muddy bottoms, where it is a predator of bivalve molluscs (Williams, 1965). It is very abundant in the Venice lagoon, locally may outnumber the autochthonous crabs Carcinus aestuarii and Pilumnus spp (CIESM, 2014).

Figure 18. Dyspanopeus sayi (http://flickr.com/photos/50608239@N03/5378633918).

Figure 19. Distribution of Dyspanopeus sayi in Mediterranean Sea (CIESM, 2014).

Rhithropanopeus harrisiiThe white-fingered mud crab Rhithropanopeus harrisii (Gould, 1841) is a highly successful estuarine invader and is one of the most widely distributed brachyuran crab species globally (Roche and Torchin 2007) (Fig. 20). Rhithropanopeus harrisii has been found in the Adriatic Sea since 1994, on the French Mediterranean coast in 2000 and on the Tunisian coast in 2003 (Roche and Torchin, 2007) (Fig. 21). Its association with international ports in most European countries indicates that it has been transferred with shipping, either as larvae in ballast water or as hull fouling.

Rhithropanopeus harrisii is a scavenger, feeding on detritus, algal material or small invertebrates, including amphipods, polychaetes and bivalves (Jensen, 2010). Evidence for negative impacts is mostly anecdotal. It may alter ecosystem functioning by competing with native crabs, being a predator on native benthic invertebrates, and being a prey of native predators. Rhithropanopeus harrisii is host for the “white spot baculovirus”, which affects penaeid shrimps and blue crabs in its native region.

Figure 20. Rhithropanopeus harrisii (Fofonoff et al., 2003).

Figure 21. Distribution of Rhithropanopeus harrisii in Mediterranean Sea (CIESM, 2014).

Charybdis luciferaThe latest record caught six miles off the Venetian coast is that of the Indo- Pacific crab Charybdis lucifera, (Mizzan and Vianello, 2009) (Fig. 22).

The original distribution of Charybdis lucifera goes from India to Japan, including Australia (Stephenson, 1972), though the ways of transportation and the causes of its introduction are unknown. It’s very likely that, owing to the increasingly busy maritime traffic, the species was unintentionally introduced by passive transportation of the planktonic larva in ballast water. As far as we know today, the finding of this single specimen represents a single, isolated case (Mizzan and Vianello, 2009).

Figure 22. Charybdis lucifera (http://zukan-bouz.com/detail.php?id=1116)

The latest additions to the inventory of the marine decapod species in the Eastern Mediterranean Sea are the pilumnid crab Eurycarcinus integrifrons (Ozcan et al., 2010) and the red swimming crab Gonioinfradens paucidentatus (Corsini-Foka et al., 2010), both of Indo-Pacific origin.

Eurycarcinus integrifronsThe alien pilumnid crab, Eurycarcinus integrifrons is native to the Red Sea-Indian Ocean reaching to the Red Sea and Madagascar (De Man 1879, Apel 2001) (Fig. 23), and was recently recorded for the first time in the Mediterranean from the Yumurtalik coast of Iskenderun Bay (Levantine Sea, Turkey) (Özcan et al. 2010). Transport in ballast is supposed the most likely vector of introduction into the eastern Mediterranean of Eurycarcinus integrifrons in Iskenderun Bay (Turkey), an area subjected to intense industrial shipping traffic.

Figure 23. Eurycarcinus integrifrons (http://www.flickr.com/photos/artour_a/5815746734/).

To date, five species of alien Pilumnidae have been recorded in the Mediterranean Sea: Actumnus globulus Heller, 1861; Glabropilumnus laevis (Dana, 1852); Pilumnopeus vauquelini (Audouin, 1826); Pilumnus minutus De Haan, 1835 (= Pilumnus hirsutus Stimpson, 1858); and Halimede tyche (Herbst, 1801) (CIESM,

2010). Three of those five species, Pilumnopeus vauquelini, Pilumnus minutus and Halimede tyche are likely to have entered the Mediterranean via the Suez canal, while two species (Glabropilumnus laevis, Actumnus globulus) probably did so through shipping activity (Galil et al., 2006).

Gonioinfradens paucidentatusGonioinfradens paucidentatus (A. Milne Edwards, 1861) is a portunid crab with a wide Indo-Pacific distribution: (Spiridonov and Neumann, 2008; Poupin, 1994; Apel and Spiridonov, 1998; Davie, 1998; Naderloo and Sari, 2007) (Fig. 24). It occurs mainly on hard substrate from shallow subdital waters to 100 m of depth and reaches a carapace length of 52.5 mm (Poupin, 1994).

Figure 24. Gonioinfradens paucidentatus (http://decapoda.free.fr/illustration.php?n=6andsp=156)

The first record for the Mediterranean Sea of the Red Sea/Indo-Pacific portunid Gonioinfradens paucidentatus (red swimming crab) was reported in Rodos Island (southeastern Aegean Sea) is given, while possible introduction vectors of the species in the area are discussed (Corsini-Foca et al., 2010).

The following alien crabs have sporadically been found in many and various areas of the Mediterranean basin: Myra subgranulata, Herbstia condyliata, Leucosia signata, Xanthias lamarckii, Macrophthalmus graeffei, Ixa monody, Atergatis roseus, Hyastenus hilgendorfi, Coleusia signata, Portunus segnis, Thalamita poissonii, Eucrate crenata, Halimede tyche, Atergatis roseus, Micippa thalia, Charybdis (Charybdis) hellerii, Carupa tenuipes, Matuta victor, Sirpus monodi, Macropodia tenuirostris.

Management practices with regard to the occurrence of ‘new’ species will require comprehensive evaluation of changing habitat conditions and will depend on the individual case. They could range from complete eradication to toleration and consideration of the ‘new’ species as an enrichment of the local biodiversity as a means to facilitate ecosystem restoration or to maintain ecosystem function as native communities re-assemble and establish under a new climate regime.

A large number of alien crabs have entered and established in the Mediterranean Sea, rendering specific measures necessary for each introduction pathway. However a more integrated and comprehensive approach to the

management of alien crab species is fundamental to ensure coherence with the wider EU maritime policy and research framework. In addition to ensure appropriate management and assess existing policies, extensive research should be directed towards the study of invasive crab species their effect on the native fauna and their impact to the ecosystem.

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