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Introduced exotic species

Hundreds of exotic species reach the Mediterranean each year. Those that survive may radically alter the native ecosystems.

It is very difficult to give the exact number. Probably in the neighbourhood of a thousand, it is in constant progression with the arrival of new species and new sightings due to :
-  better observation of the environment by scientists, fishers and amateur divers ;
-  new means of investigation or the exploration of new marine regions.

The reliability of estimates varies according to the animal or plant group considered : the appearance of a new species of algae or a small mollusc living in the sediment can more easily go unnoticed than a new species of fish, although certain families such as the gobies can surprise us when studied closely.

Even among the fishes, the figures reported are variable according to the criteria of validity of sightings. Depending on the organisms and on the methods used, the number of fish species recorded in the Mediterranean is around 700, of which 116 to 125 are considered as exotic, half of them originating in the Atlantic Ocean and the other half in Red Sea.

All the figures, updated in real time, are available on the international data base for fishes FishBase and on the CIESM atlas of exotic species in the Mediterranean : fishes, molluscs, crustaceans and macrophytes.

There are probably several hundred exotic species that reach the Mediterranean each year. They find there a hostile and unknown environment where they are in competition with native species that have long been adapted to the local ecological and environment conditions.

Most of them do not survive. Those that surmount this first test still have to ensure their descent to really become established, that is to find one or several partners and conditions favourable for the development or eggs, larvae and juveniles.

It is when the introduced species presents a serious ecological or economic impact that it is considered as invasive, but studying this impact on the existing ecosystems is time-consuming and difficult. For most of the introduced species in the Mediterranean, the ecological or economic impact has not yet been determined.

The settlement of an exotic species is linked to its capacity to find itself a vacant ecological niche, to share it with the native species that occupies it or to take its place. When it succeeds, it may radically alter the functioning of native ecosystems.

This is the case of the seaweed Caulerpa taxifolia, the strong development of which has reduced the diversity of the habitats and led to a decline in biodiversity. Another example is the invasion of the eastern basin by the rabbit fishes Siganus rivulatus and Siganus luridus originating from the Red Sea. These herbivorous fishes have conquered a niche that is little exploited in the Mediterranean by the few herbivores, the salema Sarpa salpa and the edible sea urchin Paracentrotus lividus.

The Mediterranean is also an exporter of invasive species to other seas : the red algae Dasya ballouviana to the North Sea, the red algae Polysiphonia brodiaei and the brown algae Arthrocladia villosa, Stictyosiphon soriferus and Striaria attenuata to Australia, and the fan worm Sabella spallanzani, also to Australia.

The Caulerpa are green algae (Chlorophyceae) populating temperate warm seas. They are siphonous algae, that is to say they do not possess distinct cells, the core being immersed in the cytoplasm contained in a single siphon. They are constituted of a rampant axis (stolon) fixed on the substrate by roots (rhizoids) and bearing erect fronds. They synthesize several substances that are repellent and toxic, including caulerpicine, caulerpine and caulerpenyne.

There are a hundred or so species of the genus Caulerpa. In the Mediterranean, the three main species are Caulerpa prolifera which occurs naturally, and two species that have developed there over the past few years : C. taxifolia and C. racemosa.
These three species may easily be distinguished by the appearance of their fronds : smooth and lanceolate for Caulerpa prolifera, dentrate for C. taxifolia and similar to bunches of grapes for C. racemosa.

- Caulerpa taxifolia
Its name comes from the Latin taxus (yew) owing to its resemblance to the leaves of this tree. The fronds, highly indented, resemble feathers and have a characteristic fine fluorescent green colour.

Originating in south-eastern Australia, they have an extensive range in the tropical belt of the Atlantic, Pacific and Indian oceans.

Caulerpa taxifolia is an attractive-looking seaweed that is easy to conserve in the aquarium. From the 1970s, it was exhibited in various European aquariums displaying tropical fauna and flora.

Accidentally introduced into the Mediterranean, it was observed for the first time in the natural environment in 1984, off the coast of Monaco. Probably as a result of genetic mutations, it had succeeded in adapting to the environmental conditions of the Mediterranean and had begun its spectacular expansion. Vegetal reproduction by cuttings is the only mode of reproduction of the original stump, and all the colonies present in the Mediterranean are thus genetically identical clones. A small fragment of the seaweed is enough to engender a new colony, dissemination occurring through transport of the cuttings (currents, boat anchors, fishing nets, etc.).

In the Mediterranean, the alga is more robust than in the tropical environment, it lives between the surface and fifty metres depth, on all kinds of bottom (rocky, sandy, silt, Posidonia meadow), off headlands battered by waves or deep inside sheltered creeks, in pure or polluted water. The fronds may reach a height of 80 cm ; it is resistant to low temperatures (7°C) and can survive prolonged emersion.

- History of expansion of C. taxifolia
1984, this alga was reported in the Mediterranean off the coast of the Principality of Monaco. 1990, it was found in France, at Toulon and in the vicinity.
1991, a few patches appeared along the Mediterranean coast from Menton (Côte d’Azur) to Saint-Cyprien (S-W French Mediterranean coast).
1992, the alga was sighted at Saint-Cyr-les-Lecques, Hyères (near Toulon, S.E France), Imperia (western Italian coast), the Balearic islands, and Villefranche-sur-Mer. (near Nice, Côte d’Azur).
1993, it was sighted in the Strait of Messina (Sicily).
1994, the coverage reached a surface area of anout 1 300 hectares,
2000, more than 5 000 hectares.

Since 2004, the colonisation has stabilised : 134 km of coastline affected in 2004, 137.3 in 2005, 136.5 in 2007, or 8 610 hectares.

Today, Caulerpa taxifolia is present along the coasts of seven Mediterranean countries : Croatia, Spain, France (except Corsica), Italy, Monaco, Tunisia and Turkey.

- Ecological impact
The extensive development of Caulerpa taxifolia has resulted in the homogenisation of the seabed by coverage of the different substrates, causing a reduction of the diversity of habitats and food resources. The populations of the few animals that this new environment suits have grown, but the others have been eliminated, which has resulted in the impoverishment of the biodiversity.

Various means of physical, chemical or ecological eradication (warm water, dark covers, manual uprooting, spreading of salt, copper-based processes, introduction of natural predators, etc.) have been envisaged or tested with varying degrees of effectiveness.

The surface areas affected by Caulerpa taxifolia are too extensive to envisage its total destruction. It is however possible to eliminate small isolated colonies in order to avoid the invasion of zones of patrimonial interest, such as the Port-Cros National Park where an eradication campaign is carried out every year.

- Caulerpa racemosa
This alga is characterised by short fronds bearing small vesicles (racemosa = bunch-like) and a very dense intertwining of stolons covering the substrate, mainly on sandy bottoms or dead matte, but also on rocky bottoms.
Like C. taxifolia, this species originated in Australia, but whereas C. taxifolia only reproduces in the Mediterranean in a vegetative way (cuttings), C. racemosa also uses the sexual pathway (emission of male and female gametes), producing numerous eggs that are dispersed with the currents.

C. racemosa was observed for the first time in the Mediterranean in 1990 off the port of Tripoli, Libya, then it progressed rapidly ; its first appearance on the French coasts was in 1997 at Marseille. Along the French coasts alone, 8 070 hectares were colonised by 2005, then 13 530 hectares by 2007.

By 2009, it was also present along the coasts of 14 Mediterranean countries : Albania, Algeria, Cyprus, Croatia, Spain, Greece, France (including Corsica), Italy, Libya, Malta, Monaco, Montenegro, Tunisia and Turkey ; it was also found in the Canary islands. 2010 Report on Caulerpa taxifolia and Caulerpa racemosa

- Asparagopsis armata, the hooked seaweed
This red alga (Rhodophycea) is currently found in the north-eastern Atlantic, the Mediterranean, the Black Sea and the Indo-Pacific.

The reproduction of red algae presents a succession of three generations (trigenetic cycle) : tetrasporophyte, gameophyte anad carposporophyte. In certain species, such as Asparagopsis armata, gameophytes and tetrasporophytes are morphologically very different (the tetrasporophyte type being considered as another species, Falkenbergia rufolanosa).

The hooked seaweed, in its gametophyte generation, develops in pyramid-shaped red to pale pink tufts. It has numerous small shoots giving it an asparagus-like shape (Asparagus) and spiny branchlets in the form of harpoons that enable the fronds to hook onto various supports, including diving suits. The tetrasporophyte form presents cottony pompoms a few centimetres in diameter.

It is an annual species, epiphyte on other algae, which develops in the Mediterranean from the surface down to a dozen metres depth in winter and in spring in its gametophyte form, and particularly in summer in its pompom form.

Originating in New Zealand and Australia, it was no doubt fixed on the hull of a ship that it first made its appearance in the Mediterranean in 1923. Its expansion has been rapid.

The most representative case is that if the Japanese or Portuguese oyster, Crassostrea gigas.

Originating in the Pacific, it was first introduced, probably accidentally, on the Atlantic coasts of France, Spain, Portugal and Morocco by Portuguese ships in the 16th or 17th century.

Subsequently, it was intentionally imported from Portugal to France, in particular in the Mediterranean, for the purposes of ostreiculture. Following viral epidemics that decimated the original Portuguese stocks in 1966 and 1970, the oyster farms of Thau lagoon were regenerated by means of direct importations from Japan. The planktonic larvae issued from reproduction enabled the recolonisation of the western basin of the Mediterranean and the Adriatic.

Crassostrea gigas is the hollow oyster, the main species sold on the French market ; the flat oyster, Ostraea edulis, is native to the European coasts.

Other species, also of Japanese origin, and of high commercial value, have also been imported to French lagoons in Languedoc (S.W. France Mediterranean coast), mainly in Thau lagoon : in 1965, the clam Mercenaria mercenaria and in 1981, the Venus clam Ruditapes philippinarum.

The prolific development of such organisms of course represents an economic asset. However, it may result in the introduction of associated species referred to as accompanying species. The spat of the Japanese oyster has turned out to be a redoubtable vector of penetration for algae of the same origin, of which the main species are Undaria pinnatifera and Laminaria japonica, in 1971, and Sargassum muticum, in 1980.

The first stage in the settlement of Lessepsian species, after coming through the Suez Canal, is the colonisation of the eastern Mediterranean basin. Many of them do not go beyond the Siculo-Tunisian Strait, but a few reach the western basin.

The rabbit fish Siganus luridus and the bluespotted cornetfish Fistularia commersonii, well installed in the eastern basin, have begun their exploratory route towards the west. The first sightings are rare and sporadic ; every new observation is precious.

- Fistularia commersonii
The cornetfish is recognisable by its very elongated body, up to 1.50 m in length, its long tubular muzzle in the form of a trumpet and very fine tail extended by a long filament. Generally solitary or living in small groups, it feeds on fishes and crustaceans.

Originating in the Indian and Pacific oceans, this fish was reported for the first time in the Mediterranean along the coast of Israel in 2000. Its population then grew so rapidly in the eastern basin that it was nicknamed ‘the Lessepsian sprinter’. In 2002, it was found in the Sicilian Strait, in 2003 in the south of the Tyrrhenian Sea, the following year in the north, and in 2005 on the east coast of Sardinia. By 2007, it had reached San Remo (S.W. Italy) and, to the south, also progressed very rapidly, reaching the south of Spain then the north in 2007.

Sighted several times in 2009 in south-eastern France (Alpes-Maritimes), it would appear to be well established along the coasts of Corsica and Provence : there were numerous sightings in October and November 2010 in Corsica and from Marseille to Nice along the French mainland coast.

- Siganus luridus
The rabbit fish, as its name suggests, is an herbivorous fish ; until its arrival, there was only one herbivorous fish in the western Mediterranean : the salema Sarpa salpa.

This modest-sized fish lives in very dense shoals in the juvenile state, then in smaller groups in adulthood.
It usually lives in the Indian Ocean and the Red Sea. The first sighting in the Mediterranean, in Israel, dates from 1955 ; its population grew rapidly in the eastern basin. The rare sightings in the western basin were close to the Siculo-Tunisian Strait.

In 2008, two Siganus luridus were caught by fishers at Sausset-les-Pins, near Marseille.

The absence of sightings between Sicily and the northern Mediterranean raises questions about the origin of these individuals : ballast water from a ship coming from the eastern Mediterranean, the Red Sea or the Indian Ocean, release from an aquarium or natural migration ?

À consulter

- International Commission for Scientific Exploration of the Mediterranean CIESM ATLAS OF EXOTIC SPECIES IN THE MEDITERRANEAN :
- Global Information System for Fishes :
- IUCN Invasive Species Specialist Group :