Invasion of the accidentally introduced tropical alga Caulerpa taxifolia in the Mediterranean Sea

Caulerpa taxifolia, a green alga of tropical origin, first colonized in the Mediterranean Sea in 1984 and has been spreading rapidly since. At the end of 1996, 77 stations colonized by this alien plant have been recorded along the coasts of France, Spain, Italy, Monaco and Croatia. The rapid expansion is due to its efficient vegetative reproduction and facilitated by the transport of fragments over long distances by boat anchors or fishing nets. Outstanding characteristics, such as its capacity to colonize all kinds of substratum at depths from 0,5 to 100 meters, its property to form dense, durable covers with very long fronds and stolons, and particularly its feature to contain anti-epiphytic and anti-grazing toxins, which proved to be harmful to many organisms, make C. taxifolia a strong competitor in its newly colonized marine environment. The introduction of C. taxifolia in the Mediterranean Sea has led to serious ecological consequences, such as an impoverishment of the species diversity in algal communities, a decline in sea urchin populations, and an impact on the ichthyofauna. Another major ecological risk is the invasion of the Posidonia oceanica seagrass beds by C. taxifolia, which leads to an obstruction and a clogging up of the meadow. C, taxifolia blocks the seagrass interstices and reduces light penetration, thereby altering the functions of this fragile and important ecosystem.