SPREAD OF THE GREEN-ALGA CAULERPA-TAXIFOLIA (CAULERPALES, CHLOROPHYTA) IN THE MEDITERRANEAN - POSSIBLE CONSEQUENCES OF A MAJOR ECOLOGICAL EVENT

Caulerpa taxifolia, a green alga with a circumtropical distribution, w as observed for the first time in the Mediterranean in 1984. At presen t it covers an area of 1000-2000 ha, which is increasing by a factor o f 2 to 10 annually. The Mediterranean plants have some morphological a nd physiological differences from their tropical counterparts; they co lonize a wide range of biotopes, from surface to 20-30 meters depth, a nd have been collected down to 100 meters depth. Caulerpa taxifolia ou tcompetes native seaweeds due to its high growth rate, its total subst rate occupation, its improved light access, the increased sedimentatio n rates it creates, and the synthesis of secondary metabolites (mono- and sesqui-terpenes). The concentration of these metabolites changes s easonally, and strongly affects grazing by the sea-urchin Paracentrotu s lividus. The assemblages dominated by Caulerpa taxifolia show a redu ced number of other algal species and a low diversity. Populations of sea-urchins, fishes, amphipods and polychaetes are also affected. Base d on preliminary studies, some special features concerning the functio ning of an hypothetic future ecosystem dominated by Caulerpa taxifolia are predicted. If Caulerpa taxifolia continues to spread into the Med iterranean at present rates, we will witness a major ecological event, with a strong decrease of eco-diversity (and perhaps of bio-diversity , too) in Mediterranean coastal waters.