The Mediterranean proliferation of Caulerpa taxifolia provides an opportunity to examine the extent to which an introduced species can modify biodiversity. During 1992, benthic invertebrates were sampled in Roquebrune-Cap Martin area, at depths of 6 and 10 m, from a Caulerpa station and a reference station. Species richness is slightly lower at 6 m within Caulerpa while the number of specimens is much reduced at both depths. Similarity indices compare well between reference and Caulerpa stations.

Caulerpa taxifolia, distributed in tropical seas, was recorded for the first time in the Mediterranean in 1984, as a single 1 m2 patch. This surface increased to 3 ha by 1990, 30 ha in 1991, 100 to 430 ha in 1992, and 1000 to 2000 ha at the end of 1993 (Meinesz & Hesse, 1991; Boudouresque & Meinesz, 1994).

Maximum colonization is between 2 and 6 m depth, but the alga has been observed down to 99 m. Density can reach 350 m of fronds and 14,000 leaves per m2. Caulerpa taxifolia colonizes any type of substrate and invasion results in a drastic impoverishment of the algal components of the communities (Boudouresque et al., 1992; Verlaque & Fritayre, 1994). According to these authors, maximal poverty of communities coincides with the maximal development of C. taxifolia (summer-autumn) and epiflora is negligible, except during spring.

These changes may induce functional and structural modifications of the fauna that lived in the communities before infestation. Invasion, particularly of photophilic algae and Posidonia beds (Ros et al., 1985), can modify quality and intensity of physical, chemical and hydrodynamical factors, number and quality of shelters, quantity and quality of food, and exchange of individuals between communities and populations.