Caulerpa taxifolia is an introduced green seaweed
in the Mediterranean; it is a very fast-spreading species,
is able to invade all kinds of substrata and causes regression of some seagrasses. C. taxifolia was investigated in a
small bay of the northwestern Mediterranean, where it occupies
3 distinct habitat types: (1) at the edge of Posidonia
oceanica, (2) within Cymodocea nodosa beds, and (3) on sand
and cobbles. To provide a basis for further experimental
investigations of the factors affecting its performance, a
descriptive study was carried out at this site in which both size
and density of blades were measured on 20 dates in all habitats
from October 1994 to September 1996. Great temporal
fluctuations within the same season were evident for both
variables, despite the fact that high variability was found at
small spatial and temporal scales (areas within each habitat
and time within each season). However, habitat effect in combination
with season was evident as a major factor affecting
both length and density of blades, suggesting a positive effect
of seagrasses on C. taxifolia. The greatest blade length was
found in individuals at the edge of P. oceanica, while within
C. nodosa beds, blade length was intermediate. Reduction of
blade density occurred in spring in the sand and C. nodosa
habitats, but not in the P. oceanica one. Patterns were similar
throughout the 2 years. Experimental evidence is needed
both to highlight mechanisms regulating this kind of interaction
(nurse effect vs shade-induced changes) and to investigate
whether it affects patterns of invasion and replacement
of existing species with C. taxifolia in these habitats. The
facilitative effect of seagrasses on the alga observed in the
study is also likely to cause an indirect negative effect on seagrasses themselves
