INVESTIGATING ON TRE FACTORS RESPONSIBLE FOR CAULERPA RACEMOSA

The introdueed alga Caulerpa racemosa (Forsskal) J. Agardh (Caulerpales, Chlorophyta) has become an important eomponent of roeky assemblages in the subtidal of the Mediterranean. Understanding the faetors that regulate the establishment and spread of this species is, therefore, crucial to predicting future pathways of invasion and suseeptible /oea/es. Furlher, Ihe aim of this study was to investigate on Ihe factors responsible for Ihe successful invasion of C. racemosa in the Asinara GulJ (NW-Sardinia). Key-words: Caulerpa racemosa, roeky shores, seagrass, seaweed, spreading. Introduction Several studies evaluated the effects of introduced species on native communities at different organizational level (Grosholz, 2002), while other studies tried to highIight mechanisms which regulate the success of the invasion (Mack et al., 2000). The introduced alga Caulerpa racemosa (Forsskal) J. Agardh has become an important component of rocky assemblages in the subtidal of the Mediterranean (Klein & VerIaque, 2008) and it has been indicated as a species able to change natural patterns of distribution and abundance of resident assemblages, both on rocky (Piazzi et al., 2001) and soft bottoms (Argyrou et al., 1999). Understanding the factors that regulate the establishment and spread of this species is, therefore, cruciai to predicting future pathways of invasion and susceptible locales. Although the occurrence of C. racemosa has been reported in a variety of habitat the performance seems to significant1y depend on the habitat where it establishes. This study was conducted in the Asinara Gulf (NW Sardinia) where Caulerpa racemosa has been spreading since 2002. The aim was to investigate on the factors responsible for the successful invasion of the introduced alga C. racemosa. Materials and methods In the study area 4 sites were considered: 2 outside (Porto Torres: PTI and PT2) and 2 inside (ASI and AS2) the Isola dell'Asinara Marine Protected Area. In each site 2 areas about 400 mx400 m in size were chosen, about 100s m apart. At each area data were colIected along four 30 m long transects, randomly positioned within 5-15 m of depth in rocky reefs. In each transect the extension of each type of habitat (Posidonia oceanica, dead matte, sand, coralligenous, algal turfs, barren, Cystoseira spp.) was measured. This alIowed us to estimate the relative importance of habitats. Then, bottom complexity was evaluated along the transect using a metal chain and measuring the distance between the two ends once it is laid on the bottom surface. Further, ten plots 400 cm2 in size were randomly considered along each transect (total plot=160). In each of them the structure of assemblages and the cover of C. racemosa were estimated by underwater photographs of the plot. Data were analysed using univariate, multivariate and correlative techniques to identify both the factors responsible of C. racemosa distribution and which habitats were mostly invaded. A four-way ANOVA was used to test for difference in C. racemosa cover at alI spatial scales considered: 'Protection' was fixed (no MPA and Investigating on the factors responsible for C. racemosa inrasion 281 MPA), while 'Site' (2 levels: PTI, PT2, and ASI, AS2, respectively), 'Area' (2 levels) and 'transect' (4 levels) were considered random and nested. Correlations were used to estimate the relationship among C. racemosa cover and considered factors both at plot and transect scale. Results The percent cover of Caulerpa racemosa was extremely different among sites. An overwhelmingly higher abundance was observed outside rather than inside the MPA, where the occurrence of the introduced alga was recorded just in one plot in ASI. Unfortunately, ANOVA was not run because of vari ance heterogeneity. Difference in benthic assemblages were found between no MPA and MPA sites. Specifically in unprotected sites the abundance of taxa and the habitat quality, at the plot scale and transect scale, respectively, were lower as indicated by a higher occurrence of Posidonia oceanica dead matte. At plot scale, the correlation between the mean percent cover of C. racemosa and the number of taxa highIighted a reverse negative relationship (R 2=0.72). Alike, at transect scale, a correlation was evidenced between the mean percent cover of C. racemosa and the cumulative quantity of canopy habitats (Posidonia oceanica, Cystoseira spp., photofilic algae) (R 2=0.49). Bottom complexity at transect scale was higher in MPA sites than outside but it was not found to affect the spread of C. racemosa. Conclusions The resu1ts suggest that patterns of distribution of Caulerpa racemosa on rocky reefs are largely dependent upon biotic characteristics of benthos both the diversity of taxa and habitats, depending on the scale considered. Although correlative studies cannot identify a cause-effect relationship, the results suggest that pristine Mediterranean habitats are much unlikely invaded by C. racemosa than degraded habitats (Bulleri et al., 2010).