F . Bulleri et al. J. Exp. Mar. Biol. Ecol. 255 2000 1 –19
3
overlap at heights on the shore between 0.1 and 0.2 m above the mean-low-water-level Menconi et al., 1999. Previous experiments Benedetti-Cecchi and Cinelli, 1993, 1997;
Benedetti-Cecchi et al., 1996; Benedetti-Cecchi, 2000 revealed the important role of these grazers in regulating patterns of colonization in disturbed patches. The removal of
limpets resulted in the monopolization of the substratum by filamentous algae, whereas in the presence of grazers succession proceeded with the establishment of the fleshy red
alga Rissoella verruculosa Bertolini J. Agardh, barnacles and the Cyanophyta Rivularia spp. More recently, large-scale studies employing scales similar to those of
the present work have shown considerable spatial variability in the effects of limpets among shores tens to hundreds of kilometres apart Benedetti-Cecchi et al., in press.
These experiments were carried out on rocky shores adjacent to the artificial structures studied in the present paper and the two species of limpets, which were found at
densities similar to those here reported see Section 3, were considered as a guild and excluded by means of cages Benedetti-Cecchi et al., in press.
The focus of this paper is on the effects of P . aspera and P. rustica on assemblages of
algae and barnacles developing on artificial reefs in the northwest Mediterranean. The primary aim of this study was to test the hypothesis that removing limpets from these
manufactures led to changes in the structure of assemblages similar to those observed on rocky shores. Furthermore, we examined whether there were inconsistencies in the
effects of limpets at spatial scales comparable to those investigated on rocky shores. These hypotheses were tested with a multifactorial experiment involving the orthogonal
manipulation of the presence absence of the two species of limpets. The experiment was repeated at different locations tens of kilometres apart, on different reefs within each
location a few kilometres apart, and using replicate boulders within reefs tens to hundreds of metres apart as the experimental units. This experiment also allowed us to
test the null hypotheses that P
. rustica and P. aspera had similar effects on assemblages, and that one species had no influence on the distribution of the other on the artificial
reefs these hypotheses have not been tested yet on rocky shores.
2. Materials and methods
2.1. Study site This study was done at two exposed locations Carrara, 44
8029N, 108019E and Livorno, 43
8279N, 108219E, on the northwest coast of Italy, between June 1997 and February 1998. The two locations were about 70 km apart and were characterised by the
presence of industrial developments and marinas. Two artificial reefs, about 4 km apart, were used at each location referred to as Reef 1 and Reef 2 from north to south at each
location, respectively. These reefs were 150–300 m long and run parallel to the coastline experiencing intense wave action due to western winds from southwest to
northwest. The reefs were made of transplanted carbonatic boulders, with their longer axis ranging from 1 to 3 m.
Assemblages on these boulders were qualitatively similar to those occurring in midshore and lowshore habitats of rocky coasts in the area Benedetti-Cecchi and
4 F
. Bulleri et al. J. Exp. Mar. Biol. Ecol. 255 2000 1 –19
Cinelli, 1993, 1997; Benedetti-Cecchi et al., 1996; Menconi et al., 1999; Benedetti- Cecchi, 2000. The most common algae were encrusting corallines, the brown crust
Ralfsia verrucosa Areschoug J. Agardh, Cyanophyta of the genus Rivularia, and erect algae such as Rissoella verruculosa Bertolini J. Agardh, Nemalion helmintoides
Velley Batters, Porphyra leucosticta Thuret, the articulated corallines Corallina elongata Ellis and Solander and Haliptilon virgatum Zanardini Garbary and Johansen,
the coarsely branched Laurencia obtusa Hudson Lamouroux and Chondria spp. De Notaris De Toni, and the filamentous Polysiphonia spp. and Ceramium spp. The most
abundant sessile invertebrates were the barnacles Chthamalus montagui Southward and Chthmalus stellatus Poli, while the main herbivores were the limpets Patella aspera,
Patella rustica and the snail Osilinus turbinatus Von Born; these herbivores were distributed at heights on the shore ranging from
2 0.2 to 0.4 m with respect to mean-low-water-level.
2.2. Experimental designs and analysis of data Twelve boulders with the longer axis no less than 2 m in length were selected
randomly on the seaward side of each reef. These boulders were numbered with marine epoxy for identification and randomly allocated to four treatments with three replicates
each. Treatments were: 1 control, where all limpets had been left in place 1 Pa 1 Pr,
2 removal of P . aspera
2Pa 1 Pr, 3 removal of P. rustica 1 Pa 2 Pr, and 4 removal of both species
2Pa 2 Pr. Limpets were removed by hand with the aid of a screw driver; boulders were searched thoroughly at the beginning of the experiment and
every 3–4 weeks thereafter to maintain the experimental conditions. The percentage cover of sessile organisms and the density of mobile grazers were
assessed after 4 and 8 months using quadrats of 10 3 10 cm in size. Estimates of
percentage cover were obtained visually by subdividing the quadrat in 25 2 3 2cm
sub-quadrats Dethier et al., 1993; Benedetti-Cecchi et al., 1998. Densities of grazers were expressed as the number of animals present in each quadrat. Sampling was
restricted to the seaward side of the boulders, because only this side provided a habitat comparable to that of rocky shores authors’ personal observation. Three quadrats were
placed randomly on each boulder at each sampling occasion. To avoid re-sampling the same quadrats and to maintain temporal independence in the data, the first set of
replicates was marked with epoxy putty so that they were avoided when boulders were sampled the second time.
2.3. Analysis of data Data from the two locations were analysed separately, since the densities of the two
species of limpets and the assemblages were markedly different. A five-factor mixed model analysis of variance was used to test hypotheses about the
effects of limpets on artificial reefs. Factors were: P . aspera fixed and orthogonal, P.
rustica fixed and orthogonal, Time fixed and orthogonal, Reef random and orthogonal, and Boulder random, nested within the interaction Reef
3 P. aspera 3 P. rustica. The response variables for these analyses were: 1 encrusting coralline algae,
F . Bulleri et al. J. Exp. Mar. Biol. Ecol. 255 2000 1 –19
5
2 Cyanophyta, 3 articulated coralline algae, 4 filamentous algae, 5 Ralfsia verrucosa, 6 Rissoella verruculosa, and 7 Chthamalus spp. The same model of
analysis was used to determine the efficacy of the manipulation and to test for the effects of one species of limpet on the density of the other. In these tests, the factor
corresponding to the species of limpet which was also the response variable in the analysis, was termed Removal and tested for the efficacy of the experimental manipula-
tion. Cochran’s C-test Winer, 1971; Underwood, 1997 was used to check the assumption of homogeneity of variances. In some cases it was necessary to transform the
data square root or logarithmic scale to meet this assumption. Pooling procedures were also used when appropriate, according to Winer 1971. Student–Newman–Keuls tests
SNK were used for a posteriori comparisons of the means.
3. Results
