Botanica Marina 48 (2005): 30–37  2005 by Walter de Gruyter • Berlin • New York. DOI 10.1515/BOT.2005.012 Analysis of geographical variation in species richness within the genera Audouinella (Rhodophyta), Cystoseira (Phaeophyceae) and Cladophora (Chlorophyta) in the western Mediterranean Sea Jose ´ C. Ba ´ ez 1 , Jesu ´ s Olivero 2 , Raimundo Real 2 , J. Mario Vargas 2 and Antonio Flores-Moya 1, * 1 Departamento de Biologı´a Vegetal (Bota ´ nica), Facultad de Ciencias, Universidad de Ma ´ laga, Campus de Teatinos s/n, E-29071 Ma ´ laga, Spain, email: floresa@uma.es 2 Departamento de Biologı´a Animal (Zoologı´a), Facultad de Ciencias, Universidad de Ma ´ laga, Campus de Teatinos s/n, E-29071 Ma ´ laga, Spain *Corresponding author Abstract Geographical variation in species richness in the genera Audouinella (Rhodophyta), Cystoseira (Phaeophyceae) and Cladophora (Chlorophyta) in the western Mediterra- nean Sea was analysed by using a hypothetical-deduc- tive procedure. For this purpose, the continental shelves close to ten hydrographical watersheds were used as operational geographic units, and the number of species in each of the three genera, as well as the species of the three genera pooled together, were correlated with sev- eral environmental variables. No significant relationship was found between species richness in Audouinella and the environmental variables considered in the study, whereas a U-shaped relationship was found between Cystoseira richness and current flow. For the genus Cla- dophora, a linear relationship between species richness and current flow was obtained. For the three genera pooled together, a multiple linear model was obtained for species richness as a function of current flow and the distance from the Strait of Gibraltar, following the main current in the Mediterranean Sea. Keywords: Audouinella; Cladophora; Cystoseira; Mediterranean Sea; species richness. Introduction The study of the geographical distribution of species richness is an important task when assessing ecological relationships within a given territory, because richness is a result of the spatial response of species to the envi- ronmental and biological conditions in each area (Usher 1986, Real 1992, Ricketts 2001). The analysis of species richness in marine ecosystems has been explored little in comparison to that carried out on the richness of the biota from terrestrial systems. However, two paradigms have been proposed for gradients of species richness in several marine animal taxa: (i) there is a latitudinal cline of increasing species richness from poles to tropics, and (ii) that species richness increases from coast to abyssal plain (Briggs 1991, 1994, Gray 2001). Consistent with the first paradigm, the species numbers of North Atlantic seaweeds are latitudinally correlated with the highest sur- face water temperature in August (A ´ lvarez et al. 1988). Moreover, at a smaller geographical range, such as the Iberian Peninsula, the length of the shoreline was corre- lated both with the richness of brown seaweeds and with the whole seaweed flora, while the green macroalgal spe- cies number correlated with the February water salinity (A ´ lvarez et al. 1989). However, these studies lack explan- atory value about the causes of the gradients of species richness, because they were exclusively descriptive, whereas those carried out on other groups of organisms have demonstrated the effectiveness of a hypothetical- deductive modelling approach with explanatory power (Ball 1975, Real 1992). Therefore, it should be interesting to employ the hypothetical-deductive approach to elu- cidate species richness in seaweeds. Moreover, this approach could be the best choice for geographical ranges without extensive latitudinal range, such as within the Mediterranean Sea. In fact, the objective, hypotheti- cal-deductive approach has been successfully applied to discriminate biotic regions in seaweeds from the western Mediterranean and Adriatic Seas (Ba ´ ez et al. 2004). We studied the species richness of the genera Audoui- nella (Rhodophyta), Cystoseira (Phaeophyceae) and Cla- dophora (Chlorophyta) in the western Mediterranean Sea. Since the Mediterranean has an extensive longitudinal axis but extends less than 108 in latitude, the paradigm of the latitudinal changes of species richness (see above) cannot be applied and, therefore, different initial hypo- theses must be proposed to explain possible spatial richness patterns. Moreover, the paradigm of species richness increasing from coast to abyssal plain cannot be used to explain the species richness in seaweeds because they are restricted to the photic zone in shallow- er waters. Consequently, our approach tested several hypotheses relating species richness and gradients in environmental variables, and the best relationships were selected on objective, statistically significant criteria. Materials and methods Study area and selected taxa The study area was restricted to the western Mediterra- nean Sea due to the scarcity of floristic studies in the eastern area. Rather than political-administrative divi-