MEPS 367:193-202 (2008)  -  DOI: https://doi.org/10.3354/meps07596

ABSTRACT: We analysed the distribution patterns of megabenthic species of marine molluscs from the Uruguayan shelf in relation to 3 interacting gradients, namely depth, salinity and temperature of sea bottom waters. Our results were generated from a data set comprising the complete list of megabenthic gastropods (adult shell length >50 mm; 22 species) from a study area that encompassed latitudes from 34 to 39° south, between 4 and 800 m depth. Multiple regressions were used to evaluate the contribution of 3 classes of explanatory variables to gastropod richness at sampling points along the gradients: (1) geometric constraints on the distribution of species’ tolerance ranges for environmental variables (the mid-domain effect, MDE), (2) the species–area effect driven by the spatial extent of scaled environmental variables, and (3) the environmental variables themselves. In addition, we explored multivariate explanations for the bathymetric pattern of gastropod species richness, incorporating depth-based area, temperature, salinity and geometric constraints as predictor variables. Examination of Moran’s I for regression residuals confirmed that results were not biased by spatial autocorrelation. Overall, the environmental variables per se were the most important contributors to the observed patterns. However, a significant signature of the MDE was revealed, by the entry of geometric constraints into nearly all models. In contrast, the species–area effect was less important. These results suggest that: (1) MDE may operate in any kind of bounded gradient, (2) both stochastic and deterministic processes are responsible for the pattern observed, and (3) the relative contribution of the MDE to the patterns is dependent on the gradient considered and the way in which species richness is measured.

KEY WORDS: Mid-domain effect · Species richness · Continental shelf · Gastropods · Uruguay