Depth-range distribution and diversity patterns of a polychaete (Annelida) community in the continental shelf of the Southern Gulf of Mexico

Abstract

We tested the mid-domain effect to evaluate the bathymetric patterns of annelid polychaete species richness from the Southern Gulf of Mexico as a possible hypothesis to explain the species richness gradient, exploring the overlapping of species depth ranges towards the middle continental shelf. The bathymetric gradient of the number of species was estimated using the depth ranges of 259 polychaete species and the mid-domain effect was tested using a Monte Carlo simulation program at bands of 10 m depth. The carbonate (234 species) and the terrigenous (121 species) Gulf shelves showed clear differences in species richness which was characterized by a highly significant presence of polychaetes with short depth ranges (<10 m). The distribution of richness could be described as a cubic polynomial curve but the maximum values for all case studied (200, 181 and 84 species, for the whole study area, and the carbonate and terrigenous shelves, respectively) are strongly skewed to shallow waters (40 m). This is not consistent with the peak of diversity at 60–70 m predicted by the model. Therefore, the observed patterns cannot be reproduced by the mid-domain effect, suggesting the existence of non-random factors affecting the species richness gradients in the Gulf. We also found a gradient from west to east in species richness with increasing species richness in the carbonate shelf.