Patterns of endemism in south-eastern Pacific benthic polychaetes of the Chilean coast

Abstract

Aim In this study we evaluate patterns of endemism for benthic polychaete species along the southeastern Pacific coast of Chile. Our goals were (1) to describe latitudinal gradients of endemism and identify areas of high endemism, (2) to evaluate the effect of biogeographical limits on endemism patterns, and (3) to evaluate indirectly the role played by evolutionary dynamics on patterns of endemism. Location South-eastern Pacific coast of Chile, ranging from Arica (18 degrees S) to Cape Horn (56 degrees S).

Methods We used a list of 178 species of endemic, shallow benthic polychaetes to evaluate patterns of endemism. Parsimony analysis of endemicity (PAE) and the endemism index (EI) were used to evaluate hierarchical relationships of endemism between different latitudinal bands, and to identify areas with high degrees of endemism and differences in endemism. We evaluated the effect of biogeographical limits on endemic polychaete fauna by testing for the existence of geometric constraints (mid-domain effect). The role of evolutionary dynamics on latitudinal patterns of endemism was evaluated with nestedness analysis (NA) using the temperature index.

Results The PAE analysis indicated two large, separate areas of endemism: (1) the northern area between 18 degrees S and 38 degrees S, and (2) the southern area between 39 degrees S and 56 degrees S. The endemism index showed a maximum value (32 species) around 39 degrees-41 degrees S. Species-richness curves of each 3 degrees band of latitude showed a clear mid-domain effect (69%), but the two maximum points of species richness at mid-latitudes (36 degrees S to 38 degrees S and 39 degrees S to 41 degrees S) did not correspond to the mid-domain peak in species richness, presenting a greater number of species than expected by the mid-domain effect. The nestedness analysis showed that the number of genera reaches a maximum of 70 at mid-latitudes (36 degrees-41 degrees S), decreasing towards both the northern and southern areas. The spatial distribution of the entire data set of endemic species showed a nested pattern (T degrees = 24.5 degrees, P < 0.0001).

Main conclusions Our results strongly support the existence of a latitudinal gradient of endemism for benthic polychaete species along the Chilean coast. The shape of this gradient is clearly non-linear, with a marked peak of endemism occurring at mid-latitudes (36 degrees-41 degrees S, endemism hotspot), which also corresponds to a peak in species richness. Furthermore, this hotspot is the midpoint separating two distinct areas of endemism to the north and south. We suggest that the observed pattern of endemism for benthic polychaete taxa of the Chilean coast can be explained by a combination of geometric constraints and historical mechanisms, such as the processes that affected the Chilean coast during the Neogene (e.g. ENSO, oxygen minimum zone, glaciations).