Spatial changes of macrobenthic polychaete assemblages from the southern Mexican Pacific shelf: diversity patterns and feeding guilds composition

Polychaetes account for most of the macrobenthic species found in soft bottoms, but in the Tropical Eastern Pacific, and particularly in the southern Mexican Pacific shelf, their composition and diversity patterns are very poorly known. So, the aim of this study was to analyze how the polychaetes are distributed in relation to the environmental variables across the continental shelf in that area, and to classify their assemblages as defined by the species present and their variations. For this purpose, we examined 3741 individuals of 82 species, collected there at 38 stations between 42-109 m depth. Using univariate and multivariate statistical approaches, we found that depth was the main structuring variable, to separate the faunal assemblages, with the sediment texture as secondary, but also statistically important. The distribution of the assemblages and the largest changes in diversity and variety of feeding modes were highly linked to shelf levels. The assemblage Paraprionospio pinnata-Magelona marianae (PpMm) was defined for the inner shelf, with the highest diversity and dominated by surface deposit feeder species. In the middle and outer shelves, the diversity values decreased, and the motile carnivore species gradually replaced the detritivores, determining the presence of the assemblages Linopherus kristiani-P. pinnata (PpLk) and L. kristiani (Lk), respectively. The sandy bottoms, well oxygenated and with low concentrations of nutrients from the inner shelf, correlated well with the rich assemblage PpMm, while in the deeper zones with muddy bottoms, higher in nutrients and poorly oxygenated, the less diverse assemblages PpLk and Lk occurred. Knowing how the polychaete assemblages change with depth and sediment composition in this poorly explored marine region, can be useful to develop future research, by examining their occurrence on distinct habitats and to monitor long-term faunal and environmental changes.