DNA barcoding reveals diversity patterns of earthworm communities in remote tropical forests of French Guiana

Despite representing a key component of terrestrial biota, soil invertebrates in tropical rainforests have been poorly studied from both a taxonomic and an ecological perspective when compared to other groups of terrestrial animals. We sampled earthworm communities in a range of sampling locations in two different study sites of the Nouragues Natural Reserve in French Guiana, focusing on lowland to plateau and hilltop forests as well as on savannah-like vegetation of the Nouragues granitic inselberg. We used the barcode region of the COI gene to delimit Molecular Taxonomic Units (MOTUs), further validated using species-level diagnostic morphological characters. A total of 651 sequences was obtained, most of them corresponding to juveniles that cannot be identified to the species level from morphology alone. We found a total of 48 MOTUs, and both rarefaction curves and diversity estimators (Chao1 and ACE) suggested that 60 species could occur in the study area, representing the highest earthworm richness ever recorded worldwide. Beta-diversity analyses highlighted a strong species turnover between sampling locations. Except in a few specific cases, species richness was usually limited to 12 species at the scale of a given location, which likely indicates the influence of competitive interactions during community assembly process. Community structure was dominated by species living in the upper soil layers and in surface microhabitats, with some of them able to colonize epiphytic soils up to more than 40 m above ground level. These results suggest the importance of long-term diversification processes and current ecological factors for the structuring and the diversity of earthworm communities in tropical rainforests of French Guiana.