Resolving latitudinal diversity gradients in earthworms using DNA-based species delimitation and diversity indices

Although a latitudinal gradient of species diversity is known for diverse animal groups, few detailed studies on this topic exist for soil animals. We aimed to clarify the formation process of latitudinal diversity gradients for soil animals from dispersal and evolutionary perspectives using earthworms. We used 4074 earthworms from 131 sites in the Japanese archipelago between 31.7°N and 45.1°N. Because morphological classification was difficult for these earthworms, especially for juveniles, we employed DNA-based species delimitation. We used DNA data for phylogenetic diversity indices and population genetic analysis. We analysed latitudinal changes in local and regional species diversity, and the replacement and nested structures among species assemblages using phylogenetic diversity indices, and examined whether environmental factors and the ecological traits of earthworms related to dispersal ability contribute to geographic diversity patterns. Our samples comprised 113 megascolecid and nine lumbricid molecular operational taxonomic units (MOTUs). Megascolecid species assemblage presented higher γ-, β- and α-diversity at lower latitudes affected by temperature, precipitation and snow depth. Overall β-diversity was greater at lower latitudes, reflecting greater spatial turnover due to greater γ-diversity with more local species at lower latitudes. Relatively high nestedness was observed at highest latitudes, suggesting that high-latitude species assemblages were formed through the range expansion of some species from lower latitudes. Such expansion was likely facilitated for potentially-parthenogenetic species. In contrast, Lumbricidae presented greater γ-diversity at higher latitudes and a converse nested structure from high to low latitudes. Our study demonstrated that DNA-based species delimitation is necessary to understand the exact geographic diversity pattern and its formation process for organisms whose morphological classification is difficult. The contradistinctive species-assemblage patterns between related earthworm groups might be considered to have resulted from their low dispersal ability, different biogeographical histories, and characteristic topography in the Japanese Archipelago, which covers a latitudinally wide range.