A comparison of latitudinal species diversity patterns between riverine and terrestrial earthworms from the North American temperate zone

Aim Latitudinal clines of species diversity are widely documented in terrestrial and marine ecosystems. However, the processes governing species diversity gradients in riverine ecosystems have not been well-studied. We addressed this issue by comparing species diversity between riverine aquatic and terrestrial earthworm groups (genus Sparganophilus and Diplocardia, respectively). Location North American temperate zone. Taxon Sparganophilus and Diplocardia. Methods We collected 556 Sparganophilus earthworms from 64 sites spanning 27 degrees of latitude (18.77 degrees-45.90 degrees N), and 165 Diplocardia earthworms from 23 sites (21 degrees, 19.77 degrees-41.20 degrees N). We split potential species from the phylogenetic trees based on four genes and compared the latitudinal pattern of species diversity between these two groups. Results We estimated the number of potential species to be 10 for Sparganophilus and 8 for Diplocardia, respectively, from 526 haplotypes (403 in Sparganophilus and 123 in Diplocardia). Sparganophilus species diversity was higher at mid-latitudes (32 degrees to 40 degrees) due to a preponderance of species with limited geographical distributions, whereas all specimens collected north of 40 degrees belonged to broadly distributed species. Species with limited geographical distributions were more often collected at higher elevations than broadly distributed species in Sparganophilus. For Diplocardia, species diversity was higher at lower latitudes (28 degrees to 32 degrees). Main Conclusions These results suggest that, in Sparganophilus, species composition at higher latitudes above 40 degrees is derived from range expansion by broadly distributed species from lower latitudes. The high elevation area in the native distribution range of Sparganophilus is limited to the Appalachian Mountains, which ranges above 33 degrees in latitude. The high species diversity of Sparganophilus with limited distributions at mid-latitude (32 degrees-40 degrees) suggests that the headwater regions at the Appalachian Mountains are sites for more frequent speciation.