Ecological drivers influence the distributions of two cryptic lineages in an earthworm morphospecies

Substantial genetic diversity exists within earthworm morphotypes, such that traditional species designations may be incomplete. It is, however, currently not known whether these different genetic variants show ubiquity or specialty in their distribution across separated sites subject to different climatic, biotic or soil physicochemical factors. Here we report on the results of a survey in which individuals of the Lumbricus rubellus morphotype, a species known to comprise two deeply divergent genetic lineages in England and Wales, were sampled from 26 plots. Sequences from the mitochondrial cytochrome oxidase I gene were used to distinguish lineages for 787 individuals. In conjunction, a range of geographic, climatic, biotic and soil physiochemical variables were also collected for each locality. Genotyping indicated that Lineage A was more common than Lineage B, comprising 58% of the collected L. rubellus. Six site populations comprised only Lineage A, while only a single site comprised entirely Lineage B. The remaining 20 sites contained both lineages. A multivariate ordination of site variables identified major difference between sites were associated with low pH, organic-rich soils in Western wet upland areas and pollutant levels associated with sites in the South. Earthworm genotype (as proportion of Lineage A) was not correlated with either of these major environmental axes. When individual variables of soil pH and the percentage of soil organic matter, which are known to be key driver of soil species distributions, were investigated as single variables significant relationship with lineage frequency were found. Soil organic matter content was significantly negatively correlated with Lineage A proportion, while pH was significantly positively correlated. This lineage preference may be related to lineage metabolism and/or behavioral differences. Measurement of tissue metal concentrations in worms from 17 sites identified a significant site effect in all cases, but a lineage effect only for arsenic (higher Lineage B). Tissue arsenic concentrations varied between lineages, supporting previous observations that there are differences in the way the two lineages have adapted to manage exposure to this metalloid.