Pseudo-cryptic speciation in the subterranean medium: A new species of Stylodrilus Claparède, 1862, with a revision of the status of Bichaeta Bretscher, 1900 (Annelida, Clitellata, Lumbriculidae)

The subterranean aquatic medium is known to harbor a large diversity of cryptic species. We describe a new aquatic clitellate, Stylodrilus mariae n. sp., from groundwaters in the Cantabrian region, northern Spain, using integrative taxonomy (morphological and molecular data). The new species shows inconspicuous morphological differences with Stylodrilus parvus s. lat. The tree topologies based on sequences of the barcoding gene Cytochrome Oxidase I (COI) and two nuclear markers i.e., internal transcribed spacer (ITS) region and histone 3 (H3) were concordant and revealed that S. parvus s. lat. and the new species are genetically well-differentiated. They are non-sister lineages, showing a maximum genetic (uncorrected p) distance of 20% for COI. Constrained phenotypic evolution is apparent in the clade within which S. mariae n. sp. and S. parvus are placed; this can explain the decoupling of morphological and molecular evolution in these two species although we cannot completely discard convergent evolution. Based on molecular data, the former Bichaeta sanguinea clusters within the genus Stylodrilus, and since this is concordant with the morphology, Stylodrilus sanguineus ( Bretscher, 1900) n. comb. is proposed.