Cryptic characters for cryptic taxa: On the taxonomic utility of the genital chaetae in earthworms (Oligochaeta, Hormogastridae)

Hard anatomical characters have successfully been used in invertebrate systematics. Besides their chaetae, this kind of characters is virtually absent from earthworms (a taxonomically challenging group). This work explores the taxonomic utility of genital chaetae, which have been demonstrated to play an important role in earthworm reproduction. Genital chaetae were collected from different clades in the family Hormogastridae, some of them comprising cryptic complexes. The morphology of these structures was studied in detail with scanning electron microscopy and differential interference contrast microscopy with the aim of finding relevant phylogenetic information at different levels: putative diagnostic characters for clades, phylogenetic relationships between clades, and support for cryptic species delimitation. Our results revealed wide morphological variability in the distal tip of the genital chaetae between the different clades (summarized in five qualitative characters) and cryptic lineages. Canonical Variate Analysis based on dorsal landmarks was able to separate chaetae from the cryptic lineages in non-overlapping clusters and Discriminant Function Analysis allowed to assess its reliability. Some complex structures in the tip of genital chaetae, such as denticles and pores, highlighted the necessity of further research on their role in the reproduction of the different earthworm families. Our results support the potential of genital chaetae as a systematic character to resolve conflicting phylogenetic relationships of earthworms, being also potentially suitable for disentangling cryptic species complexes.