They come in all sizes: integrative systematics and morphological radiation of Scherotheca (Lumbricidae, Crassiclitellata)

Abstract

Scherotheca is amongst the most diverse earthworm genera within Lumbricidae, exhibiting striking ecomorphological radiation—from small epigeic-like forms to giant anecic species. Accurate systematics is critical for exploring such diversification. We sequenced 16 previously unrepresented species plus additional populations across their range for five molecular markers, inferring phylogenetic relationships via maximum likelihood, Bayesian inference and time-calibrated analyses, as well as ancestral area and character state reconstructions. Scherotheca was recovered as monophyletic, but previous subgeneric divisions were unsupported. With the addition of four new species, Scherotheca currently includes 56 species: Cryptic diversity within Scherotheca aquitania may increase this number upon further research. Biogeographic analyses supported a middle Eocene origin (~47 Mya) on the Corsica–Sardinia–Provence microplate, followed by westward expansion driven by paleogeographic events, such as Pyrenean uplift and marine introgressions. Multiple Corsican clades evolved independently, displaying morphological radiation under insular pressures. Ancestral reconstructions indicated the last common ancestor resembled the mid-sized Scherotheca portcrosana. The radiation of body size was particularly pronounced in Atlantic regions, likely shaped by ecological, climatic and biotic pressures. This study provides the most comprehensive evolutionary framework for Scherotheca, serving as a reference for further research on Western European earthworm diversification, functional trait evolution and biogeographic history.