Syntopy in subterranean fauna: Trophic specialisation in two new species of Rhyacodrilus Bretscher, 1901 (Annelida, Clitellata, Rhyacodrilinae)

Trophic generalism in the subterranean fauna is assumed to be an adaptation to food scarcity or irregular availability, but studies on the feeding biology of subterranean species are almost nonexistent. Herein we (i) describe two new species, Rhyacodrilus arenivorus sp. nov. and Rhyacodrilus moulis sp. nov. (Annelida, Clitellata, Rhyacodrilinae), occurring in syntopy in the subterranean river of Labouiche, France; and (ii) report trophic and morphological specialisation in these two congeneric oligochaete species. The gut contents of R. arenivorus sp. nov. and R. moulis sp. nov. were similar, i.e., mineral particles and detritus. However, a detailed analysis revealed specialised foraging strategies in both species. Niche breadth value for ingested mineral particles was low (0.28) in R. moulis sp. nov. and intermediate (0.52) in R. arenivorus sp. nov. Moreover, slight interspecific morphological differences were found in two characters associated with foraging strategies in oligochaetes, the pharynx and the pharyngeal glands. A relatively high degree of niche overlap (≥0.58), however, was detected between R. arenivorus sp. nov. and R. moulis sp. nov. in relation to the size of ingested mineral particles.