Ancient diversity within Diporodrilus (Crassiclitellata, Annelida) clarify the historical biogeography of Corso-Sardinian earthworms

Abstract

Corsica and Sardinia are amongst the largest islands of the Western Mediterranean. Their complex geological history included belonging to the European–Iberian continental margin (close to current-day Catalonia and Provence) and varying degrees of isolation for the last 30 million years, leading to peculiar, highly endemic faunas and floras. This is especially true for their earthworm faunas, which include endemic species of several Lumbricoidea genera and the endemic family (or subfamily) Diporodrilidae. Only three species have been described for the morphologically unique Diporodrilus, but there exists evidence for wide morphological variability within them and the existence of several species-level genetic lineages within Corsica. This work aimed to investigate the genetic diversity and phylogenetic relationships between the genetic lineages of Diporodrilus from Corsica and Sardinia (based on the sequences of 5 mito-nuclear markers), to perform an integrative systematics revision combining species delimitation techniques and morphological data, and to obtain a time-calibrated phylogeny of Diporodrilus and other Corso-Sardinian Lumbricoidea. Within 15 populations of the morphospecies Diporodrilus omodeoi and Diporodrilus pilosus across Corsica and Sardinia, 10 species-level genetic lineages were detected. Phylogenetic independence, high genetic divergence and morphological differences provided the support for the description of five new pseudocryptic species: Diporodrilus rotundus sp. nov., Di. jorgei sp. nov., Di. minor sp. nov., Di. meridionalis sp. nov. and Di. telti sp. nov. Time-calibrated phylogenetic inference estimated the age for genus Diporodrilus at 65.9 Mya; even if other Corso-Sardinian genera were significantly younger, all of them presented deep divergences predating the break-off of the microplate from the continent. The almost threefold increase in the known diversity of Diporodrilus stresses the need for comprehensive earthworm diversity inventories in both Sardinia and Corsica, and for studies on their ecological role and conservation status. The close correspondence between some geological and cladogenetic events suggest that the distribution of Corso-Sardinian earthworms could be used to inform standing geological controversies.