An integrative approach to the taxonomy of the pigmented European Pseudomonocelis Meixner, 1943 (Platyhelminthes: Proseriata)

Only a few species belonging to the Proseriata (Platyhelminthes) show a parenchymatic pigmentation, which may aid identification. Among these, Pseudomonocelis agilis has a yellowish body and is provided with a reddish-brown girdle in front of the statocyst. The species is known for limited areas of northern Europe and the Mediterranean. The present study was conducted to assess both the taxonomic status of populations attributed to the species across the unusually wide range for an interstitial flatworm, which lacks an obvious means of dispersal, and the levels of genetic variability within and among populations, by employing an integrative approach that included the analyses, on six populations, of three molecular markers (small subunit ribosomal 18S-like gene, inter-simple sequence repeat, allozymes), karyotypes, and 11 morphological characters. Furthermore, crossbreeding experiments were carried out on the Mediterranean populations. The results obtained revealed the existence of four highly divergent genotypic clusters, accompanied by karyological differences, with complete intersterility among the clusters tested. The combination of approaches adopted strongly supports the conclusion that the wide-ranging European pigmented species P. agilis is actually composed of four species: P. agilis in the Baltic area; Pseudomonocelis cetinae in the Adriatic; and Pseudomonocelis sp. nov. A and Pseudomonocelis sp. nov. B in the western and eastern Mediterranean, respectively. The latter two species are morphologically indistinguishable for the parameters essayed. Reconstruction of the phylogenetic relationships of these taxa, including congeneric and consubfamilial outgroups, showed that pigmentation is a plesiomorphic condition for the genus Pseudomonocelis and that Pseudomonocelis sp. nov. A shares a previously undetected, sister-group relationship with species of the unpigmented P. ophiocephala complex. The present study thus depicts complex speciation processes in a mesopsammic species, which involves allopatric divergence operating on different scales and ecological shifts, and highlights that the contribution of microturbellarians to marine biodiversity may be seriously underestimated.