DNA barcodes, cryptic diversity and phylogeography of a W Mediterranean assemblage of thermosbaenacean crustaceans

We assess the occurrence of crypticism and analyse the phylogeography of a thermosbaenacean crustacean, the monodellid Tethysbaena scabra, endemic to the Balearic Islands (W Mediterranean). This species occurs only in mixohaline waters of coastal wells and caves adjacent to the seashore. We have used the mitochondrial DNA barcode region to assess its genetic population structure throughout the anchialine environment of the islands. Maximum likelihood phylogenetic analyses showed that the Balearic Tethysbaena and those from the NW Italian Peninsula form a monophyletic assemblage subdivided into several lineages. Cytochrome c oxidase subunit 1 (cox1) p-distances among the more divergent Mallorcan lineages are remarkably high and on par with those established between the formally described species T.scabra from Menorca and T.argentarii from Italy. This result and the application of the generalised mixed Yule coalescence model (GMYC) suggest that at least some of the Mallorcan lineages represent cryptic species. A clear-cut phylogeographic pattern is displayed by this anchialine assemblage: six of its seven lineages appear in allopatry, with the exception of a Mallorcan lineage limited to a single cave nested within the geographic range of another lineage. All lineages show a distribution reduced to a single cave or to short portions of coast not exceeding 60km in length. Our coalescence estimations suggest an early Tortonian (10.7Ma) origin for the Balearic+Italy Tethysbaena clade, an age that is largely prior to the onset of the eustatic oscillations associated with the Quaternary glaciations. Only the diversification that took place within some of the Mallorcan lineages could be coeval with the broad glacio-eustatic oscillations of the Quaternary.