A molecular approach to the reconstruction of the pre-Lessepsian fauna of the Isthmus of Suez: The case of the interstitial flatworm Monocelis lineata sensu lato (Platyhelminthes: Proseriata)

Since the opening of the Suez Canal in 1869 the Mediterranean Sea has been affected by a gradually increasing
influx of species of Erythraean origin, which significantly modified species composition and ecological interactions
of entire communities. Most studies have focused on macrofaunal species. Nevertheless, given the sandy
nature of its shores, the Suez Canal is an ideal habitat for many interstitial taxa, whose study has been so far
neglected. During a number of sampling campaigns, aimed to appreciate interstitial flatworms fauna in the Suez
Canal, specimens of the Monocelis lineata (Platyhelminthes: Proseriata) species complex were collected in Lake
Timsah.
Here we performed molecular analyses by using the 18S and 28S nuclear rDNA genes. Species delimitation
methods agree in considering this population as a distinct entity (Monocelis sp. nov.). Molecular phylogeny
places Monocelis sp. nov. as the sister taxon of the Mediterranean populations of the Monocelis lineata species
complex. Molecular dating results based on a minimum age calibration point, date the common ancestor of
Monocelis sp. nov. and Mediterranean populations around the Bramertonian Interglacial optimum (1.8–2.3 myr
ago). In that stage, due to high eustatic sea levels, the geographic limit in the Isthmus of Suez shifted southward,
with a new biogeographic barrier placed about the southern edge of the Gulf of Suez. It is thus conceivable that
the ancestor of present-day Monocelis sp. nov. became isolated from its Mediterranean counterparts in the following
regression, during the Pre-Pastonian glaciation. Data presented here suggest that populations inhabiting
the Isthmus should be carefully screened as they may reveal a long history of persistence in the area and help to
reconstruct past evolutionary histories of Mediterranean and Red Sea species. Considering the recent project
aimed to widen the Suez Canal, which may significantly modify present ecological conditions, gathering of
genetic data from populations of the water bodies of the Isthmus becomes an urgent task, before they are lost
forever.