The nunatak and tabula rasa hypotheses may be compatible: the European phylogeography of a riparian earthworm.
The tabula rasa hypothesis of postglacial immigration supports the notion that species now found in northern European areas must have been recently recolonized from historical refugia. Until the 1960s, however, there was almost complete consensus that disjunctions and endemism in the North Atlantic region of Europe could not be explained without in situ survival during glacial periods (the nunatak hypothesis). Although some earthworms can survive in permafrost and tolerate cold conditions, it is generally believed that most earthworms were eradicated from northern latitudes during the Last Glacial Maximum. To test which hypothesis explains the phylogeography of the riparian and parthenogenetic earthworm Eiseniella tetraedra, we collected 1,640 specimens from 19 different countries in Europe. We examined three molecular markers (COI, 16S and 28S) and their morphology. Eleven lineages were found, nested in five clades. Clade I was more prevalent in cold biogeographical regions such as the continental, the Atlantic or even the Arctic, while clade II was prevalent in Mediterranean regions. We investigated their potential niches through Species Distribution Models, which agreed with the distribution trends. The presence of restricted clades in the Iberian and Scandinavian peninsulas, as well as in Eastern Europe, suggests that these three regions served as refugia during the Last Glacial Maximum. Thus, both hypotheses were necessary to explain the actual distribution of this shore-dwelling earthworm.