Age and origin of the Niphargidae
Niphargids are almost completely bound to subterranean waters except for a few species recorded from surface habitats or the bottom of deep lakes (Karaman and Ruffo, 1986). The family occurs only in the Palaearctic west of the Caspian Sea and currently comprises ten valid genera, of which Niphargus, with 408 species, is by far the most species-rich. The rest embraces only 16 species (Horton and Lowry, 2013). Only Haploginglymus is allopatric with respect to the rest of niphargids, although it overlaps with Niphargus (exceptionally coexisting with it) in two small disjunct areas on both edges of the Pyrenees (Pretus and Sabater, 1990; Notenboom, 1991; Karaman, 2015a; b).
Karaman and Ruffo (1986) suggested that the diversification of the family began in the basins of the Paratethys Sea during the Tertiary Period, from which European fresh waters were subsequently colonised. But the discovery of casts of niphargids in Late Eocene Baltic amber (ca. 45-50 Myr old; Coleman and Myers, 2001; Coleman and Ruffo, 2002; Jażdżewski and Kupryjanowicz, 2010) rules out this hypothesis since the birth of Parathethys as an enclosed basin with reduced salinity and endemic faunas took place only afterwards, at the early Oligocene at most (Rögl, 1997). McInerney et al. (2014) have attributed a much older, late Cretaceous age to the family, and an origin in NW Europe rather than in the Balkan area followed by a gradual range expansion across central Europe to reach its current distribution range. A third, alternative scenario where niphargids colonized freshwaters directly from the sea multiple times independently during their evolutionary history, and where the lineage of Haploginglymus settled in Iberia when it was an island during the late Cretaceous is not favored here since, if this were the case: (1) it is hardly conceivable that Niphargus was not established throughout the entire Peninsula when it is present in French territories adjacent to the Atlantic and the Mediterranean, and vice-versa, that Haploginglymus is not present in France; (2) there are no niphargids on North Atlantic oceanic islands, nor on Mediterranean islands such as the Balearics, disconnected from the continents at least since the end of the Oligocene, but where ulterior marine transgressive pulses have constrained severely the extension of emerged land; (3) the distribution of the family is not limited to areas covered by epicontinental seas in the geological past (Notenboom, 1991); and (4) brackish water species are exceptional among members of the family (Notenboom, 1991).
Our study reveals a strongly supported sister relationship between niphargids and pseudoniphargids that deserves a comment since it is relevant for the origin and biogeography of the Niphargidae. Contrary to niphargids, which are mainly limnic and are present on territories not formerly occupied by the sea, the Pseudoniphargidae are undeniably thalassoid, being present even on oceanic islands that have never been connected to the continents (Bermuda, Canaries, Madeira, Azores; Stock et al., 1986; Stock, 1988; Stock and Abreu, 1992; Stock, 1980). The study of the phylogenetic relationships among members of the family based on both morphological (Notenboom, 1988) and molecular features (analyses currently under way in our lab) has lead to the identification of a monophyletic cluster of species of Pseudoniphargus Chevreux, 1901 cantoned on the western edge of the Pyrennees – Basque Country, bordering the Gulf of Biscay – as the most primitive lineage within the family. Remarkably, the most primitive niphargids are also found in an area adjacent to the Gulf of Biscay (Great Britain; Mcinerney et al., 2014). Accordingly, we suggest that niphargids – contrary to pseudoniphargids – have colonized continental waters only once, and from a marine ancestor common to both families, and that this common ancestor most probably lived on the NE Atlantic coasts at the end of the Cretaceous. Once established in continental waters, niphargids proceeded to spread across Europe, with the colonisation of Iberia by the ancestor of Haploginglymus taking place before the rise of the Pyrenees 55-47 Myr ago. Niphargids should continue to be considered as a primary limnic group for biogeographic purposes, despite its presumed relatively recent (late Cretaceous) marine origin and sister relationship with the (thalassoid) Pseudoniphargidae.