Through systematic, faunistic and zoogeographical relationships among the most significant stygobitic and eustygophilic taxa, more than one rout of colonization, as well as different temporal derivations, can be suggested for the groundwater fauna of Italy.

REGRESSION MODEL EVOLUTION (According to Stock’s proposition).

It regards such ancient stygobiontes as the thermosbaenaceans, the mysids, the microparasellid and cirolanid isopods, the decapod genus Typhlocaris, the gammarids Bogidiella, Hadzia and some Salentinella species and, maybe, the ostracod Pseudolimnocythere hypogaea, that originated gradually from marine ancestors that got stranded and successively adapted to inland ground waters during regression periods of the Tertiary; in fact, these groups are actually distributed in areas once covered by Tertiary seas, as well as they show a wide distribution and plastic ecology.

ACTIVE MIGRATION MODEL

It refers to those stygobiontes, such as numerous cyclopid and harpacticoid copepods, the amphipod genera Pseudoniphargus and Rhipidogammarus and some water mites that dispersed in inland uncolonized areas from the sea, through brackish water environments,due to their high degree of salinity tolerance.

SEA-SURFACE FRESH WATERS-GROUND WATERS MIGRATION MODEL

This model concerns some italian stygobitic taxa, such as the amphipods of the genus Carinurella, and the syncarids, that followed a route of colonization leading them from the sea into surface fresh waters, and from there into underground continental aquatic systems.

CLIMATIC REFUGIUM MODEL

It is related to numerous italian stygobiontes and eustygophiles (asellid isopods, ostracods except Pseudolimnocythere, the copepod genera Speocyclops, Graeteriella, Elaphoidella and other representatives of the family Canthocamptidae, the amphipods Niphargus and Ilvanella and the idrobioid gastropods) that colonized the ground waters directly from surface fresh waters, somewhere at the end of Pliocene, owing to drastic surface climatic changes (e.g. the Quaternary cooling).

Some species, such as the amphipod Bogidiella chappuisi and the microparasellid isopod Microcharon marinus, as well as harpacticoid copepods of the families Ectinosomatidae and Cylindropsyllidae, cannot properly be placed in the above models since they have living marine ancestors or they are still colonizing the underground freshwater systems through mixohaline interstitial environments.