Marine Regressions and the Evolution of Groundwater Dwelling Amphipods (Crustacea)

The origin of marine lineages of amphipods in continental and insular ground waters is treated, and the im- portance in it of relative sea-level lowering is weighed. Pat- terns of distribution and relationships of Hadzioidea, Salentinellidae, Bogidielloidea and Niphargidae are re- viewed, with emphasis on the evidence for their direct marine origin, and on their dispersal abilities. The process of troglobitization, linked with subterranean evolution, in amphipods is discussed. A two-step model of the evolution of thalassoid lineages is discerned. During the first phase, marine populations colonize crevicular or interstitial habitats. These habitats are physiographically connected with the inland stygohabi- tats, and are characterized by severe ecological conditions. During the second phase, certain lineages overcome the salinity boundary and colonize inland ground waters through active migration or passive isolation, or through a combination of both. Marine regressions seem of little significance in the first evolutionary phase, where they primarily explain the isola- tion in coastal ground waters of marine littoral populations, already adapted to subterranean conditions. Vicariance by marine regressions is considered of relatively great evolu- tionary significance in thalassoid groups of low vagility and dispersal ability. This type of vicariance is followed by peripatric speciation, and a largely polychotomous pattern of descent is predicted.